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+The Project Gutenberg EBook of A Critique of the Theory of Evolution, by 
+Thomas Hunt Morgan
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Critique of the Theory of Evolution
+
+Author: Thomas Hunt Morgan
+
+Release Date: December 17, 2009 [EBook #30701]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CRITIQUE OF THEORY OF EVOLUTION ***
+
+
+
+
+Produced by Bryan Ness, Keith Edkins, and the Online
+Distributed Proofreading Team at https://www.pgdp.net. Pages
+scanned by Bryan Ness.
+
+
+
+
+
+Transcriber's note: A few typographical errors have been corrected: they
+are listed at the end of the text.
+
+       *       *       *       *       *
+
+
+Princeton University
+
+THE LOUIS CLARK VANUXEM FOUNDATION
+LECTURES FOR 1915-1916
+
+       *       *       *       *       *
+
+The Louis Clark Vanuxem Foundation of Princeton University
+
+was established in 1912 with a bequest of $25,000 under the will of Louis
+Clark Vanuxem, of the Class of 1879. By direction of the executors of Mr.
+Vanuxem's estate, the income of the foundation is to be used for a series
+of public lectures delivered in Princeton annually, at least one half of
+which shall be on subjects of current scientific interest. The lectures are
+to be published and distributed among schools and libraries generally.
+
+The following lectures have already been published or are in press:
+
+    1912-13 The Theory of Permutable Functions, by Vito Volterra
+
+    1913-14 Lectures delivered in connection with the dedication of the
+    Graduate College of Princeton University by Emile Boutroux, Alois
+    Riehl, A. D. Godley, and Arthur Shipley
+
+    1914-15 Romance, by Sir Walter Raleigh
+
+    1915-16 A Critique of the Theory of Evolution, by Thomas Hunt Morgan
+
+       *       *       *       *       *
+
+LOUIS CLARK VANUXEM FOUNDATION
+
+A CRITIQUE
+
+OF THE
+
+THEORY OF EVOLUTION
+
+BY
+
+THOMAS HUNT MORGAN
+
+PROFESSOR OF EXPERIMENTAL ZOOLOGY IN
+COLUMBIA UNIVERSITY
+
+LECTURES DELIVERED AT PRINCETON UNIVERSITY
+FEBRUARY 24, MARCH 1, 8, 15, 1916
+
+PRINCETON UNIVERSITY PRESS
+PRINCETON
+LONDON: HUMPHREY MILFORD
+OXFORD UNIVERSITY PRESS
+1916
+
+       *       *       *       *       *
+
+Copyright, 1916, by
+PRINCETON UNIVERSITY PRESS
+Published October, 1916
+
+[Illustration]
+
+       *       *       *       *       *
+
+
+PREFACE
+
+Occasionally one hears today the statement that we have come to realize
+that we know nothing about evolution. This point of view is a healthy
+reaction to the over-confident belief that we knew everything about
+evolution. There are even those rash enough to think that in the last few
+years we have learned more about evolution than we might have hoped to know
+a few years ago. A _critique_ therefore not only becomes a criticism of the
+older evidence but an appreciation of the new evidence.
+
+In the first lecture an attempt is made to put a new valuation on the
+traditional evidence for evolution. In the second lecture the most recent
+work on heredity is dealt with, for only characters that are inherited can
+become a part of the evolutionary process. In the third lecture the
+physical basis of heredity and the composition of the germ plasm stream are
+examined in the light of new observations; while in the fourth lecture the
+thesis is developed that chance variation combined with a property of
+living things to manifold themselves is the key note of modern evolutionary
+thought.
+
+T. H. MORGAN
+
+_July, 1916_
+
+       *       *       *       *       *
+
+
+                      TABLE OF CONTENTS
+
+                          CHAPTER I
+             A REVALUATION OF THE EVIDENCE ON
+          WHICH THE THEORY OF EVOLUTION WAS BASED
+
+                                                      PAGE
+  PREFACE                                                v
+
+  1. THREE KINDS OF EVOLUTION                          1-7
+
+  2. THE EVIDENCE FOR ORGANIC EVOLUTION               7-27
+     a. The Evidence from Comparative Anatomy         7-14
+     b. The Evidence from Embryology                 14-23
+     c. The Evidence from Paleontology               24-27
+
+  3. THE FOUR GREAT HISTORICAL SPECULATIONS          27-39
+     a. The Environment                              27-31
+        Geoffroy St. Hilaire
+     b. Use and Disuse                               31-34
+        From Lamarck to Weismann
+     c. The Unfolding Principle                      34-36
+        Nägeli and Bateson
+     d. Natural Selection                            36-39
+        Darwin
+
+                         CHAPTER II
+             THE BEARING OF MENDEL'S DISCOVERY ON
+               THE ORIGIN OF HEREDITY CHARACTERS
+
+  1. Mendel's First Discovery--Segregation           41-52
+
+  2. Mendel's Second Discovery--Independent
+     Assortment                                      52-59
+
+  3. The Characters of Wild Animals and Plants
+     Follow the Same Laws of Inheritance as do
+     the Characters of Domesticated Animals and
+     Plants                                          59-84
+     a. Sexual Dimorphism                            61-64
+        Eosin eye color of Drosophila                61-62
+        Color of the Clover Butterfly, Colias
+        philodice                                    62-63
+        Color of Papilio turnus                      63
+        Color pattern of Papilio polytes             63-64
+     b. Duplication of parts                         65-66
+        Thorax of Drosophila                         65
+        Legs of Drosophila                           65-66
+     c. Loss of characters                           66-68
+        "Eyeless" of Drosophila                      66-67
+        Vestigial wings of Drosophila                67
+        Bar eye of Drosophila                        67-68
+     d. Small changes of characters                  68-70
+        "Speck"                                      68
+        Bristles of "club"                           70
+     e. Manifold effects of same factor              71
+     f. Constant but trivial effects may be the
+        product of factors having other vital
+        aspect                                       73
+     g. Sex-linked inheritance                       75-80
+        in Drosophila ampelophila                    75-76
+        in the wild species D. repleta               76
+        in man                                       77
+        in domesticated Fowls                        77-78
+        in the wild moth, Abraxas                    78-80
+    h. Multiple allelomorphs                         81-84
+        in the wild Grouse Locust                    81-83
+        in domesticated mice and rabbits             83
+        in Drosophila ampelophila                    84
+
+  4. MUTATION AND EVOLUTION                          84-88
+
+                         CHAPTER III
+               THE FACTORIAL THEORY OF HEREDITY
+             AND THE COMPOSITION OF THE GERM PLASM
+
+  1. THE CELLULAR BASIS OF ORGANIC EVOLUTION
+     AND HEREDITY                                    89-98
+
+  2. THE MECHANISM OF MENDELIAN HEREDITY
+     DISCOVERED IN THE BEHAVIOR OF THE
+     CHROMOSOMES                                    98-102
+
+  3. THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA
+     AMPELOPHILA                                   103-118
+     a. Group I.                                   104-109
+     b. Group II.                                  109-112
+     c. Group III.                                 112-115
+     d. Group IV.                                  115-118
+
+  4. LOCALIZATION OF FACTORS IN THE CHROMOSOMES    118-142
+     a. The Evidence from Sex Linked Inheritance   118-137
+     b. The Evidence from Interference             137-138
+     c. The Evidence from Non-Disjunction          139-142
+
+  5. HOW MANY GENETIC FACTORS ARE THERE IN
+     THE GERM-PLASM OF A SINGLE INDIVIDUAL?        142-143
+
+  6. CONCLUSIONS                                   144
+
+                         CHAPTER IV
+                   SELECTION AND EVOLUTION
+
+  1. THE THEORY OF NATURAL SELECTION               145-161
+
+  2. HOW HAS SELECTION IN DOMESTICATED ANIMALS
+     AND PLANTS BROUGHT ABOUT ITS RESULTS?         161-165
+
+  3. ARE FACTORS CHANGED THROUGH SELECTION?        165-187
+
+  4. HOW DOES NATURAL SELECTION INFLUENCE
+     THE COURSE OF EVOLUTION?                      187-193
+
+  5. CONCLUSIONS                                   193-194
+
+  INDEX                                            195-197
+
+       *       *       *       *       *
+
+
+CHAPTER I
+
+A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED
+
+We use the word evolution in many ways--to include many different kinds of
+changes. There is hardly any other scientific term that is used so
+carelessly--to imply so much, to mean so little.
+
+THREE KINDS OF EVOLUTION
+
+We speak of the evolution of the stars, of the evolution of the horse, of
+the evolution of the steam engine, as though they were all part of the same
+process. What have they in common? Only this, that each concerns itself
+with the _history_ of something. When the astronomer thinks of the
+_evolution_ of the earth, the moon, the sun and the stars, he has a picture
+of diffuse matter that has slowly condensed. With condensation came heat;
+with heat, action and reaction within the mass until the chemical
+substances that we know today were produced. This is the nebular hypothesis
+of the astronomer. The astronomer explains, or tries to explain, how this
+evolution took place, by an appeal to the physical processes that have been
+worked out in the laboratory, processes which he thinks have existed
+through all the eons during which this evolution was going on and which
+were its immediate causes.
+
+When the biologist thinks of the evolution of animals and plants, a
+different picture presents itself. He thinks of series of animals that have
+lived in the past, whose bones (fig. 1) and shells have been preserved in
+the rocks. He thinks of these animals as having in the past given birth,
+through an unbroken succession of individuals, to the living inhabitants of
+the earth today. He thinks that the old, simpler types of the past have in
+part changed over into the more complex forms of today.
+
+He is thinking as the historian thinks, but he sometimes gets confused and
+thinks that he is explaining evolution when he is only describing it.
+
+[Illustration: FIG. 1. A series of skulls and feet. Eohippus, Mesohippus,
+Meryhippus, Hipparion and Equus. (American Museum of Natural History. After
+Matthews.)]
+
+A third kind of evolution is one for which man himself is responsible, in
+the sense that he has brought it about, often with a definite end in view.
+
+His mind has worked slowly from stage to stage. We can often trace the
+history of the stages through which his psychic processes have passed. The
+evolution of the steam-boat, the steam engine, paintings, clothing,
+instruments of agriculture, of manufacture, or of warfare (fig. 2)
+illustrates the history of human progress. There is an obvious and striking
+similarity between the evolution of man's inventions and the evolution of
+the shells of molluscs and of the bones of mammals, yet in neither case
+does a knowledge of the order in which these things arose explain them. If
+we appeal to the psychologist he will probably tell us that human
+inventions are either the result of happy accidents, that have led to an
+unforeseen, but discovered use; or else the use of the invention was
+foreseen. It is to the latter process more especially that the idea of
+_purpose_ is applied. When we come to review the four great lines of
+evolutionary thought we shall see that this human idea of purpose recurs in
+many forms, suggesting that man has often tried to explain how organic
+evolution has taken place by an appeal to the method which he believes he
+makes use of himself in the inorganic world.
+
+[Illustration: FIG. 2. Evolution of pole arms. (Metropolitan Museum. After
+Dean.)]
+
+What has the evolution of the stars, of the horse and of human inventions
+in common? Only this, that in each case from a simple beginning through a
+series of changes something more complex, or at least different, has come
+into being. To lump all these kinds of changes into one and call them
+evolution is no more than asserting that you believe in consecutive series
+of events (which is history) causally connected (which is science); that
+is, that you believe in history and that you believe in science. But let us
+not forget that we may have complete faith in both without thereby offering
+any explanation of either. It is the business of science to find out
+_specifically_ what kinds of events were involved when the stars evolved in
+the sky, when the horse evolved on the earth, and the steam engine was
+evolved from the mind of man.
+
+Is it not rather an empty generalization to say that any kind of change is
+a process of evolution? At most it means little more than that you want to
+intimate that miraculous intervention is not necessary to account for such
+kinds of histories.
+
+We are concerned here more particularly with the biologists' ideas of
+evolution. My intention is to review the evidence on which the old theory
+rested its case, in the light of some of the newer evidence of recent
+years.
+
+Four great branches of study have furnished the evidence of organic
+evolution. They are:
+
+  Comparative anatomy.
+  Embryology.
+  Paleontology.
+  Experimental Breeding or Genetics.
+
+_The Evidence from Comparative Anatomy_
+
+When we study animals and plants we find that they can be arranged in
+groups according to their resemblances. This is the basis of comparative
+anatomy, which is only an accurate study of facts that are superficially
+obvious to everyone.
+
+The groups are based not on a single difference, but on a very large number
+of resemblances. Let us take for example the group of vertebrates.
+
+[Illustration: FIG. 3. Limb skeletons of extinct and living animals,
+showing the homologous bones: 1, salamander; 2, frog; 3, turtle; 4,
+Aetosaurus; 5, Pleisiosaurus; 6, Ichthyosaurus; 7, Mesosaurus; 8, duck.
+(After Jordan and Kellogg.)]
+
+The hand and the arm of man are similar to the hand and arm of the ape. We
+find the same plan in the forefoot of the rat, the elephant, the horse and
+the opossum. We can identify the same parts in the forefoot of the lizard,
+the frog (fig. 3), and even, though less certainly, in the pectoral fins of
+fishes. Comparison does not end here. We find similarities in the skull and
+back bones of these same animals; in the brain; in the digestive system; in
+the heart and blood vessels; in the muscles.
+
+Each of these systems is very complex, but the same general arrangement is
+found in all. Anyone familiar with the evidence will, I think, probably
+reach the conclusion either that these animals have been created on some
+preconceived plan, or else that they have some other bond that unites them;
+for we find it difficult to believe that such complex, yet similar things
+could have arisen independently. But we try to convince our students of the
+truth of the theory of evolution not so much by calling their attention to
+this relation as by tracing each organ from a simple to a complex
+structure.
+
+I have never known such a course to fail in its intention. In fact, I know
+that the student often becomes so thoroughly convinced that he resents any
+such attempt as that which I am about to make to point out that the
+evidence for his conviction is not above criticism.
+
+[Illustration: FIG. 4. Drosophila ampelophila. a, Female and b, male.]
+
+Because we can often arrange the series of structures in a line extending
+from the very simple to the more complex, we are apt to become unduly
+impressed by this fact and conclude that if we found the complete series we
+should find all the intermediate steps and that they have arisen in the
+order of their complexity. This conclusion is not necessarily correct. Let
+me give some examples that have come under my own observation. We have bred
+for five years the wild fruit fly Drosophila ampelophila (fig. 4) and we
+have found over a hundred and twenty-five new types that breed true. Each
+has arisen independently and suddenly. Every part of the body has been
+affected by one or another of these mutations. For instance many different
+kinds of changes have taken place in the wings and several of these involve
+the size of the wings. If we arrange the latter arbitrarily in the order of
+their size there will be an almost complete series beginning with the
+normal wings and ending with those of apterous flies. Several of these
+types are represented in figure 5. The order in which these mutations
+occurred bears no relation to their size; each originated independently
+from the wild type.
+
+[Illustration: FIG. 5. Mutants of Drosophila ampelophila arranged in order
+of size of wings: (a) cut; (b) beaded; (c) stumpy; (d) another individual
+of stumpy; (f) vestigial (g) apterous.]
+
+The wings of the wild fly are straight (fig. 4). Several types have arisen
+in which the wings are bent upwards and in the most extreme type the wings
+are curled over the back, as seen in figure 54 (g), yet there is no
+historical connection between these stages.
+
+Mutations have occurred involving the pigmentation of the body and wings.
+The head and thorax of the wild Drosophila ampelophila are grayish yellow,
+the abdomen is banded with yellow and black, and the wings are gray. There
+have appeared in our cultures several kinds of darker types ranging to
+almost black flies (fig. 20) and to lighter types that are quite yellow. If
+put in line a series may be made from the darkest flies at one end to the
+light yellow flies at the other. These types, with the fluctuations that
+occur within each type, furnish a complete series of gradations; yet
+historically they have arisen independently of each other.
+
+Many changes in eye color have appeared. As many as thirty or more races
+differing in eye color are now maintained in our cultures. Some of them are
+so similar that they can scarcely be separated from each other. It is
+easily possible beginning with the darkest eye color, sepia, which is deep
+brown, to pick out a perfectly graded series ending with pure white eyes.
+But such a serial arrangement would give a totally false idea of the way
+the different types have arisen; and any conclusion based on the existence
+of such a series might very well be entirely erroneous, for the fact that
+such a series exists bears no relation to the order in which its members
+have appeared.
+
+Suppose that evolution "in the open" had taken place in the same way, by
+means of _discontinuous_ variation. What value then would the evidence from
+comparative anatomy have in so far as it is based on a continuous series of
+variants of any organ?
+
+No one familiar with the entire evidence will doubt for a moment that these
+125 races of Drosophila ampelophila belong to the same species and have had
+a common origin, for while they may differ mainly in one thing they are
+extremely alike in a hundred other things, and in the general relation of
+the parts to each other.
+
+It is in this sense that the evidence from comparative anatomy can be used
+I think as an argument for evolution. It is the resemblances that the
+animals or plants in any group have in common that is the basis for such a
+conclusion; it is not because we can arrange in a continuous series any
+particular variations. In other words, our inference concerning the common
+descent of two or more species is based on the totality of such
+resemblances that still remain in large part after each change has taken
+place. In this sense the argument from comparative anatomy, while not a
+demonstration, carries with it, I think, a high degree of probability.
+
+_The Evidence from Embryology_
+
+In passing from the egg to the adult the individual goes through a series
+of changes. In the course of this development we see not only the
+beginnings of the organs that gradually enlarge and change into those of
+the adult animal, but also see that organs appear and later disappear
+before the adult stage is reached. We find, moreover, that the young
+sometimes resemble in a most striking way the adult stage of groups that we
+place lower in the scale of evolution.
+
+Many years before Darwin advanced his theory of evolution through natural
+selection, the resemblance of the young of higher animals to the adults of
+lower animals had attracted the attention of zoölogists and various views,
+often very naïve, had been advanced to account for the resemblance. Among
+these speculations there was one practically identical with that adopted by
+Darwin and the post-Darwinians, namely that the higher animals repeat in
+their development the _adult stages_ of lower animals. Later this view
+became one of the cornerstones of the theory of organic evolution. It
+reached its climax in the writings of Haeckel, and I think I may add
+without exaggeration that for twenty-five years it furnished the chief
+inspiration of the school of descriptive embryology. Today it is taught in
+practically all textbooks of biology. Haeckel called this interpretation
+the Biogenetic Law.
+
+[Illustration: FIG. 6. Young trout (Trutta fario) six days after hatching.
+(After Ziegler.)]
+
+It was recognized, of course, that many embryonic stages could not possibly
+represent ancestral animals. A young fish with a huge yolk sac attached
+(fig. 6) could scarcely ever have led a happy, free life as an adult
+individual. Such stages were interpreted, however, as _embryonic_ additions
+to the original ancestral type. The embryo had done something on its own
+account.
+
+In some animals the young have structures that attach them to the mother,
+as does the placenta of the mammals. In other cases the young develop
+membranes about themselves--like the amnion of the chick (fig. 7) and
+mammal--that would have shut off an adult animal from all intercourse with
+the outside world. Hundreds of such embryonic adaptations are known to
+embryologists. These were explained as adaptations and as falsifications of
+the ancestral records.
+
+[Illustration: FIG. 7. Diagram of chick showing relations of amnion,
+allantois and yolk. (After Lillie.)]
+
+At the end of the last century Weismann injected a new idea into our views
+concerning the origin of variations. He urged that variations are germinal,
+i.e. they first appear in the egg and the sperm as changes that later bring
+about modifications in the individual. The idea has been fruitful and is
+generally accepted by most biologists today. It means that the offspring of
+a pair of animals are not affected by the structure or the activities of
+their parents, but the germ plasm is the unmodified stream from which both
+the parent and the young have arisen. Hence their resemblance. Now, it has
+been found that a variation arising in the germ plasm, no matter what its
+cause, may affect any stage in the development of the next individuals that
+arise from it. There is no reason to suppose that such a change produces a
+new character that always sticks itself, as it were, on to the end of the
+old series. This idea of germinal variation therefore carried with it the
+death of the older conception of evolution by superposition.
+
+In more recent times another idea has become current, mainly due to the
+work of Bateson and of de Vries--the idea that variations are
+discontinuous. Such a conception does not fall easily into line with the
+statement of the biogenetic "law"; for actual experience with discontinuous
+variation has taught us that new characters that arise do not add
+themselves to the end of the line of already existing characters but if
+they affect the adult characters they change them without, as it were,
+passing through and beyond them.
+
+[Illustration: FIG. 8. Diagram of head of chick A and B, showing gill
+slits, and aortic arches; and head of fish C showing aortic arches. (After
+Hesse.)]
+
+[Illustration: FIG. 9. Human embryo showing gill slits and aortic arches.
+(After His; from Marshall.)]
+
+I venture to think that these new ideas and this new evidence have played
+havoc with the biogenetic "law". Nevertheless, there is an interpretation
+of the facts that is entirely compatible with the theory of evolution. Let
+me illustrate this by an example.
+
+[Illustration: FIG. 10. Young fish, dorsal view, and side view, showing
+gill slits. (After Kopsch.)]
+
+The embryos of the chick (fig. 8) and of man (fig. 9) possess at an early
+stage in their development gill-slits on the sides of the neck like those
+of fishes. No one familiar with the relations of the parts will for a
+moment doubt that the gill slits of these embryos and of the fish represent
+the same structures. When we look further into the matter we find that
+young fish also possess gill slits (fig. 10 and 11)--even in young stages
+in their development. Is it not then more probable that the mammal and bird
+possess this stage in their development simply because it has never been
+lost? Is not this a more reasonable view than to suppose that the gill
+slits of the embryos of the higher forms represent the adult gill slits of
+the fish that in some mysterious way have been pushed back into the embryo
+of the bird?
+
+[Illustration: FIG. 11. Side views of head of embryo sharks, showing gill
+slits.]
+
+I could give many similar examples. All can be interpreted as embryonic
+survivals rather than as phyletic contractions. Not one of them calls for
+the latter interpretation.
+
+The study of the cleavage pattern of the segmenting egg furnishes the most
+convincing evidence that a different explanation from the one stated in the
+biogenetic law is the more probable explanation.
+
+[Illustration: FIG. 12. Cleavage stages of four types of eggs, showing the
+origin of the mesenchyme cells (stippled) and mesoderm cells (darker); a,
+Planarian; b, Annelid (Podarke); c, Mollusc (Crepidula), d, Mollusc
+(Unio).]
+
+It has been found that the cleavage pattern has the same general
+arrangement in the early stages of flat worms, annelids and molluscs (fig.
+12). Obviously these stages have never been adult ancestors, and obviously
+if their resemblance has any meaning at all, it is that each group has
+retained the same general plan of cleavage, possessed by their common
+ancestor.
+
+Accepting this view, let us ask, does the evidence from embryology favor
+the theory of evolution? I think that it does very strongly. The embryos of
+the mammal, bird, and lizard have gill slits today because gill slits were
+present in the embryos of their ancestors. There is no other view that
+explains so well their presence in the higher forms.
+
+Perhaps someone will say, Well! is not this all that we have contended for!
+Have you not reached the old conclusion in a roundabout way? I think not.
+To my mind there is a wide difference between the old statement that the
+higher animals living today have the original adult stages telescoped into
+their embryos, and the statement that the resemblance between certain
+characters in the embryos of higher animals and corresponding stages in the
+embryos of lower animals is most plausibly explained by the assumption that
+they have descended from the same ancestors, and that their common
+structures are embryonic survivals.
+
+_The Evidence from Paleontology_
+
+The direct evidence furnished by fossil remains is by all odds the
+strongest evidence that we have in favor of organic evolution. Paleontology
+holds the incomparable position of being able to point directly to the
+evidence showing that the animals and plants living in past times are
+connected with those living at the present time, often through an unbroken
+series of stages. Paleontology has triumphed over the weakness of the
+evidence, which Darwin admitted was serious, by filling in many of the
+missing links.
+
+Paleontology has been criticised on the ground that she cannot pretend to
+show the actual ancestors of living forms because, if in the past genera
+and species were as abundant and as diverse as we find them at present, it
+is very improbable that the bones of any individual that happened to be
+preserved are the bones of just that species that took part in the
+evolution. Paleontologists will freely admit that in many cases this is
+probably true, but even then the evidence is, I think, still just as
+valuable and in exactly the same sense as is the evidence from comparative
+anatomy. It suffices to know that there lived in the past a particular
+"group" of animals that had many points in common with those that preceded
+them and with those that came later. Whether these are the actual ancestors
+or not does not so much matter, for the view that from such a group of
+species the later species have been derived is far more probable than any
+other view that has been proposed.
+
+With this unrivalled material and splendid series of gradations,
+paleontology has constructed many stages in the past history of the globe.
+But paleontologists have sometimes gone beyond this descriptive phase of
+the subject and have attempted to formulate the "causes", "laws" and
+"principles" that have led to the development of their series. It has even
+been claimed that paleontologists are in an incomparably better position
+than zoölogists to discover such principles, because they know both the
+beginning and the end of the evolutionary series. The retort is obvious. In
+his sweeping and poetic vision the paleontologist may fail completely to
+find out the nature of the pigments that have gone into the painting of his
+picture, and he may confuse a familiarity with the different views he has
+enjoyed of the canvas with a knowledge of how the painting is being done.
+
+My good friend the paleontologist is in greater danger than he realizes,
+when he leaves descriptions and attempts explanation. He has no way to
+check up his speculations and it is notorious that the human mind without
+control has a bad habit of wandering.
+
+When the modern student of variation and heredity--the geneticist--looks
+over the different "continuous" series, from which certain "laws" and
+"principles" have been deduced, he is struck by two facts: that the gaps,
+in some cases, are enormous as compared with the single changes with which
+he is familiar, and (what is more important) that they involve numerous
+parts in many ways. The geneticist says to the paleontologist, since you do
+not know, and from the nature of your case can never know, whether your
+differences are due to one change or to a thousand, you can not with
+certainty tell us anything about the hereditary units which have made the
+process of evolution possible. And without this knowledge there can be no
+understanding of the causes of evolution.
+
+THE FOUR GREAT HISTORICAL SPECULATIONS
+
+Looking backward over the history of the evolution theory we recognize that
+during the hundred and odd years that have elapsed since Buffon, there have
+been four main lines of _speculation_ concerning evolution. We might call
+them the four great cosmogonies or the four modern epics of evolution.
+
+THE ENVIRONMENT
+
+_Geoffroy St. Hilaire_
+
+About the beginning of the last century Geoffroy St. Hilaire, protégé, and
+in some respects a disciple of Buffon, was interested as to how living
+species are related to the animals and plants that had preceded them. He
+was familiar with the kind of change that takes place in the embryo if it
+is put into new or changed surroundings, and from this knowledge he
+concluded that as the surface of the earth slowly changed--as the carbon
+dioxide contents in the air altered--as land appeared--and as marine
+animals left the water to inhabit it, they or their embryos responded to
+the new conditions and those that responded favorably gave rise to new
+creations. As the environment changed the fauna and flora changed--change
+for change. Here we have a picture of progressive evolution that carries
+with it an idea of mechanical necessity. If there is anything mystical or
+even improbable in St. Hilaire's argument it does not appear on the
+surface; for he did not assume that the response to the new environment was
+always a favorable one or, as we say, an adaptation. He expressly stated
+that _if_ the response was unfavorable the individual or the race died out.
+He assumed that _sometimes_ the change might be favorable, i.e., that
+certain species, entire groups, would respond in a direction favorable to
+their existence in a new environment and these would come to inherit the
+earth. In this sense he anticipated certain phases of the natural selection
+theory of Darwin, but only in part; for his picture is not one of strife
+within and without the species, but rather the escape of the species from
+the old into a new world.
+
+If then we recognize the intimate bond in chemical constitution of living
+things and of the world in which they develop, what is there improbable in
+St. Hilaire's hypothesis? Why, in a word is not more credit given to St.
+Hilaire in modern evolutionary thought? The reasons are to be found, I
+think, first, in that the evidence to which he appealed was meagre and
+inconclusive; and, second, in that much of his special evidence does not
+seem to us to be applicable. For example the monstrous forms that
+development often assumes in a strange environment, and with which every
+embryologist is only too familiar, rarely if ever furnish combinations, as
+he supposed, that are capable of living. On the contrary, they lead rather
+to the final catastrophe of the organism. And lastly, St. Hilaire's appeal
+to sudden and great transformations, such as a crocodile's egg hatching
+into a bird, has exposed his view to too easy ridicule.
+
+But when all is said, St. Hilaire's conception of evolution contains
+elements that form the background of our thinking to-day, for taken
+broadly, the interaction between the organism and its environment was a
+mechanistic conception of evolution even though the details of the theory
+were inadequate to establish his contention.
+
+In our own time the French metaphysician Bergson in his _Evolution
+Creatrice_ has proposed in mystical form a thought that has at least a
+superficial resemblance to St. Hilaire's conception. The response of living
+things is no longer hit in one species and miss in another; it is precise,
+exact; yet not mechanical in the sense at least in which we usually employ
+the word mechanical. For Bergson claims that the one chief feature of
+living material is that it responds favorably to the situation in which it
+finds itself; at least so far as lies within the possible physical
+limitations of its organization. Evolution has followed no preordained
+plan; it has had no creator; it has brought about its own creation by
+responding adaptively to each situation as it arose.
+
+But note: the man of science believes that the organism responds today as
+it does, because at present it has a chemical and physical constitution
+that gives this response. We find a specific chemical composition and
+generally a specific physical structure already existing. We have no reason
+to suppose that such particular reactions would take place until a specific
+chemical configuration had been acquired. Where did this constitution come
+from? This is the question that the scientist asks himself. I suppose
+Bergson would have to reply that it came into existence at the moment that
+the first specific stimulus was applied. But if this is the answer we have
+passed at once from the realm of observation to the realm of fancy--to a
+realm that is foreign to our experience; for such a view assumes that
+chemical and physical reactions are guided by the needs of the organism
+when the reactions take place inside living beings.
+
+USE AND DISUSE
+
+_From Lamarck to Weismann_
+
+The second of the four great historical explanations appeals to a change
+not immediately connected with the outer world, but to one within the
+organism itself.
+
+Practice makes perfect is a familiar adage. Not only in human affairs do we
+find that a part through use becomes a better tool for performing its task,
+and through disuse degenerates; but in the field of animal behavior we find
+that many of the most essential types of behavior have been learned through
+repeated associations formed by contact with the outside.
+
+It was not so long ago that we were taught that the instincts of animals
+are the inherited experience of their ancestors--lapsed intelligence was
+the current phrase.
+
+Lamarck's name is always associated with the application of the theory of
+the inheritance of acquired characters. Darwin fully endorsed this view and
+made use of it as an explanation in all of his writings about animals.
+Today the theory has few followers amongst trained investigators, but it
+still has a popular vogue that is widespread and vociferous.
+
+To Weismann more than to any other single individual should be ascribed the
+disfavor into which this view has fallen. In a series of brilliant essays
+he laid bare the inadequacy of the supposed evidence on which the
+inheritance of acquired characters rested. Your neighbor's cat, for
+instance, has a short tail, and it is said that it had its tail pinched off
+by a closing door. In its litter of kittens one or more is found without a
+tail. Your neighbor believes that here is a case of cause and effect. He
+may even have known that the mother and grandmother of the cat had natural
+tails. But it has been found that short tail is a dominant character;
+therefore, until we know who was the father of the short-tailed kittens the
+accident to its mother and the normal condition of her maternal ancestry is
+not to the point.
+
+Weismann appealed to common sense. He made few experiments to disprove
+Lamarck's hypothesis. True, he cut off the tails of some mice for a few
+generations but got no tailless offspring and while he gives no exact
+measurements with coefficients of error he did not observe that the tails
+of the descendants had shortened one whit. The combs of fighting cocks and
+the tails of certain breeds of sheep have been cropped for many generations
+and the practice continues today, because their tails are still long. While
+in Lamarck's time there was no evidence opposed to his ingenious theory,
+based as it was on an appeal to the acknowledged facts of improvement that
+take place in the organs of an individual through their own functioning (a
+fact that is as obvious and remarkable today as in the time of Lamarck),
+yet now there is evidence as to whether the effects of use and disuse are
+inherited, and this evidence is not in accord with Lamarck's doctrine.
+
+THE UNFOLDING PRINCIPLE
+
+_Nägeli and Bateson_
+
+I have ventured to put down as one of the four great historical
+explanations, under the heading of the unfolding principle, a conception
+that has taken protean forms. At one extreme it is little more than a
+mystic sentiment to the effect that evolution is the result of an inner
+driving force or principle which goes under many names such as
+Bildungstrieb, nisus formativus, vital force, and orthogenesis.
+Evolutionary thought is replete with variants of this idea, often naïvely
+expressed, sometimes unconsciously implied. Evolution once meant, in fact,
+an unfolding of what pre-existed in the egg, and the term still carries
+with it something of its original significance.
+
+Nägeli's speculation written several years after Darwin's "Origin of
+Species" may be taken as a typical case. Nägeli thought that there exists
+in living material an innate power to grow and expand. He vehemently
+protested that he meant only a mechanical principle but as he failed to
+refer such a principle to any properties of matter known to physicists and
+chemists his view seems still a mysterious affirmation, as difficult to
+understand as the facts themselves which it purports to explain.
+
+Nägeli compared the process of evolution to the growth of a tree, whose
+ultimate twigs represent the living world of species. Natural selection
+plays only the rôle of the gardener who prunes the tree into this or that
+shape but who has himself _produced_ nothing. As an imaginative figure of
+speech Nägeli's comparison of the tree might even today seem to hold if we
+substituted "mutations" for "growth", but although we know so little about
+what causes mutations there is no reason for supposing them to be due to an
+inner impulse, and hence they furnish no justification for such a
+hypothesis.
+
+In his recent presidential address before the British Association Bateson
+has inverted this idea. I suspect that his effort was intended as little
+more than a _tour de force_. He claims for it no more than that it is a
+possible line of speculation. Perhaps he thought the time had come to give
+a shock to our too confident views concerning evolution. Be this as it may,
+he has invented a striking paradox. Evolution has taken place through the
+steady loss of inhibiting factors. Living matter was stopped down, so to
+speak, at the beginning of the world. As the stops are lost, new things
+emerge. Living matter has changed only in that it has become simpler.
+
+NATURAL SELECTION
+
+_Darwin_
+
+Of the four great historical speculations about evolution, the doctrine of
+Natural Selection of Darwin and Wallace has met with the most widespread
+acceptance. In the last lecture I intend to examine this theory critically.
+Here we are concerned only with its broadest aspects.
+
+Darwin appealed to _chance variations_ as supplying evolution with the
+material on which natural selection works. If we accept, for the moment,
+this statement as the cardinal doctrine of natural selection it may appear
+that evolution is due, (1) _not_ to an _orderly_ response of the organism
+to its environment, (2) _not_ in the main to the activities of the animal
+through the use or disuse of its parts, (3) _not_ to any innate principle
+of living material itself, and (4) above all _not_ to purpose either from
+within or from without. Darwin made quite clear what he meant by chance. By
+chance he did not mean that the variations were not causal. On the contrary
+he taught that in Science we mean by chance only that the particular
+combination of causes that bring about a variation are not known. They are
+accidents, it is true, but they are causal accidents.
+
+In his famous book on "Animals and Plants under Domestication", Darwin
+dwells at great length on the nature of the conditions that bring about
+variations. If his views seem to us today at times vague, at times
+problematical, and often without a secure basis, nevertheless we find in
+every instance, that Darwin was searching for the _physical causes of
+variation_. He brought, in consequence, conviction to many minds that there
+are abundant indications, even if certain proof is lacking, that the causes
+of variation are to be found in natural processes.
+
+Today the belief that evolution takes place by means of natural processes
+is generally accepted. It does not seem probable that we shall ever again
+have to renew the old contest between evolution and special creation.
+
+But this is not enough. We can never remain satisfied with a negative
+conclusion of this kind. We must find out what natural causes bring about
+variations in animals and plants; and we must also find out what kinds of
+variations are inherited, and how they are inherited. If the circumstantial
+evidence for organic evolution, furnished by comparative anatomy,
+embryology and paleontology is cogent, we should be able to observe
+evolution going on at the present time, i.e. we should be able to observe
+the occurrence of variations and their transmission. This has actually been
+done by the geneticist in the study of mutations and Mendelian heredity, as
+the succeeding lectures will show.
+
+       *       *       *       *       *
+
+
+CHAPTER II
+
+THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITARY CHARACTERS
+
+Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied
+the heredity of certain characters of the common edible pea, in the garden
+of the monastery at Brünn.
+
+In his account of his work written in 1868, he said:
+
+    "It requires indeed some courage to undertake a labor of such a
+    far-reaching extent; it appears, however, to be the only right way by
+    which we can finally reach the solution of a question the importance of
+    which cannot be over-estimated in connection with the history of the
+    evolution of organic forms."
+
+He tells us also why he selected peas for his work:
+
+    "The selection of the plant group which shall serve for experiments of
+    this kind must be made with all possible care if it be desired to avoid
+    from the outset every risk of questionable results."
+
+    "The experimental plants must necessarily
+
+    1. Possess constant differentiating characters.
+
+    2. The hybrids of such plants must, during the flowering period, be
+    protected from the influence of all foreign pollen, or be easily
+    capable of such protection."
+
+Why do biologists throughout the world to-day agree that Mendel's discovery
+is one of first rank?
+
+A great deal might be said in this connection. What is essential may be
+said in a few words. Biology had been, and is still, largely a descriptive
+and speculative science. _Mendel showed by experimental proof that heredity
+could be explained by a simple mechanism. His discovery has been
+exceedingly fruitful._
+
+Science begins with naïve, often mystic conceptions of its problems. It
+reaches its goal whenever it can replace its early guessing by verifiable
+hypotheses and predictable results. This is what Mendel's law did for
+heredity.
+
+MENDEL'S FIRST DISCOVERY--SEGREGATION
+
+[Illustration: FIG. 13. Diagram illustrating a cross between a red (dark)
+and a white variety of four o'clock (Mirabilis jalapa).]
+
+Let us turn to the demonstration of his first law--the law of segregation.
+The first case I choose is not the one given by Mendel but one worked out
+later by Correns. If the common garden plant called four o'clock (Mirabilis
+jalapa) with red flowers is crossed to one having white flowers, the
+offspring are pink (fig. 13). The hybrid, then, is intermediate in the
+color of its flowers between the two parents. If these hybrids are inbred
+the offspring are white, pink and red, in the proportion of 1:2:1. All of
+these had the same ancestry, yet they are of three different kinds. If we
+did not know their history it would be quite impossible to state what the
+ancestry of the white or of the red had been, for they might just as well
+have come from pure white and pure red ancestors respectively as to have
+emerged from the pink hybrids. Moreover, when we test them we find that
+they are as pure as are white or red flowering plants that have had all
+white or all red flowering ancestors.
+
+Mendel's Law explains the results of this cross as shown in figure 14.
+
+The egg cell from the white parent carries the factor for white, the pollen
+cell from the red parent carries the factor for red. The hybrid formed by
+their union carries both factors. The result of their combined action is to
+produce flowers intermediate in color.
+
+When the hybrids mature and their germ cells (eggs or pollen) ripen, each
+carries only one of these factors, either the red or the white, but not
+both. In other words, the two factors that have been brought together in
+the hybrid separate in its germ cells. Half of the egg cells are white
+bearing, half red bearing. Half of the pollen cells are white bearing, half
+red bearing. Chance combinations at fertilization give the three classes of
+individuals of the second generation.
+
+[Illustration: FIG. 14. Diagram illustrating the history of the factors in
+the germ cells of the cross shown in Fig. 13.]
+
+The white flowering plants should forever breed true, as in fact they do.
+The red flowering plants also breed true. The pink flowering plants, having
+the same composition as the hybrids of the first generation, should give
+the same kind of result. They do, indeed, give this result i.e. one white
+to two pink to one red flowered offspring.
+
+[Illustration: FIG. 15. Diagram illustrating a cross between special races
+of white and black fowls, producing the blue (here gray) Andalusian.]
+
+Another case of the same kind is known to breeders of poultry. One of the
+most beautiful of the domesticated breeds is known as the Andalusian. It is
+a slate blue bird shading into blue-black on the neck and back. Breeders
+know that these blue birds do not breed true but produce white, black, and
+blue offspring.
+
+[Illustration: FIG. 16. Diagram showing history of germ cells of cross of
+Fig. 15. The larger circles indicate the color of the birds; their enclosed
+small circles the nature of the factors in the germ cells of such birds.]
+
+The explanation of the failure to produce a pure race of Andalusians is
+that they are like the pink flowers of the four o'clock, i.e., they are a
+hybrid type formed by the meeting of the white and the black germ cells. If
+the whites produced by the Andalusians are bred to the blacks (both being
+pure strains), all the offspring will be blue (fig. 15); if these blues are
+inbred they will give 1 white, to 2 blues, to 1 black. In other words, the
+factor for white and the factor for black separate in the germ cells of the
+hybrid Andalusian birds (fig. 16).
+
+[Illustration: FIG. 17. Diagram of Mendel's cross between yellow (dominant)
+and green (recessive) peas.]
+
+The third case is Mendel's classical case of yellow and green peas (fig.
+17). He crossed a plant belonging to a race having yellow peas with one
+having green peas. The hybrid plants had yellow seeds. These hybrids inbred
+gave three yellows to one green. The explanation (fig. 18) is the same in
+principle as in the preceding cases. The only difference between them is
+that the hybrid which contains both the yellow and the green factors is in
+appearance not intermediate, but like the yellow parent stock. Yellow is
+said therefore to be dominant and green to be recessive.
+
+[Illustration: FIG. 18. Diagram illustrating the history of the factors in
+the cross shown in Fig. 17.]
+
+Another example where one of the contrasted characters is dominant is shown
+by the cross of Drosophila with vestigial wings to the wild type with long
+wings (fig. 19). The F_1 flies have long wings not differing from those of
+the wild fly, so far as can be observed. When two such flies are inbred
+there result three long to one vestigial.
+
+[Illustration: FIG. 19. Diagram illustrating a cross between a fly
+(Drosophila ampelophila) with long wings and a mutant fly with vestigial
+wings.]
+
+The question as to whether a given character is dominant or recessive is a
+matter of no theoretical importance for the principle of segregation,
+although from the notoriety given to it one might easily be misled into the
+erroneous supposition that it was the discovery of this relation that is
+Mendel's crowning achievement.
+
+Let me illustrate by an example in which the hybrid standing between two
+types overlaps them both. There are two mutant races in our cultures of the
+fruit fly Drosophila that have dark body color, one called sooty, another
+which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives
+offspring that are intermediate in color. Some of them are so much like
+sooty that they cannot be distinguished from sooty. At the other extreme
+some of the hybrids are as dark as the lightest of the ebony flies. If
+these hybrids are inbred there is a continuous series of individuals,
+sooties, intermediates and ebonies. Which color here shall we call the
+dominant? If the ebony, then in the second generation we count three
+ebonies to one sooty, putting the hybrids with the ebonies. If the dominant
+is the sooty then we count three sooties to one ebony, putting the hybrids
+with the sooties. The important fact to find out is whether there actually
+exist three classes in the second generation. This can be ascertained even
+when, as in this case, there is a perfectly graded series from one end to
+the other, by testing out individually enough of the flies to show that
+one-fourth of them never produce any descendants but ebonies, one-fourth
+never any but sooties, and one-half of them give rise to both ebony and
+sooty.
+
+[Illustration: FIG. 20. Cross between two allelomorphic races of
+Drosophila, sooty and ebony, that give a completely graded series in F_2.]
+
+MENDEL'S SECOND DISCOVERY--INDEPENDENT ASSORTMENT
+
+Besides his discovery that there are pairs of characters that disjoin, as
+it were, in the germ cells of the hybrid (law of segregation) Mendel made a
+second discovery which also has far-reaching consequences. The following
+case illustrates Mendel's second law.
+
+If a pea that is yellow and round is crossed to one that is green and
+wrinkled (fig. 21), all of the offspring are yellow and round. Inbred,
+these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green
+wrinkled. All the yellows taken together are to the green as 3:1. All the
+round taken together are to the wrinkled as three to one; but some of the
+yellows are now wrinkled and some of the green are now round. There has
+been a recombination of characters, while at the same time the results, for
+each pair of characters taken separately, are in accord with Mendel's Law
+of Segregation, (fig. 22). The second law of Mendel may be called the law
+of independent assortment of different character pairs.
+
+[Illustration: FIG. 21. Cross between yellow-round and green-wrinkled peas,
+giving the 9: 3: 3: 1 ratio in F_2.]
+
+We can, as it were, take the characters of one organism and recombine them
+with those of a different organism. We can explain this result as due to
+the assortment of factors for these characters in the germ cells according
+to a definite law.
+
+[Illustration: FIG. 22. Diagram to show the history of the factor pairs
+yellow-green and round-wrinkled of the cross in Fig. 21.]
+
+As a second illustration let me take the classic case of the combs of
+fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23),
+the offspring have a comb different from either. It is called a walnut
+comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1
+single. This proportion shows that the grandparental types differed in
+respect to two pairs of characters.
+
+[Illustration: FIG. 23. Cross between pea and rose combed fowls. (Charts of
+Baur and Goldschmidt.)]
+
+A fourth case is shown in the fruit fly, where an ebony fly with long wings
+is mated to a grey fly with vestigial wings (fig. 24). The offspring are
+gray with long wings. If these are inbred they give 9 gray long, 3 gray
+vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).
+
+[Illustration: FIG. 24. Cross between long ebony and gray vestigial flies.]
+
+The possibility of interchanging characters might be illustrated over and
+over again. It is true not only when two pairs of characters are involved,
+but when three, four, or more enter the cross.
+
+[Illustration: FIG. 25. Diagram to show the history of the factors in the
+cross shown in Fig. 24.]
+
+It is as though we took individuals apart and put together parts of two,
+three or more individuals by substituting one part for another.
+
+Not only has this power to make whatever combinations we choose great
+practical importance, it has even greater theoretical significance; for, it
+follows that the individual is not in itself the unit in heredity, but that
+within the germ-cells there exist smaller units concerned with the
+transmission of characters.
+
+The older mystical statement of the individual as a unit in heredity has no
+longer any interest in the light of these discoveries, except as a past
+phase of biological history. We see, too, more clearly that the sorting out
+of factors in the germ plasm is a very different process from the influence
+of these factors on the development of the organism. There is today no
+excuse for confusing these two problems.
+
+If mechanistic principles apply also to embryonic development then the
+course of development is capable of being stated as a series of
+chemico-physical reactions and the "_individual_" is merely a term to
+express the sum total of such reactions and should not be interpreted as
+something different from or more than these reactions. So long as so little
+is known of the actual processes involved in development the use of the
+term "individuality", while giving the appearance of profundity, in reality
+often serves merely to cover ignorance and to make a mystery out of a
+mechanism.
+
+THE CHARACTERS OF WILD ANIMALS AND PLANTS FOLLOW THE SAME LAWS OF
+INHERITANCE AS DO THE CHARACTERS OF DOMESTICATED ANIMALS AND PLANTS.
+
+Darwin based many of his conclusions concerning variation and heredity on
+the evidence derived from the garden and from the stock farm. Here he was
+handicapped to some extent, for he had at times to rely on information much
+of which was uncritical, and some of which was worthless.
+
+Today we are at least better informed on _two_ important points; one
+concerning the _kinds_ of variations that furnish to the cultivator the
+materials for his selection; the other concerning the modes of inheritance
+of these variations. We know now that new characters are continually
+appearing in domesticated as well as in wild animals and plants, that these
+characters are often sharply marked off from the original characters, and
+whether the differences are great or whether they are small they are
+transmitted alike according to Mendel's law.
+
+Many of the characteristics of our domesticated animals and cultivated
+plants originated long ago, and only here and there have the records of
+their first appearance been preserved. In only a few instances are these
+records clear and definite, while the complete history of any large group
+of our domesticated products is unknown to us.
+
+Within the last five or six years, however, from a common wild species of
+fly, the fruit fly, Drosophila ampelophila, which we have brought into the
+laboratory, have arisen over a hundred and twenty-five new types whose
+origin is completely known. Let me call attention to a few of the more
+interesting of these types and their modes of inheritance, comparing them
+with wild types in order to show that the kinds of inheritance found in
+domesticated races occur also in wild types. The results will show beyond
+dispute that the characters of wild types are inherited in precisely the
+same way as are the characters of the mutant types--a fact that is not
+generally appreciated except by students of genetics, although it is of the
+most far-reaching significance for the theory of evolution.
+
+A mutant appeared in which the eye color of the female was different from
+that of the male. The eye color of the mutant female is a dark eosin color,
+that of the male yellowish eosin. From the beginning this difference was as
+marked as it is to-day. Breeding experiments show that eosin eye color
+differs from the red color of the eye of the wild fly by a single mutant
+factor. Here then at a single step a type appeared that was sexually
+dimorphic.
+
+Zoölogists know that sexual dimorphism is not uncommon in wild species of
+animals, and Darwin proposed the theory of sexual selection to account for
+the difference between the sexes. He assumed that the male preferred
+certain kinds of females differing from himself in a particular character,
+and thus in time through sexual selection, the sexes came to differ from
+each other.
+
+[Illustration: FIG. 26. Clover butterfly (Colias philodice) with two types
+of females, above; and one type of male, below.]
+
+In the case of eosin eye color no such process as that postulated by Darwin
+to account for the differences between the sexes was involved; for the
+single mutation that brought about the change also brought in the
+dimorphism with it.
+
+In recent years zoölogists have carefully studied several cases in which
+two types of female are found in the same species. In the common clover
+butterfly, there is a yellow and a white type of female, while the male is
+yellow (fig. 26). It has been shown that a single factor difference
+determines whether the female is yellow or white. The inheritance is,
+according to Gerould, strictly Mendelian.
+
+[Illustration: FIG. 27. Papilio turnus with two types of females above and
+one type of male below.]
+
+In Papilio turnus there exist, in the southern states, two kinds of
+females, one yellow like the male, one black (fig. 27). The evidence here
+is not so certain, but it seems probable that a single factor difference
+determines whether the female shall be yellow or black.
+
+Finally in Papilio polytes of Ceylon and India three different types of
+females appear, (fig. 28 to right) only one of which is like the male. Here
+the analysis of the breeding data shows the possibility of explaining this
+case as due to two pairs Mendelian factors which give in combination the
+three types of female.
+
+[Illustration: FIG. 28. Papilio polytes, with three types of female to
+right and one type of male above to left.]
+
+Taking these cases together, they furnish a much simpler explanation than
+the one proposed by Darwin. They show also that characters like these shown
+by wild species may follow Mendel's law.
+
+[Illustration: FIG. 29. Mutant race of fruit fly with intercalated
+duplicate mesothorax on dorsal side.]
+
+There has appeared in our cultures a fly in which the third division of the
+thorax with its appendages has changed into a segment like the second (fig.
+29). It is smaller than the normal mesothorax and its wings are imperfectly
+developed, but the bristles on the upper surface may have the typical
+arrangement of the normal mesothorax. The mutant shows how great a change
+may result from a single factor difference.
+
+A factor that causes duplication in the legs has also been found. Here the
+interesting fact was discovered (Hoge) that duplication takes place only in
+the cold. At ordinary temperatures the legs are normal.
+
+[Illustration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a'
+normal eye.]
+
+In contrast to the last case, where a character is doubled, is the next one
+in which the eyes are lost (fig. 30). This change also took place at a
+single step. All the flies of this stock however, cannot be said to be
+eyeless, since many of them show pieces of the eye--indeed the variation is
+so wide that the eye may even appear like a normal eye unless carefully
+examined. Formerly we were taught that eyeless animals arose in caves. This
+case shows that they may also arise suddenly in glass milk bottles, by a
+change in a single factor.
+
+I may recall in this connection that wingless flies (fig. 5 f) also arose
+in our cultures by a single mutation. We used to be told that wingless
+insects occurred on desert islands because those insects that had the best
+developed wings had been blown out to sea. Whether this is true or not, I
+will not pretend to say, but at any rate wingless insects may also arise,
+not through a slow process of elimination, but at a single step.
+
+The preceding examples have all related to recessive characters. The next
+one is dominant.
+
+[Illustration: FIG. 31. Mutant race of fruit fly called bar to the right
+(normal to the left). The eye is a narrow vertical bar, the outline of the
+original eye is indicated.]
+
+A single male appeared with a narrow vertical red bar (fig. 31) instead of
+the broad red oval eye. Bred to wild females the new character was found to
+dominate, at least to the extent that the eyes of all its offspring were
+narrower than the normal eye, although not so narrow as the eye of the pure
+stock. Around the bar there is a wide border that corresponds to the region
+occupied by the rest of the eye of the wild fly. It lacks however the
+elements of the eye. It is therefore to be looked upon as a rudimentary
+organ, which is, so to speak, a by-product of the dominant mutation.
+
+The preceding cases have all involved rather great changes in some one
+organ of the body. The following three cases involve slight changes, and
+yet follow the same laws of inheritance as do the larger changes.
+
+[Illustration: FIG. 32. Mutant race of fruit fly, called speck. There is a
+minute black speck at base of wing.]
+
+At the base of the wings a minute black speck appeared (fig. 32). It was
+found to be a Mendelian character. In another case the spines on the thorax
+became forked or kinky (fig. 52b). This stock breeds true, and the
+character is inherited in strictly Mendelian fashion.
+
+[Illustration: FIG. 33. Mutant race of fruit fly called club. The wings
+often remain unexpanded and two bristles present in wild fly (b) are absent
+on side of thorax (c).]
+
+In a certain stock a number of flies appeared in which the wing pads did
+not expand (fig. 33). It was found that this peculiarity is shown in only
+about twenty per cent of the individuals supposed to inherit it. Later it
+was found that this stock lacked two bristles on the sides of the thorax.
+By means of this knowledge the heredity of the character was easily
+determined. It appears that while the expansion of the wing pads fails to
+occur once in five times--probably because it is an environmental effect
+peculiar to this stock,--yet the minute difference of the presence or
+absence of the two lateral bristles is a constant feature of the flies that
+carry this particular factor.
+
+In the preceding cases I have spoken as though a factor influenced only one
+part of the body. It would have been more accurate to have stated that the
+_chief_ effect of the factor was observed in a particular part of the body.
+Most students of genetics realize that a factor difference usually affects
+more than a single character. For example, a mutant stock called
+rudimentary wings has as its principle characteristic very short wings
+(fig. 34). But the factor for rudimentary wings also produces other effects
+as well. The females are almost completely sterile, while the males are
+fertile. The viability of the stock is poor. When flies with rudimentary
+wings are put into competition with wild flies relatively few of the
+rudimentary flies come through, especially if the culture is crowded. The
+hind legs are also shortened. All of these effects are the results of a
+single factor-difference.
+
+[Illustration: FIG. 34. Mutant race of fruit fly, called rudimentary.]
+
+One may venture the guess that some of the specific and varietal
+differences that are characteristic of wild types and which at the same
+time appear to have no survival value, are only by-products of factors
+whose most important effect is on another part of the organism where their
+influence is of vital importance.
+
+It is well known that systematists make use of characters that are constant
+for groups of species, but which do not appear in themselves to have an
+adaptive significance. If we may suppose that the constancy of such
+characters may be only an index of the presence of a factor whose _chief_
+influence is in some other direction or directions, some physiological
+influence, for example, we can give at least a reasonable explanation of
+the constancy of such characters.
+
+I am inclined to think that an overstatement to the effect that each factor
+may affect the entire body, is less likely to do harm than to state that
+each factor affects only a particular character. The reckless use of the
+phrase "unit character" has done much to mislead the uninitiated as to the
+effects that a single change in the germ plasm may produce on the organism.
+Fortunately, the expression "unit character" is being less used by those
+students of genetics who are more careful in regard to the implications of
+their terminology.
+
+There is a class of cases of inheritance, due to the XY chromosomes, that
+is called sex linked inheritance. It is shown both by mutant characters and
+characters of wild species.
+
+For instance, white eye color in Drosophila shows sex linked inheritance.
+If a white eyed male is mated to a wild red eyed female (fig. 35) all the
+offspring have red eyes. If these are inbred, there are three red to one
+white eyed offspring, but white eyes occur only in the males. The
+grandfather has transmitted his peculiarity to half of his grandsons, but
+to none of his granddaughters.
+
+[Illustration: FIG. 35. Diagram showing a cross between a white eyed male
+and a red eyed female of the fruit fly. Sex linked inheritance.]
+
+The reciprocal cross (fig. 36) is also interesting. If a white eyed female
+is bred to a red eyed male, all of the daughters have red eyes and all of
+the sons have white eyes. We call this criss-cross inheritance. If these
+offspring are inbred, they produce equal numbers of red eyed and white eyed
+females and equal numbers of red eyed and white eyed males. The ratio is 1:
+1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the usual 3:1
+Mendelian ratio. Yet, as will be shown later, the result is in entire
+accord with Mendel's principle of segregation.
+
+[Illustration: FIG. 36. Diagram illustrating a cross between a red eyed
+male and white eyed female of the fruit fly (reciprocal cross of that shown
+in Fig. 35).]
+
+It has been shown by Sturtevant that in a wild species of Drosophila, viz.,
+D. repleta, two varieties of individuals exist, in one of which the thorax
+has large splotches and in the other type smaller splotches (fig. 37). The
+factors that differentiate these varieties are sex linked.
+
+[Illustration: FIG. 37. Two types of markings on thorax of Drosophila
+repleta, both found "wild". They show sex linked inheritance.]
+
+Certain types of color blindness (fig. 38) and certain other abnormal
+conditions in man such as haemophilia, are transmitted as sex linked
+characters.
+
+[Illustration: FIG. 38, A. Diagram illustrating inheritance of color
+blindness in man; the iris of the color-blind eye is here black.]
+
+[Illustration: FIG. 38, B. Reciprocal of cross in Fig. 38 a.]
+
+In domestic fowls sex linked inheritance has been found as the
+characteristic method of transmission for at least as many as six
+characters, but here the relation of the sexes is in a sense reversed. For
+instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock
+(fig. 39), the offspring are all barred. If these are inbred half of the
+daughters are black and half are barred; all of the sons are barred. The
+grandmother has transmitted her color to half of her granddaughters but to
+none of her grandsons.
+
+[Illustration: FIG. 39. Sex-linked inheritance in domesticated birds shown
+here in a cross between barred Plymouth Rock male and black Langshan
+female.]
+
+[Illustration: FIG. 40. Reciprocal of Fig. 39.]
+
+In the reciprocal cross (fig. 40) black cock by barred hen, the daughters
+are black and the sons barred--criss-cross inheritance. These inbred give
+black hens and black cocks, barred hens and barred cocks.
+
+There is a case comparable to this found in a wild species of moth, Abraxas
+grossulariata. A wild variation of this type is lighter in color and is
+known as A. lacticolor. When these two types are crossed they exhibit
+exactly the same type of heredity as does the black-barred combination in
+the domestic fowl. As shown in figure 41, lacticolor female bred to
+grossulariata male gives grossulariata sons and daughters. These inbred
+give grossulariata males and females and lacticolor females. Reciprocally
+lacticolor male by grossulariata female, (fig. 42) gives lacticolor
+daughters and grossulariata sons and these inbred give grossulariata males
+and females and lacticolor males and females.
+
+[Illustration: FIG. 41. Sex-linked inheritance in the wild moth, Abraxas
+grossulariata (darker) and A. lacticolor.]
+
+[Illustration: FIG. 42. Reciprocal of Fig. 41.]
+
+[Illustration: FIG. 43. Four wild types of Paratettix in upper line with
+three hybrids below.]
+
+It has been found that there may be even more than two factors that show
+Mendelian segregation when brought together in pairs. For example, in the
+southern States there are several races of the grouse locust (Paratettix)
+that differ from each other markedly in color patterns (fig. 43). When any
+two individuals of these races are crossed they give, as Nabours has shown,
+in F_2 a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there
+are here at least nine characters, any two of which behave as a Mendelian
+pair. These races have arisen in nature and differ definitely and
+strikingly from each other, yet any two differ by only one factor
+difference.
+
+[Illustration: FIG. 44. Diagram illustrating four allelomorphs in mice,
+viz. gray bellied gray (wild type) (above, to left); white bellied gray
+(above, to right); yellow (below, to right); and black (below, to left).]
+
+Similar relations have been found in a number of domesticated races. In
+mice there is a quadruple system represented by the gray house mouse, the
+white bellied, the yellow and the black mouse (fig. 44). In rabbits there
+is probably a triple system, that includes the albino, the Himalayan, and
+the black races. In the silkworm moth there have been described four types
+of larvae, distinguished by different color markings, that form a system of
+quadruple allelomorphs. In Drosophila there is a quintuple system of
+factors in the sex chromosome represented by eye colors, a triple system of
+body colors, and a triple system of factors for eye colors in the third
+chromosome.
+
+MUTATION AND EVOLUTION
+
+What bearing has the appearance of these new types of Drosophila on the
+theory of evolution may be asked. The objection has been raised in fact
+that in the breeding work with Drosophila we are dealing with artificial
+and unnatural conditions. It has been more than implied that results
+obtained from the breeding pen, the seed pan, the flower pot and the milk
+bottle do not apply to evolution in the "open", nature "at large" or to
+"wild" types. To be consistent, this same objection should be extended to
+the use of the spectroscope in the study of the evolution of the stars, to
+the use of the test tube and the balance by the chemist, of the
+galvanometer by the physicist. All these are unnatural instruments used to
+torture Nature's secrets from her. I venture to think that the real
+antithesis is not between unnatural and natural treatment of Nature, but
+rather between controlled or verifiable data on the one hand, and
+unrestrained generalization on the other.
+
+If a systematist were asked whether these new races of Drosophila are
+comparable to wild species, he would not hesitate for a moment. He would
+call them all one species. If he were asked why, he would say, I think,
+"These races differ only in one or two striking points, while in a hundred
+other respects they are identical even to the minutest details." He would
+add, that as large a group of wild species of flies would show on the whole
+the reverse relations, _viz._, they would differ in nearly every detail and
+be identical in only a few points. In all this I entirely agree with the
+systematist, for I do not think such a group of types differing by one
+character each, is comparable to most wild groups of species because the
+difference between wild species is due to a large number of such single
+differences. The characters that have been accumulated in wild species are
+of significance in the maintenance of the species, or at least we are led
+to infer that even though the visible character that we attend to may not
+itself be important, one at least of the other effects of the factors that
+represent these characters is significant. It is, of course, hardly to be
+expected that _any_ random change in as complex a mechanism as an insect
+would improve the mechanism, and as a matter of fact it is doubtful whether
+any of the mutant types so far discovered are better adapted to those
+conditions to which a fly of this structure and habits is already adjusted.
+But this is beside the mark, for modern genetics shows very positively that
+adaptive characters are inherited in exactly the same way as are those that
+are not adaptive; and I have already pointed out that we cannot study a
+single mutant factor without at the same time studying one of the factors
+responsible for normal characters, for the two together constitute the
+Mendelian pair.
+
+And, finally, I want to urge on your attention a question that we are to
+consider in more detail in the last lecture. Evolution of wild species
+appears to have taken place by modifying and improving bit by bit the
+structures and habits that the animal or plant already possessed. We have
+seen that there are thirty mutant factors at least that have an influence
+on eye color, and it is probable that there are at least as many normal
+factors that are involved in the production of the red eye of the wild fly.
+
+Evolution from this point of view has consisted largely in introducing new
+factors that influence characters already present in the animal or plant.
+
+Such a view gives us a somewhat different picture of the process of
+evolution from the old idea of a ferocious struggle between the individuals
+of a species with the survival of the fittest and the annihilation of the
+less fit. Evolution assumes a more peaceful aspect. New and advantageous
+characters survive by incorporating themselves into the race, improving it
+and opening to it new opportunities. In other words, the emphasis may be
+placed less on the competition between the individuals of a species
+(because the destruction of the less fit does not _in itself_ lead to
+anything that is new) than on the appearance of new characters and
+modifications of old characters that become incorporated in the species,
+for on these depends the evolution of the race.
+
+       *       *       *       *       *
+
+
+CHAPTER III
+
+THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM
+
+The discovery that Mendel made with edible peas concerning heredity has
+been found to apply everywhere throughout the plant and animal kingdoms--to
+flowering plants, to insects, snails, crustacea, fishes, amphibians, birds,
+and mammals (including man).
+
+There must be something that these widely separated groups of plants and
+animals have in common--some simple mechanism perhaps--to give such
+definite and orderly series of results. There is, in fact, a mechanism,
+possessed alike by animals and plants, that fulfills every requirement of
+Mendel's principles.
+
+THE CELLULAR BASIS OF ORGANIC EVOLUTION AND HEREDITY
+
+In order to appreciate the full force of the evidence, let me first pass
+rapidly in review a few familiar, historical facts, that preceded the
+discovery of the mechanism in question.
+
+[Illustration: FIG. 45. Typical cell showing the cell wall, the protoplasm
+(with its contained materials); the nucleus with its contained chromatin
+and nuclear sap. (After Dahlgren.)]
+
+Throughout the greater part of the last century, while students of
+evolution and of heredity were engaged in what I may call the more general,
+or, shall I say, the _grosser_ aspects of the subject, there existed
+another group of students who were engaged in working out the minute
+structure of the material basis of the living organism. They found that
+organs such as the brain, the heart, the liver, the lungs, the kidneys,
+etc., are not themselves the units of structure, but that all these organs
+can be reduced to a simpler unit that repeats itself a thousand-fold in
+every organ. We call this unit a cell (fig. 45).
+
+The egg is a cell, and the spermatozoon is a cell. The act of fertilization
+is the union of two cells (fig. 47, upper figure). Simple as the process of
+fertilization appears to us today, its discovery swept aside a vast amount
+of mystical speculation concerning the rôle of the male and of the female
+in the act of procreation.
+
+Within the cell a new microcosm was revealed. Every cell was found to
+contain a spherical body called the nucleus (fig. 46a). Within the nucleus
+is a network of fibres, a sap fills the interstices of the network. The
+network resolves itself into a definite number of threads at each division
+of the cell (fig. 46 b-e). These threads we call chromosomes. Each species
+of animals and plants possesses a characteristic number of these threads
+which have a definite size and sometimes a specific shape and even
+characteristic granules at different levels. Beyond this point our
+strongest microscopes fail to penetrate. Observation has reached, for the
+time being, its limit.
+
+[Illustration: FIG. 46. A series of cells in process of cell division. The
+chromosomes are the black threads and rods. (After Dahlgren.)]
+
+The story is taken up at this point by a new set of students who have
+worked in an entirely different field. Certain observations and experiments
+that we have not time to consider now, led a number of biologists to
+conclude that the chromosomes are the bearers of the hereditary units. If
+so, there should be many such units carried by _each_ chromosome, for the
+number of chromosomes is limited while the number of independently
+inherited characters is large. In Drosophila it has been demonstrated not
+only that there are exactly as many groups of characters that are inherited
+together as there are pairs of chromosomes, but even that it is possible to
+locate one of these groups in a particular chromosome and to state the
+_relative position_ there of the factors for the characters. If the
+validity of this evidence is accepted, the study of the cell leads us
+finally in a mechanical, but not in a chemical sense, to the ultimate units
+about which the whole process of the transmission of the hereditary factors
+centers.
+
+But before plunging into this somewhat technical matter (that is difficult
+only because it is unfamiliar), certain facts which are familiar for the
+most part should be recalled, because on these turns the whole of the
+subsequent story.
+
+[Illustration: FIG. 47. An egg, and the division of the egg--the so-called
+process of cleavage. (After Selenka.)]
+
+The thousands of cells that make up the cell-state that we call an animal
+or plant come from the fertilized egg. An hour or two after fertilization
+the egg divides into two cells (fig. 47). Then each half divides again.
+Each quarter next divides. The process continues until a large number of
+cells is formed and out of these organs mould themselves.
+
+[Illustration: FIG. 48. Section of the egg of the beetle, Calligrapha,
+showing the pigment at one end where the germ cells will later develop as
+shown in the other two figures. (After Hegner.)]
+
+At every division of the cell the chromosomes also divide. Half of these
+have come from the mother, half from the father. Every cell contains,
+therefore, the sum total of all the chromosomes, and if these are the
+bearers of the hereditary qualities, every cell in the body, whatever its
+function, has a common inheritance.
+
+At an early stage in the development of the animal certain cells are set
+apart to form the organs of reproduction. In some animals these cells can
+be identified early in the cleavage (fig. 48).
+
+The reproductive cells are at first like all the other cells in the body in
+that they contain a full complement of chromosomes, half paternal and half
+maternal in origin (fig. 49). They divide as do the other cells of the body
+for a long time (fig. 49, upper row). At each division each chromosome
+splits lengthwise and its halves migrate to opposite poles of the spindle
+(fig. 49 c).
+
+But there comes a time when a new process appears in the germ cells (fig 49
+e-h). It is essentially the same in the egg and in the sperm cells. The
+discovery of this process we owe to the laborious researches of many
+workers in many countries. The list of their names is long, and I shall not
+even attempt to repeat it. The chromosomes come together in pairs (fig. 49
+a). Each maternal chromosome mates with a paternal chromosome of the same
+kind.
+
+[Illustration: FIG. 49. In the upper row of the diagram a typical process
+of nuclear division, such as takes place in the early germ cells or in the
+body cells. In the lower row the separation of the chromosomes that have
+paired. This sort of separation takes place at one of the two reduction
+divisions.]
+
+Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the
+divisions the double chromosomes separate so that each resulting cell comes
+to contain some maternal and some paternal chromosomes, i.e. one or the
+other member of each pair. At the other division each chromosome simply
+splits as in ordinary cell division.
+
+[Illustration: FIG. 50. The two maturation divisions of the sperm cell.
+Four sperms result, each with half (haploid) the full number (diploid) of
+chromosomes.]
+
+The upshot of the process is that the ripe eggs (fig. 51) and the ripe
+spermatozoa (fig. 50) come to contain only half the total number of
+chromosomes.
+
+[Illustration: FIG. 51. The two maturation divisions of the egg. The
+divisions are unequal, so that two small polar bodies are formed one of
+these subsequently divides. The three polar bodies and the egg are
+comparable to the four sperms.]
+
+When the eggs are fertilized the whole number of chromosomes is restored
+again.
+
+THE MECHANISM OF MENDELIAN HEREDITY DISCOVERED IN THE BEHAVIOR OF THE
+CHROMOSOMES
+
+If the factors in heredity are carried in the chromosomes and if the
+chromosomes are definite structures, we should anticipate that there should
+be as many _groups_ of characters as there are kinds of chromosomes. In
+only one case has a sufficient number of characters been studied to show
+whether there is any correspondence between the number of hereditary groups
+of characters and the number of chromosomes. In the fruit fly, Drosophila
+ampelophila, we have found about 125 characters that are inherited in a
+perfectly definite way. On the opposite page is a list of some of them.
+
+It will be observed in this list that the characters are arranged in four
+groups, Groups I, II, III and IV. Three of these groups are equally large
+or nearly so; Group IV contains only two characters. The characters are put
+into these groups because in heredity the members of each group tend to be
+inherited together, i.e., if two or more enter the cross together they tend
+to remain together through subsequent generations. On the other hand, any
+member of one group is inherited entirely independently of any member of
+the other groups; in the same way as Mendel's yellow-green pair of
+characters is inherited independently of the round-wrinkled pair.
+
+  _Group I_       _Group II_        _Group III_        _Group IV_
+   Abnormal        Antlered          Band               Bent
+   Bar             Apterous          Beaded             Eyeless
+   Bifid           Arc               Cream III
+   Bow             Balloon           Deformed
+   Cherry          Black             Dwarf
+   Chrome          Blistered         Ebony
+   Cleft           Comma             Giant
+   Club            Confluent         Kidney
+   Depressed       Cream II          Low crossing over
+   Dot             Curved            Maroon
+   Eosin           Dachs             Peach
+   Facet           Extra vein        Pink
+   Forked          Fringed           Rough
+   Furrowed        Jaunty            Safranin
+   Fused           Limited           Sepia
+   Green           Little crossover  Sooty
+   Jaunty          Morula            Spineless
+   Lemon           Olive             Spread
+   Lethals, 13     Plexus            Trident
+   Miniature       Purple            Truncate intensifier
+   Notch           Speck             Whitehead
+   Reduplicated    Strap             White ocelli
+   Ruby            Streak
+   Rudimentary     Trefoil
+   Sable           Truncate
+   Shifted         Vestigial
+   Short
+   Skee
+   Spoon
+   Spot
+   Tan
+   Truncate intensifier
+   Vermilion
+   White
+   Yellow
+
+If the factors for these characters are carried by the chromosomes, then we
+should expect that those factors that are carried by the same chromosome
+would be inherited together, provided the chromosomes are definite
+structures in the cell.
+
+[Illustration: FIG. 52. Chromosomes (diploid) of D. ampelophila. The sex
+chromosomes are XX in the female and XY in the male. There are three other
+pairs of chromosomes.]
+
+In the chromosome group of Drosophila, (fig. 52) there are _four_ pairs of
+chromosomes, three of nearly the same size and one much smaller. Not only
+is there agreement between the number of hereditary groups and the number
+of the chromosomes, but even the size relations are the same, for there are
+three great groups of characters and three pairs of large chromosomes, and
+one small group of characters and one pair of small chromosomes.
+
+THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA AMPELOPHILA
+
+The following description of the characters of the wild fly may be useful
+in connection with the account of the modifications of these characters
+that appear in the mutants.
+
+The head and thorax of the wild fly are grayish-yellow, the abdomen is
+banded with alternate stripes of yellow and black. In the male, (fig. 4 to
+right), there are three narrow bands and a black tip. In the female there
+are five black bands (fig. 4 to left). The wings are gray with a surface
+texture of such a kind that at certain angles they are iridescent. The eyes
+are a deep, solid, brick-red. The minute hairs that cover the body have a
+very definite arrangement that is most obvious on the head and thorax.
+There is a definite number of larger hairs called bristles or chaetae which
+have a characteristic position and are used for diagnostic purposes in
+classifying the species. On the foreleg of the male there is a comb-like
+organ formed by a row of bristles; it is absent in the female. The comb is
+a secondary sexual character, and it is, so far as known, functionless.
+
+Some of the characters of the mutant types are shown in figures 53, 54, 55,
+56. The drawing of a single fly is often used here to illustrate more than
+one character. This is done to economize space, but of course there would
+be no difficulty in actually bringing together in the same individual any
+two or more characters belonging to the same group (or to different
+groups). Without colored figures it is not possible to show many of the
+most striking differences of these mutant races; at most dark and light
+coloring can be indicated by the shading of the body, wings, or eyes.
+
+_Group I_
+
+In the six flies drawn in figure 53 there are shown five different wing
+characters. The first of these types (a) is called cut, because the ends of
+the wings look as though they had been cut to a point. The antennae are
+displaced downward and appressed and their bristle-like aristae are
+crumpled.
+
+[Illustration: FIG. 53. Group I. (See text)]
+
+The second figure (b) represents a fly with a notch in the ends of the
+wings. This character is dominant, but the same factor that produces the
+notch in the wings is also a recessive lethal factor; because of this
+latter effect of the character no males of this race exist, and the females
+of the race are never pure but hybrid. Every female with notch wings bred
+to a wild male, will produce in equal numbers notch winged daughters and
+daughters with normal wings. There will be half as many sons as daughters.
+The explanation of this peculiar result is quite simple. Every notch winged
+female has one X chromosome that carries the factor for notch and one X
+chromosome that is "normal". Daughters receiving the former chromosomes are
+notched because the factor for notch is dominant, but they are not killed
+since the lethal effect of the notch factor is recessive to the normal
+allelomorph carried by the other chromosome that the daughters get from
+their father. This normal factor is recessive for notch but dominant for
+life. This same figure (b) is used here to show three other sex linked
+characters. The spines on the thorax are twisted or kinky, which is due to
+a factor called "forked". The effect is best seen on the thorax, but all
+spines on the body are similarly modified; even the minute hairs are also
+affected. Ruby eye color might be here represented--if the eyes in the
+figure were colored. The lighter color of the body and antennae is intended
+to indicate that the character tan is also present. The light color of the
+antennae is the most certain way of identifying tan. The tan flies are
+interesting because they have lost the positive heliotropism that is so
+marked a feature in the behavior of D. ampelophila. As this peculiarity of
+the tan flies is inherited like all the other sex linked characters, it
+follows that when a tan female is bred to a wild male all the sons inherit
+the recessive tan color and indifference to light, while the daughters show
+the dominant sex linked character of their father, i.e., they are "gray",
+and go to the light. Hence when such a brood is disturbed the females fly
+to the light, but the males remain behind.
+
+One of the first mutants that appeared in D. ampelophila was called
+rudimentary on account of the condition of the wings (c). The same mutation
+has appeared independently several times. In the drawing (c) the dark body
+color is intended to indicate "sable" and the lighter color of the eyes is
+intended to indicate eosin. This eye color, which is an allelomorph of
+white, is also interesting because in the female the color is deeper than
+in the male. In other cases of sex linked factors the character is the same
+in the two sexes.
+
+In the fourth figure (d) the third and fourth longitudinal veins of the
+wing are _fused_ into one vein from the base of the wing to the level of
+the first cross-vein and in addition converge and meet near their outer
+ends. The shape of the eye is represented in the figure as different from
+the normal, due to another factor called "bar". This is a dominant
+character, the hybrid condition being also narrow, but not so narrow as the
+pure type. Vermilion eye color might also be here represented--due to a
+factor that has appeared independently on several occasions.
+
+In the fifth figure (e) the wings are shorter and more pointed than in the
+wild fly. This character is called miniature. The light color of the
+drawing may be taken to represent yellow body color, and the light color of
+the eye white eye color.
+
+In the last figure (f) the wings are represented as pads, essentially in
+the same condition that they are in when the fly emerges from the pupa
+case. Not all the flies of this stock have the wings in this condition;
+some have fully expanded wings that appear normal in all respects.
+Nevertheless, about the same percentage of offspring show the pads
+irrespective of whether the parents had pads or expanded wings.
+
+The flies of this stock show, however, another character, which is a
+product of the same factor, and which is constant, i.e., repeated in all
+individuals. The two bristles on the sides of the thorax are constantly
+absent in this race. The lighter color of the eye in the figure may be
+taken to indicate buff--a faint yellowish color. The factor for this eye
+color is another allelomorph of white.
+
+There are many other interesting characters that belong to the first group,
+such as abnormal abdomen, short legs, duplication of the legs, etc. In
+fact, any part of the body may be affected by a sex-linked factor.
+
+_Group II_
+
+In the first figure (a) of figure 54 that contains members of Group II the
+wings are almost entirely absent or "vestigial". This condition arose at a
+single step and breeds true, although it appears to be influenced to some
+extent by temperature, also by modifiers that sometimes appear in the
+stock. Purple eye color belongs in Group II; it resembles the color of the
+eye of the wild fly but is darker and more translucent.
+
+[Illustration: FIG. 54. Group II. (See text.)]
+
+In the second figure (b) the wing is again long and narrow and sometimes
+bent back on itself, as shown here. In several respects the wing resembles
+strap (d) but seems to be due to another factor, called antler,
+insufficiently studied as yet.
+
+In the third figure (c) the wings turn up at the end. This is brought about
+by the presence of the factor called jaunty.
+
+In the fourth figure the wings are long and narrow and several of the veins
+are unrepresented. This character, "strap", is very variable and has not
+yet been thoroughly studied. On the thorax there is a deep black mark
+called trefoil. Even in the wild fly there is a three pronged mark on the
+thorax present in many individuals. Trefoil is a further development and
+modification of this mark and is due to a special factor.
+
+In the fifth figure (e) the wings are arched. The factor is called arc. The
+dark color of the body, and especially of the wings, indicates the factor
+for black.
+
+The sixth figure (f) shows the wings "curved" downwards. In addition there
+is present a minute black speck at the base of each wing, due to another
+factor called speck.
+
+In the seventh figure (g) the wing is truncate. Its end is obliquely
+squared instead of rounded; it may be longer than the body, or shorter when
+other modifying factors are present. The mutation that produces this type
+of wing is of not infrequent occurrence. It has been shown by Muller and
+Altenburg that there are at least two factors that modify this
+character--the chief factor is present in the second chromosome; alone it
+produces the truncate wing in only a certain percentage of cases, but when
+the modifiers are also present about ninety percent of the individuals may
+show the truncate condition of the wing. But the presence of these factors
+makes the stock very infertile, so that it is difficult to maintain.
+
+In the eighth figure (h) the legs are shortened owing to the absence of a
+segment of the tarsus. The stock is called dachs--a nickname given to it
+because the short legs suggested the dachshund.
+
+_Group III_
+
+In figure 55, (a), a mutant type called bithorax is shown. The old
+metathorax is replaced by another mesothorax thrust in between the normal
+mesothorax and the abdomen. It carries a pair of wings that do not
+completely unfold. On this new mesothorax the characteristic arrangement of
+the bristles is shown. Thus at a single step a typical region of the body
+has doubled. The character is recessive.
+
+[Illustration: FIG. 55. Group III. (See text.)]
+
+The size of the adult fly of D. ampelophila varies greatly according to the
+amount of nourishment obtained by the larva. After the fly emerges its size
+remains nearly constant, as in many insects. Two races have, however, been
+separated by Bridges that are different in size as a result of a genetic
+factor. The first of these, called dwarf, is represented by figure 55, (b).
+
+The race is minute, although of course its size is variable, depending on
+food and other conditions. The same figure shows the presence of another
+factor, "sooty", that makes the fly very dark. Maroon eye color might be
+here represented, due to still another factor.
+
+In the third figure (c) the other mutation in size is shown. It is called
+"giant". The flies are twice the size of wild flies. An eye color, called
+peach, might here be represented. It is an allelomorph of pink.
+
+In the fourth figure (d) the mutant called dichaete is shown. It is
+characterized by the absence of two of the bristles on the thorax. Other
+bristles may also be absent, but not so constantly as the two just
+mentioned. Another effect of the same factor is the spread-out condition of
+the wings. The very dark eye color in this figure may be taken to indicate
+the presence of another factor, "sepia", which causes the eyes to assume a
+brown color that becomes black with age. Most of the other mutations in eye
+color that have occurred tend to give a lighter color: this one, which is
+also recessive, makes the eye darker.
+
+In the fifth figure (e) the color of the darkest fly is due to a factor
+called ebony, which is an allelomorph of sooty.
+
+In the sixth figure (f) the wings are beaded, i.e., the margin is defective
+at intervals, giving a beaded-like outline to the wings. This condition is
+very variable and much affected by other factors that influence the shape
+of the wings. The lighter eye color of the drawing may be taken to
+represent pink.
+
+In the seventh figure (g) the wings are curled up over the back. This is a
+recessive character.
+
+_Group IV_
+
+Only two mutants have been obtained that do not belong to any of the
+preceding groups; these are put together in Group IV. It has been shown
+that they are linked to each other and the linkage is so close that it has
+thus far been impossible to obtain the dominant recessive. One of these
+mutants, called "eyeless" (fig. 56, a, a^1), is variable--the eyes are
+often entirely absent or represented by one or more groups of ommatidia.
+The outline of the original eye, so to speak, is strongly marked out and
+its area might be called a rudimentary organ, if such a statement has any
+meaning here.
+
+[Illustration: FIG. 56. Group IV. (See text.)]
+
+The other figure (b) represents "bent", so called from the shape of the
+wings. This mutant is likewise very variable, often indistinguishable from
+the wild type, yet when well developed strikingly different from any other
+mutant.
+
+This brief account of a few of the mutant races that can be most easily
+represented by uncolored figures will serve to show how all parts of the
+body may change, some of the changes being so slight that they would be
+overlooked except by an expert, others so great that in the character
+affected the flies depart far from the original species.
+
+_It is important to note that mutations in the first chromosome are not
+limited to any part of the body nor do they affect more frequently a
+particular part. The same statement holds equally for all of the other
+chromosomes. In fact, since each factor may affect visibly several parts of
+the body at the same time there are no grounds for expecting any special
+relation between a given chromosome and special regions of the body. It can
+not too insistently be urged that when we say a character is the product of
+a particular factor we mean no more than that it is the most conspicuous
+effect of the factor._
+
+If, then, as these and other results to be described point to the
+chromosomes as the bearers of the Mendelian factors, and if, as will be
+shown presently, these factors have a definite location in the chromosomes
+it is clear that the location of the factors in the chromosomes bears no
+spatial relation to the location of the parts of the body to each other.
+
+LOCALIZATION OF FACTORS IN THE CHROMOSOMES
+
+_The Evidence from Sex Linked Inheritance_
+
+When we follow the history of pairs of chromosomes we find that their
+distribution in successive generations is paralleled by the inheritance of
+Mendelian characters. This is best shown in the sex chromosomes (fig. 57).
+In the female there are two of these chromosomes that we call the X
+chromosomes; in the male there are also two but one differs from those of
+the female in its shape, and in the fact that it carries none of the normal
+allelomorphs of the mutant factors. It is called the Y chromosome.
+
+The course followed by the sex chromosomes and that by the characters in
+the case of sex linked inheritance are shown in the next diagram of
+Drosophila illustrating a cross between a white eyed male and a red eyed
+female.
+
+[Illustration: FIG. 57. Scheme of sex determination in Drosophila type.
+Each _mature_ egg contains one X, each mature sperm contains one X, or a Y
+chromosome. Chance union of any egg with any sperm will give either XX
+(female) or XY (male).]
+
+[Illustration: FIG. 58. Cross between white eyed male of D. ampelophila and
+red eyed female. The sex chromosomes are indicated by the rods. A black rod
+indicates that the chromosome carries the factor for red; the open
+chromosome the factor for white eye color.]
+
+The first of these represents a cross between a white eyed male and a red
+eyed female (fig. 58, top row). The X chromosome in the male is represented
+by an open bar, the Y chromosome is bent. In the female the two X
+chromosomes are black. Each egg of such a female will contain one "black" X
+after the polar bodies have been thrown off. In the male there will be two
+classes of sperm--the female-producing, carrying the (open) X, and the
+male-producing, carrying the Y chromosome. Any egg fertilized by an X
+bearing sperm will produce a female that will have red eyes because the X
+(black) chromosome it gets from the mother carries the dominant factor for
+red. Any egg fertilized by a Y-bearing sperm will produce a male that will
+also have red eyes because he gets his (black) X chromosome from his
+mother.
+
+When, then, these two F_1 flies (second row) are inbred the following
+combinations are expected. Each egg will contain a black X (red eye
+producing) or a white X (white eye producing) after the polar bodies have
+been extruded. The male will produce two kinds of sperms, of which the
+female producing will contain a black X (red eye producing). Since any egg
+may by chance be fertilized by any sperm there will result the four classes
+of individuals shown on the bottom row of the diagram. All the females will
+have red eyes, because irrespective of the two kinds of eggs involved all
+the female-producing sperm carry a black X. Half of the males have red eyes
+because half of the eggs have had each a red-producing X chromosome. The
+other half of the males have white eyes, because the other half of the eggs
+had each a white-producing X chromosome. Other evidence has shown that the
+Y chromosome of the male is indifferent, so far as these Mendelian factors
+are concerned.
+
+[Illustration: FIG. 59. Cross between red eyed male and white eyed female;
+reciprocal cross of Fig. 58.]
+
+The reciprocal experiment is illustrated in figure 59. A white eyed female
+is mated to a red eyed male (top row). All the mature eggs of such a female
+contain one white-producing X chromosome represented by the open bar in the
+diagram. The red eyed male contains female-producing X-bearing sperm that
+carry the factor for red eye color, and male-producing Y chromosomes. Any
+egg fertilized by an X-bearing sperm will become a red eyed female because
+the X chromosome that comes from the father carries the dominant factor for
+red eye color. Any egg fertilized by a Y-bearing sperm will become a male
+with white eyes because the only X chromosome that the male contains comes
+from his mother and is white producing.
+
+When these two F_1 flies are inbred (middle row) the following combinations
+are expected. Half the eggs will contain each a white producing X
+chromosome and half red producing. The female-producing sperms will each
+contain a white X and the male-producing sperms will each contain an
+indifferent Y chromosome. Chance meetings of egg and sperm will give the
+four F_2 classes (bottom row). These consist of white eyed and red eyed
+females and white eyed and red eyed males. The ratio here is 1:1 and not
+three to one (3:1) as in other Mendelian cases. But Mendel's law of
+segregation is not transgressed, as the preceding analysis has shown; for,
+the chromosomes have followed strictly the course laid down on Mendel's
+principle for the distribution of factors. The peculiar result in this case
+is due to the fact that the F_1 male gets his single factor for eye color
+from his mother only and it is linked to or contained in a body (the X
+chromosome) that is involved in producing the females, while the mate of
+this body--the Y chromosome--is indifferent with regard to these factors,
+yet active as a mate to X in synapsis.
+
+[Illustration: FIG. 60. Diagram of sex determination in type with XX female
+and XO male (after Wilson).]
+
+In man there are several characters that show exactly this same kind of
+inheritance. Color blindness, or at least certain kinds of color blindness,
+appear to follow the same scheme. A color blind father transmits through
+his daughters his peculiarity to half of his grandsons, but to none of his
+grand-daughters (fig. 38A). The result is the same as in the case of the
+white eyed male of Drosophila. Color blind women are rather unusual, which
+is expected from the method of inheritance of this character, but in the
+few known cases where such color blind women have married normal husbands
+the sons have inherited the peculiarity from the mother (fig. 38B). Here
+again the result is the same as for the similar combination in Drosophila.
+
+[Illustration: FIG. 61. Spermatogenesis in man. There are 47 chromosomes
+(diploid) in the male. After reduction half of the sperm carry 24
+chromosomes (one of which is X) and half carry 23 chromosomes (no X).]
+
+In man the sex formula appears to be XX for the female and XO for the male
+(fig. 60), and since the relation is essentially the same as that in
+Drosophila the chromosome explanation is the same. According to von
+Winiwarter there are 48 chromosomes in the female and 47 in the male (fig.
+61). After the extrusion of the polar bodies there are 24 chromosomes in
+the egg. In the male at one of the two maturation divisions the X
+chromosome passes to one pole undivided (fig. 61, C). In consequence there
+are two classes of sperms in man; female producing containing 24
+chromosomes, and male producing containing 23 chromosomes. If the factor
+for color blindness is carried by the X chromosome its inheritance in man
+works out on the same chromosome scheme and in the same way as does white
+eye color (or any other sex linked character) in the fly, for the O sperm
+in man is equivalent to the Y sperm in the fly.
+
+In these cases we have been dealing with a single pair of characters. Let
+us now take a case where two pairs of sex linked characters enter the cross
+at the same time, and preferably a case where the two recessives enter the
+cross from the same parent.
+
+If a female with white eyes and yellow wings is crossed to a wild male with
+red eyes and gray wings (fig. 62), the sons are yellow and have white eyes
+and the daughters are gray and have red eyes. If two F_1 flies are mated
+they will produce the following classes.
+
+[Illustration: FIG. 62. Cross between a white eyed, yellow winged female of
+D. ampelophila and a red eyed, gray winged male. Two pairs of sex linked
+characters, viz., white-red and yellow-gray are involved. (See text.)]
+
+  Yellow  Gray  Yellow  Gray
+  White   Red   Red     White
+  ------------  -------------
+        |             |
+       99.%          1.%
+
+Not only have the two grandparental combinations reappeared, but in
+addition two new combinations, viz., grey white and yellow red. The two
+original combinations far exceed in numbers the new or exchange
+combinations. If we follow the history of the X chromosomes we discover
+that the _larger classes_ of grandchildren appear in accord with the way in
+which the X chromosomes are transmitted from one generation to the next.
+
+The _smaller classes_ of grandchildren, the exchange combinations or
+cross-overs, as we call them, can be explained by the assumption that at
+some stage in their history an interchange of parts has taken place between
+the chromosomes. This is indicated in the diagrams.
+
+The most important fact brought out by the experiment is that the factors
+that went in together tend to stick together. It makes no difference in
+what combination the members of the two pairs of characters enter, they
+tend to remain in that combination.
+
+If one admits that the sex chromosomes carry these factors for the
+sex-linked characters--and the evidence is certainly very strong in favor
+of this view--it follows necessarily from these facts that at some time in
+their history there has been an interchange between the two sex chromosomes
+in the female.
+
+There are several stages in the conjugation of the chromosomes at which
+such an interchange between the members of a pair might occur. There is
+further a small amount of direct evidence, unfortunately very meagre at
+present, showing that an interchange does actually occur.
+
+At the ripening period of the germ cell the members of each pair of
+chromosomes come together (fig. 49, e). In several forms they have been
+described as meeting at one end and then progressively coming to lie side
+by side as shown in fig. 63, e, f, g, h, i. At the end of the process they
+appear to have completely united along their length (fig. 63, j, k, l). It
+is always a maternal and a paternal chromosome that meet in this way and
+always two of the same kind. It has been observed that as the members of a
+pair come together they occasionally twist around each other (fig. 63, g,
+l, and 64, and 65). In consequence a part of one chromosome comes to be now
+on one side and now on the other side of its mate.
+
+[Illustration: FIG. 63. Conjugation of chromosomes (side to side union) in
+the spermatogenesis of Batracoseps. (After Janssens.)]
+
+When the chromosomes separate at the next division of the germ cell the
+part on one side passes to one pole, the part on the other to the opposite
+pole, (figs. 64 and 65). Whenever the chromosomes do not untwist at this
+time there must result an interchange of pieces where they were crossed
+over each other.
+
+[Illustration: FIG. 64. Scheme to illustrate a method of crossing over of
+the chromosomes.]
+
+Janssens has found at the time of separation evidence in favor of the view
+that some such interchange probably takes place.
+
+We find this same process of interchange of characters taking place in each
+of the other three groups of Drosophila. An example will show this for the
+Group II.
+
+[Illustration: FIG. 65. Scheme to illustrate double crossing over.]
+
+If a black vestigial male is crossed to a gray long-winged female (fig. 66)
+the offspring are gray long. If an F_1 female is back-crossed to a black
+vestigial male the following kinds of flies are produced:
+
+  Black        Gray       Black   Gray
+  vestigial    long       long    vestigial
+  -----------------       -----------------
+          |                      |
+         83%                    17%
+
+The combinations that entered are more common in the F_2 generations than
+the cross-over classes, showing that there is linkage of the factors that
+entered together.
+
+Another curious fact is brought out if instead of back-crossing the F_1
+female we back-cross the F_1 male to a black vestigial female. Their
+offspring are now of only two kinds, black vestigial and gray long. This
+means that in the male there is no crossing-over or interchange of pieces.
+This relation holds not only for the Group II but for all the other groups
+as well.
+
+Why interchange takes place in the female of Drosophila and not in the male
+we do not know at present. We might surmise that when in the male the
+members of a pair come together they do not twist around each other, hence
+no crossing-over results.
+
+[Illustration: FIG. 66. Cross between black vestigial and gray long flies.
+Two pairs of factors involved in the second group. The F_1 female is back
+crossed (to right) to black vestigial male; and the F_1 male is back
+crossed to black vestigial female (to left). Crossing over takes place in
+the F_1 female but not in the F_1 male.]
+
+Crossing-over took place between white and yellow only once in a hundred
+times. Other characters show different values, but the same value under the
+same conditions is obtained from the same pair of characters.
+
+[Illustration: FIG. 67. Map of four chromosomes of D. ampelophila locating
+those factors in each group that have been most fully studied.]
+
+If we assume that the nearer together the factors lie in the chromosome the
+less likely is a twist to occur between them, and conversely the farther
+apart they lie the more likely is a twist to occur between them, we can
+understand how the linkage is different for different pairs of factors.
+
+On this basis we have made out chromosomal maps for each chromosome (fig.
+67). The diagram indicates those loci that have been most accurately
+placed.
+
+_The Evidence from Interference_
+
+There is a considerable body of information that we have obtained that
+corroborates the location of the factors in the chromosome. This evidence
+is too technical to take up in any detail, but there is one result that is
+so important that I must attempt to explain it. If, as I assume, crossing
+over is brought about by twisting of the chromosomes, and if owing to the
+material of the chromosomes there is a most frequent distance of internode,
+then, when crossing over between nodes takes place at same level at a-b in
+figure 68, the region on each side of that point, a to A and b to B, should
+be protected, so to speak, from further crossing over. This in fact we have
+found to be the case. No other explanation so far proposed will account for
+this extraordinary relation.
+
+[Illustration: FIG. 68. Scheme to indicate that when the members of a pair
+of chromosomes cross (at a-b) the region on each side is protected
+inversely to the distance from a-b.]
+
+What advantage, may be asked, is there in obtaining numerical data of this
+kind? It is this:--whenever a new character appears we need only determine
+in which of the four groups it lies and its distance from two members
+within that group. With this information we can predict with a high degree
+of probability what results it will give with any other member of any
+group. Thus we can do on paper what would require many months of labor by
+making the actual experiment. In a word we can predict what will happen in
+a situation where prediction is impossible without this numerical
+information.
+
+_The Evidence from Non-Disjunction_
+
+In the course of the work on Drosophila exceptions appeared in one strain
+where certain individuals did not conform to the scheme of sex linked
+inheritance. For a moment the hypothesis seemed to fail, but a careful
+examination led to the suspicion that in this strain something had happened
+to the sex chromosomes. It was seen that if in some way the X chromosomes
+failed to disjoin in certain eggs, the exceptions could be explained. The
+analysis led to the suggestion that if the Y chromosome had got into the
+female line the results would be accounted for, since its presence there
+would be expected to cause this peculiar non-disjunction of the X
+chromosomes.
+
+That this was the explanation was shown when the material was examined. The
+females that gave these results were found by Bridges to have two X's and a
+Y chromosome.
+
+The normal chromosome group of the female is shown in figure 52 and the
+chromosome group of one of the exceptional females is shown in figure 69.
+In a female of this kind there are three sex chromosomes X X Y which are
+homologous in the sense that in normal individuals the two present are
+mates and separate at the reduction division. If in the X X Y individual X
+and X conjugate and separate at reduction and the unmated Y is free to move
+to either pole of the spindle, two kinds of mature eggs will result, viz.,
+X and XY. If, on the other hand, X and Y conjugate and separate at
+reduction and the remaining X is free to go to either pole, four kinds of
+eggs will result--XY--X--XX--Y. As a total result four kinds of eggs are
+expected: viz. many XY and X eggs and a few XX and Y eggs.
+
+[Illustration: FIG. 69. Figure of the chromosome group of an XXY female,
+that gives non-disjunction.]
+
+These four kinds of eggs may be fertilized either by female-producing
+sperms or male-producing sperms, as indicated in the diagram (fig. 70).
+
+[Illustration: FIG. 70. Scheme showing the results of fertilizing white
+bearing eggs (4 kinds) resulting from non-disjunction. The upper half of
+the diagram gives the results when these eggs are fertilized by normal red
+bearing, female producing sperm, the lower half by normal, male producing
+sperm.]
+
+If such an XXY female carried white bearing Xs (open X in the figures), and
+the male carried a red bearing X (black X in the figures) it will be seen
+that there should result an exceptional class of sons that are red, and an
+exceptional class of daughters that are white. Tests of these exceptions
+show that they behave subsequently in heredity as their composition
+requires. Other tests may also be made of the other classes of offspring.
+Bridges has shown that they fulfill all the requirements predicted. Thus a
+result that seemed in contradiction with the chromosome hypothesis has
+turned out to give a brilliant confirmation of that theory both genetically
+and cytologically.
+
+HOW MANY GENETIC FACTORS ARE THERE IN THE GERM-PLASM OF A SINGLE INDIVIDUAL
+
+In passing I invite your attention to a speculation based on our maps of
+the chromosomes--a speculation which I must insist does not pretend to be
+more than a guess but has at least the interest of being the first guess
+that we have ever been in position to make as to how many factors go
+towards the makeup of the germ plasm.
+
+We have found practically no factors less than .04 of a unit apart. If our
+map includes the entire length of the chromosomes and if we assume factors
+are uniformly distributed along the chromosome at distances equal to the
+shortest distance yet observed, viz. .04, then we can calculate roughly how
+many hereditary factors there are in Drosophila. The calculation gives
+about 7500 factors. The reader should be cautioned against accepting the
+above assumptions as strictly true, for crossing-over values are known to
+differ according to different environmental conditions (as shown by Bridges
+for age), and to differ even in different parts of the chromosome as a
+result of the presence of specific genetic factors (as shown by
+Sturtevant). Since all the chromosomes except the X chromosomes are double
+we must double our estimate to give the _total_ number of factors, but the
+half number is the number of the different kinds of factors of Drosophila.
+
+CONCLUSIONS
+
+I have passed in review a long series of researches as to the nature of the
+hereditary material. We have in consequence of this work arrived within
+sight of a result that seemed a few years ago far beyond our reach. The
+mechanism of heredity has, I think, been discovered--discovered not by a
+flash of intuition but as the result of patient and careful study of the
+evidence itself.
+
+With the discovery of this mechanism I venture the opinion that the problem
+of heredity has been solved. We know how the factors carried by the parents
+are sorted out to the germ cells. The explanation does not pretend to state
+how factors arise or how they influence the development of the embryo. But
+these have never been an integral part of the doctrine of heredity. The
+problems which they present must be worked out in their own field. So, I
+repeat, the mechanism of the chromosomes offers a satisfactory solution of
+the traditional problem of heredity.
+
+       *       *       *       *       *
+
+
+CHAPTER IV
+
+SELECTION AND EVOLUTION
+
+Darwin's Theory of Natural Selection still holds today first place in every
+discussion of evolution, and for this very reason the theory calls for
+careful scrutiny; for it is not difficult to show that the expression
+"natural selection" is to many men a metaphor that carries many meanings,
+and sometimes different meanings to different men. While I heartily agree
+with my fellow biologists in ascribing to Darwin himself, and to his work,
+the first place in biological philosophy, yet recognition of this claim
+should not deter us from a careful analysis of the situation in the light
+of work that has been done since Darwin's time.
+
+THE THEORY OF NATURAL SELECTION
+
+In his great book on the _Origin of Species_, Darwin tried to do two
+things: first, to show that the evidence bearing on evolution makes that
+explanation probable. No such great body of evidence had ever been brought
+together before, and it wrought, as we all know, a revolution in our modes
+of thinking.
+
+Darwin also set himself the task of showing _how_ evolution might have
+taken place. He pointed to the influence of the environment, to the effects
+of use and disuse, and to natural selection. It is to the last theory that
+his name is especially attached. He appealed to a fact familiar to
+everyone, that no two individuals are identical and that some of the
+differences that they show are inherited. He argued that those individuals
+that are best suited to their environment are the most probable ones to
+survive and to leave most offspring. In consequence their descendants
+should in time replace through competition the less well-adapted
+individuals of the species. This is the process Darwin called natural
+selection, and Spencer the survival of the fittest.
+
+Stated in these general terms there is nothing in the theory to which
+anyone is likely to take exception. But let us examine the argument more
+critically.
+
+[Illustration: FIG. 71. Series of leaves of a tree arranged according to
+size. (After de Vries.)]
+
+If we measure, or weigh, or classify any character shown by the individuals
+of a population, we find differences. We recognize that some of the
+differences are due to the varied experiences that the individuals have
+encountered in the course of their lives, i.e. to their environment, but we
+also recognize that some of the differences may be due to individuals
+having different inheritances--different germ plasms. Some familiar
+examples will help to bring home this relation.
+
+If the leaves of a tree are arranged according to size (fig. 71), we find a
+continuous series, but there are more leaves of medium size than extremes.
+If a lot of beans be sorted out according to their weights, and those
+between certain weights put into cylinders, the cylinders, when arranged
+according to the size of the beans, will appear as shown in figure 72. An
+imaginary line running over the tops of the piles will give a curve (fig.
+73) that corresponds to the curve of probability (fig. 74).
+
+[Illustration: FIG. 72. Beans put into cylindrical jars according to the
+sizes of the beans. The jars arranged according to size of contained beans.
+(After de Vries.)]
+
+[Illustration: FIG. 73. A curve resulting from arrangement of beans
+according to size. (After de Vries.)]
+
+If we stand men in lines according to their height (fig. 75) we get a
+similar arrangement.
+
+[Illustration: FIG. 74. Curve of probability.]
+
+[Illustration: FIG. 75. Students arranged according to size. (After
+Blakeslee.)]
+
+The differences in size shown by the individual beans or by the individual
+men are due in part to heredity, in part to the environment in which they
+have developed. This is a familiar fact of almost every-day observation. It
+is well shown in the following example. In figure 76 the two boys and the
+two varieties of corn, which they are holding, differ in height. The
+pedigrees of the boys (fig. 77) make it probable that their height is
+largely inherited and the two races of corn are known to belong to a tall
+and a short race respectively. Here, then, the chief effect or difference
+is due to heredity. On the other hand, if individuals of the same race
+develop in a favorable environment the result is different from the
+development in an unfavorable environment, as shown in figure 78. Here to
+the right the corn is crowded and in consequence dwarfed, while to the left
+the same kind of corn has had more room to develop and is taller.
+
+[Illustration: FIG. 76. A short and a tall boy each holding a stalk of
+corn--one stalk of a race of short corn, the other of tall corn. (After
+Blakeslee.)]
+
+[Illustration: FIG. 77. Pedigree of boys shown in Fig. 76. (After
+Blakeslee.)]
+
+Darwin knew that if selection of particular kinds of individuals of a
+population takes place the next generation is affected. If the taller men
+of a community are selected _the average_ of their offspring will be taller
+than the average of the former population. If selection for tallness again
+takes place, still taller men will _on the average_ arise. If, amongst
+these, selection again makes a choice the process would, he thought,
+continue (fig. 79).
+
+[Illustration: FIG. 78. A race of corn reared under different conditions.]
+
+We now recognize that this statement contains an important truth, but we
+have found that it contains only a part of the truth. Any one who repeats
+for himself this kind of selection experiment will find that while his
+average class will often change in the direction of his selection, the
+process slows down as a rule rather suddenly (fig. 80). He finds, moreover,
+that the limits of variability are not necessarily transcended as the
+process continues even although the average may for a while be increased.
+More tall men may be produced by selection of this kind, but the tallest
+men are not necessarily any taller than the tallest in the original
+population.
+
+[Illustration: FIG. 79. Curves showing how (hypothetically) selection might
+be supposed to bring about progress in direction of selection. (After
+Goldschmidt.)]
+
+Selection, then, has not produced anything new, but only more of certain
+kinds of individuals. Evolution, however, means producing more new things,
+not more of what already exists.
+
+Darwin seems to have thought that the range of variation shown by the
+offspring of a given individual about that type of individual would be as
+wide as the range shown by the original population (fig. 79), but Galton's
+work has made it clear that this is not the case in a general or mixed
+population. If the offspring of individuals continued to show, as Darwin
+seems to have thought, as wide a range on each side of their parents' size,
+so to speak, as did the original population, then it would follow that
+selection could slide successive generations along in the direction of
+selection.
+
+[Illustration: FIG. 80. Diagram illustrating the results of selection for
+extra bristles in D. ampelophila. Selection at first produces decided
+effects which soon slow down and then cease. (MacDowell.)]
+
+Darwin himself was extraordinarily careful, however, in the statements he
+made in this connection and it is rather by implication than by actual
+reference that one can ascribe this meaning to his views. His
+contemporaries and many of his followers, however, appear to have accepted
+this _sliding scale_ interpretation as the cardinal doctrine of evolution.
+If this is doubted or my statement is challenged then one must explain why
+de Vries' mutation theory met with so little enthusiasm amongst the older
+group of zoölogists and botanists; and one must explain why Johannsen's
+splendid work met with such bitter opposition from the English school--the
+biometricians--who amongst the post-Darwinian school are assumed to be the
+lineal descendants of Darwin.
+
+And in this connection we should not forget that just this sort of process
+was supposed to take place in the inheritance of use and disuse. What is
+gained in one generation forms the basis for further gains in the next
+generation. Now, Darwin not only believed that acquired characters are
+inherited but turned more and more to this explanation in his later
+writings. Let us, however, not make too much of the matter; for it is much
+less important to find out whether Darwin's ideas were vague, than it is to
+make sure that our own ideas are clear.
+
+If I have made several statements here that appear dogmatic let me now
+attempt to justify them, or at least give the evidence which seems to me to
+make them probable.
+
+The work of the Danish botanist, Johannsen, has given us the most carefully
+analyzed case of selection that has ever been obtained. There are,
+moreover, special reasons why the material that he used is better suited to
+give definite information than any other so far studied. Johannsen worked
+with the common bean, weighing the seeds or else measuring them. These
+beans if taken from many plants at random give the typical curve of
+probability (fig. 74). The plant multiplies by self-fertilization. Taking
+advantage of this fact Johannsen kept the seeds of each plant separate from
+the others, and raised from them a new generation. When curves were made
+from these new groups it was found that some of them had different modes
+from that of the original general population (fig. 81 A-E, bottom group).
+They are shown in the upper groups (A, B, C, D, E). But do not understand
+me to say that the offspring of each bean gave a different mode.
+
+[Illustration: FIG. 81. Pure lines of beans. The lower figure gives the
+general population, the other figures give the pure lines within the
+population. (After Johannsen.)]
+
+On the contrary, some of the lines would be the same.
+
+The result means that the general population is made up of definite kinds
+of individuals that may have been sorted out.
+
+That his conclusion is correct is shown by rearing a new generation from
+any plant or indeed from several plants of any one of these lines. Each
+line repeats the same modal class. There is no further breaking up into
+groups. Within the line it does not matter at all whether one chooses a big
+bean or a little one--they will give the same result. In a word, the germ
+plasm in each of these lines is pure, or homozygous, as we say. The
+differences that we find between the weights (or sizes) of the individual
+beans are due to external conditions to which they have been subjected.
+
+In a word, Johannsen's work shows that the frequency distribution of a pure
+line is due to factors that are extrinsic to the germ plasm. It does not
+matter then which individuals in a pure line are used to breed from, for
+they all carry the same germ plasm.
+
+We can now understand more clearly how selection acting on a general
+population brings about results in the direction of selection.
+
+An individual is picked out from the population in order to get a
+particular kind of germ plasm. Although the different classes of
+individuals may overlap, so that one can not always judge an individual
+from its appearance, nevertheless on the whole chance favors the picking
+out of the kind of germ plasm sought.
+
+In species with separate sexes there is the further difficulty that two
+individuals must be chosen for each mating, and superficial examination of
+them does not insure that they belong to the same group--their germ plasm
+cannot be inspected. Hence selection of biparental forms is a precarious
+process, now going forward, now backwards, now standing still. In time,
+however, the process forward is almost certain to take place if the
+selection is from a heterogeneous population. Johannsen's work was
+simplified because he started with pure lines. In fact, had he not done so
+his work would not have been essentially different from that of any
+selection experiment of a pure race of animals or plants. Whether Johannsen
+realized the importance of the condition or not is uncertain--curiously he
+laid no emphasis on it in the first edition of his "Elemente der exakten
+Erblichkeitslehre".
+
+It has since been pointed out by Jennings and by Pearl that a race that
+reproduces by self-fertilization as does this bean, automatically becomes
+pure in all of the factors that make up its germ plasm. Since
+self-fertilization is the normal process in this bean the purity of the
+germ plasm already existed when Johannsen began to experiment.
+
+HOW HAS SELECTION IN DOMESTICATED ANIMALS AND PLANTS BROUGHT ABOUT ITS
+RESULTS?
+
+If then selection does not bring about transgressive variation in a general
+population, how can selection produce anything new? If it can not produce
+anything new, is there any other way in which selection becomes an agent in
+evolution?
+
+We can get some light on this question if we turn to what man has done with
+his domesticated animals and plants. Through selection, i.e., artificial
+selection, man has undoubtedly brought about changes as remarkable as any
+shown by wild animals and plants. We know, moreover, a good deal about how
+these changes have been wrought.
+
+(1) By crossing different wild species or by crossing wild with races
+already domesticated new combinations have been made. Parts of one
+individual have been combined with parts of others, creating new
+combinations. It is possible even that characters that are entirely new may
+be produced by the interaction of factors brought into recombination.
+
+(2) New characters appear from time to time in domesticated and in wild
+species. These, like the mutants in Drosophila, are fully equipped at the
+start. Since they breed true and follow Mendel's laws it is possible to
+combine them with characters of the wild type or with those of other mutant
+races.
+
+Amongst the new mutant factors there may be some whose chief effect is on
+the character that the breeder is already selecting. Such a modification
+will be likely to attract attention. Superficially it may appear that the
+factor for the original character has varied, while the truth may be that
+another factor has appeared that has modified a character already present.
+In fact, many or all Mendelian factors that affect the same organ may be
+said to be modifiers of each other's effects. Thus the factor for vermilion
+causes the eye to be one color, and the factor for eosin another color,
+while eosin vermilion is different from both. Eosin may be said to be a
+modifier of vermilion or vermilion of eosin. In general, however, it is
+convenient to use the term "modifier" for cases in which the factor causes
+a detectable change in a character already present or conspicuous.
+
+[Illustration: FIG. 82. Scheme to indicate influence of the modifying
+factors, cream and whiting. Neither produces any effect alone but they
+modify other eye colors such as eosin.]
+
+One of the most interesting, and at the same time most treacherous, kinds
+of modifying factors is that which produces an effect _only_ when some
+other factor is present. Thus Bridges has shown that there is a factor
+called "cream" that does not affect the red color of the eye of the wild
+fly, yet makes "eosin" much paler (fig. 82). Another factor "whiting" which
+produces no effect on red makes eosin entirely white. Since cream or
+whiting may be carried by red eyed flies without their presence being seen
+until eosin is used, the experimenter must be continually on the lookout
+for such factors which may lead to erroneous conclusions unless detected.
+As yet breeders have not realized the important rôle that modifiers have
+played in their results, but there are indications at least that the
+heaping up of modifying factors has been one of the ways in which highly
+specialized domesticated animals have been produced. Selection has
+accomplished this result not by changing factors, but by picking up
+modifying factors. The demonstration of the presence of these factors has
+already been made in some cases. Their study promises to be one of the most
+instructive fields for further work bearing on the selection hypothesis.
+
+In addition to these well recognized methods by which artificial selection
+has produced new things we come now to a question that is the very crux of
+the selection theory today. Our whole conception of selection turns on the
+answer that we give to this matter and if I appear insistent and go into
+some detail it is because I think that the matter is worth very careful
+consideration.
+
+ARE FACTORS CHANGED THROUGH SELECTION?
+
+As we have seen, the variation that we find from individual to individual
+is due in part to the environment; this can generally be demonstrated.
+Other differences in an ordinary population are recognized as due to
+different genetic (hereditary) combinations. No one will dispute this
+statement. But is all the variability accounted for in these two ways? May
+not a factor itself fluctuate? Is it not _a priori_ probable that factors
+do fluctuate? Why, in a word, should we regard factors as inviolate when we
+see that everything else in organisms is more or less in amount? I do not
+know of any _a priori_ reason why a factor may not fluctuate, unless it is,
+as I like to think, a chemical molecule. We are, however, dealing here not
+with generalities but with evidence, and there are three known methods by
+means of which it has been shown that variability, other than environmental
+or recombinational, is not due to variability in a factor, nor to various
+"potencies" possessed by the same factors.
+
+(1) By making the stock uniform for all of its factors--chief factors and
+modifiers alike. Any change in such a stock produced by selection would
+then be due to a change in one or more of the factors themselves.
+Johannsen's experiment is an example of this sort.
+
+[Illustration: FIG. 83 a. Drosophila ampelophila with truncate wings.]
+
+(2) The second method is one that is capable of _demonstrating_ that the
+effects of selection are actually due to modifiers. It has been worked out
+in our laboratory, chiefly by Muller, and used in a particular case to
+demonstrate that selection produced its effect by isolating modifying
+factors. For example, a mutant type called truncate appeared, characterized
+by shorter wings, usually square at the end, (fig. 83a). The wings varied
+from those of normal length to wings much shorter (fig. 83b). For three
+years the mutant stock was bred from individuals having the shorter wings
+until at last a stock was obtained in which some of the individuals had
+wings much shorter than the body. By means of linkage experiments it was
+shown that at least three factors were present that modified the wings.
+These were isolated by means of their linkage relations, and their mutual
+influence on the production of truncate wings was shown.
+
+[Illustration: FIG. 83 b. Series of wings of different length shown by
+truncate stock of D. ampelophila.]
+
+An experiment of this kind can only be carried out in a case where the
+groups of linked gens are known. At present Drosophila is the only animal
+(or plant) sufficiently well known to make this test possible, but this
+does not prove that the method is of no value. On the contrary it shows
+that any claim that factors can themselves be changed can have no finality
+until the claim can be tested out by means of the linkage test. For
+instance, bar eye (fig. 31) arose as a mutation. All our stock has
+descended from a single original mutant. But Zeleny has shown that
+selection within our stock will make the bar eye narrower or broader
+according to the direction of selection. It remains to be shown in this
+case how selection has produced its effects, and this can be done by
+utilizing the same process that was used in the case of truncate.
+
+Another mutant stock called beaded (fig. 84), has been bred for five years
+and selected for wings showing more beading. In extreme cases the wings
+have been reduced to mere stumps (see stumpy, fig. 5), but the stock shows
+great variability. It is probable here as Dexter has shown, that a number
+of mutant factors that act as modifiers have been picked up in the course
+of the selection, and when it is recalled that during those five years over
+125 new characters have appeared elsewhere it does not seem improbable that
+factors also have appeared that modify the wings of this stock.
+
+[Illustration: FIG. 84. Two flies showing beaded wings.]
+
+(3) The third method is one that has been developed principally by East for
+plants; also by MacDowell for rabbits and flies. The method does not claim
+to prove that modifiers are present, but it shows why certain results are
+in harmony with that expectation and can not be accounted for on the basis
+that a factor has changed. Let me give an example. When a Belgian hare with
+large body was crossed to a common rabbit with a small body the hybrid was
+intermediate in size. When the hybrid was crossed back to the smaller type
+it produced rabbits of various sizes in apparently a continuous series.
+MacDowell made measurements of the range of variability in the first and in
+the second generations.
+
+    _Classification in relation to parents based on skull lengths and ulna
+    lengths, to show the relative variability of two measurements and of
+    the first generation (F_1) and the back cross (B. C.)_
+
+  CHARACTER|GENERATION|-13|-12|-11|-10| -9| -8| -7| -6| -5| -4| -3| -2| -1|
+  ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
+  Length of{   F_1    |   |   |   |   |   |   |   |   |   |   |   |   |   |
+    skull  {   B.C.   |   |   |   |   |   |   |   |   |   |   |   |   |  3|
+  Length of{   F_1    |   |   |   |   |   |   |   |   |   |   |   |   |   |
+    ulna   {   B.C.   |  1|   |   |   |   |  1|   |  2|  3|  1|  2|  4|  4|
+
+  _same table continued_
+
+  CHARACTER|GENERATION|  0|  1|  2|  3|  4|  5|  6|  7|  8|  9| 10| 11| 12|
+  ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
+  Length of{   F_1    |   |   |   |   |   |   |   |  2|  2|  8|  5| 10|  7|
+    skull  {   B.C.   |  6|  4| 13| 18| 42| 32| 38| 34| 16| 16|  8|  4|  3|
+  Length of{   F_1    |   |  1|   |   |  2|   |  1|  1|  1|  2|  2|  5|  3|
+    ulna   {   B.C.   | 12| 11| 20| 26| 17| 19| 18| 15| 12| 13| 15| 11|  5|
+
+  _same table continued_
+
+  CHARACTER|GENERATION| 13| 14| 15| 16| 17| 18| 19| 20| 21| 22| 23| 24| 25|
+  ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
+  Length of{   F_1    |  3|  2|  2|   |   |   |   |   |   |   |   |   |   |
+    skull  {   B.C.   |  1|   |   |   |   |   |   |   |   |   |   |   |   |
+  Length of{   F_1    |  1|  7|  3|  2|  1|   |   |   |  2|   |  1|   |  1|
+    ulna   {   B.C.   |  2|  4|  2|  2|   |   |  1|  1|   |   |   |   |   |
+
+He found that the variability was smaller in the first generation than in
+the second generation (back cross). This is what is expected if several
+factor-differences were involved, because the hybrids of the first
+generation are expected to be more uniform in factorial composition than
+are those in the second generation which are produced by recombination of
+the factors introduced through their grandparents. Excellent illustrations
+of the same kinds of results have been found in Indian corn. As shown in
+figure 85 the length of the cob in F_1 is intermediate between the parent
+types while in F_2 the range is wider and both of the original types are
+recovered. East states that similar relations have been found for 18
+characters in corn. Emerson has recently furnished further illustrations of
+the same relations in the length of stalks in beans.
+
+[Illustration: FIG. 85. Cross between two races of Indian corn, one with
+short cobs and one with long cobs. The range of variability in F_1 is less
+than that in F_2. (After East.)]
+
+A similar case is shown by a cross between fantail and common pigeons (fig.
+86). The latter have twelve feathers in the tail, while the selected race
+from which the fantails came had between 28 and 38 feathers in the tail.
+The F_1 offspring (forty-one individuals) showed (fig. 87) between 12 and
+20 tail feathers, while in F_2 the numbers varied between 12 and 25. Here
+one of the grand-parental types reappears in large numbers, while the
+extreme of the other grand-parental type did not reappear (in the counts
+obtained), although the F_2 number would probably overlap the lower limits
+of the race of fantail grandparents had not a selected (surviving) lot been
+taken for the figures given in the table.
+
+[Illustration: FIG. 86. Cross of pigeon with normal tail P_1 and fantail
+P_1; F_1, bird below.]
+
+[Illustration: FIG. 87. Cross of normal and fantail pigeons. (See Fig. 86.)
+The F_2 range is wider than that of F_1. The normal grand-parental type of
+12 feathers was recovered in F_2 but the higher numbers characteristic of
+fantails were not recovered.]
+
+The preceding account attempts to point out how I should prefer to
+interpret the problem of selection in the light of the most recent work on
+breeding. But I would give a very incomplete account of the whole situation
+if I neglected to include some important work which has led some of my
+fellow-workers to a very different conclusion.
+
+[Illustration: FIG. 88. Scheme to show classes of hooded rats used by
+Castle. (After Castle.)]
+
+Castle in particular is the champion of a view based on his results with
+hooded rats. Starting with individuals which have a narrow black stripe
+down the back he selected for a narrower stripe in one direction and for a
+broader stripe in the other. As the diagram shows (fig. 88) Castle has
+succeeded in producing in one direction a race in which the dorsal stripe
+has disappeared and in the other direction a race in which the black has
+extended over the back and sides, leaving only a white mark on the belly.
+Neither of these extremes occurs, he believes, in the ordinary hooded race
+of domesticated rats. In other words no matter how many of them came under
+observation the extreme types of his experiment would not be found.
+
+Castle claims that the factor for hoodedness must be a single Mendelian
+unit, because if hooded rats are crossed to wild gray rats with uniform
+coat and their offspring are inbred there are produced in F_2 three uniform
+rats to one hooded rat. Castle advances the hypothesis that factors--by
+which he means Mendelian factors--may themselves vary in much the same way
+as do the characters that they stand for. He argues, in so many words, that
+since we judge a factor by the kind of character it produces, when the
+character varies the factor that stands for it may have changed.
+
+As early as 1903 Cuénot had carried out experiments with spotted mice
+similar to those of Castle with rats. Cuénot found that spotted crossed to
+uniform coat color gave in F_2 a ratio of three uniform to one spotted, yet
+selection of those spotted mice with more white in their coat produced mice
+in successive generations that had more and more white. Conversely Cuénot
+showed that selection of those spotted mice that had more color in their
+coat produced mice with more and more color and less white. Cuénot does not
+however bring up in this connection the question as to how selection in
+these spotted mice brings about its results.
+
+Without attempting to discuss these results at the length that they deserve
+let me briefly state why I think Castle's evidence fails to establish his
+conclusion.
+
+In the first place one of the premises may be wrong. The three to one ratio
+in F_2 by no means proves that all conditions of hoodedness are due to one
+factor. The result shows at most that one factor that gives the hooded
+types is a simple Mendelian factor. The changes in this type may be caused
+by modifying factors that can show an effect only when hoodedness is itself
+present. That this is not an imaginary objection but a real one is shown by
+an experiment that Castle himself made which furnishes the ground for the
+second objection.
+
+Second. If the factor has really changed its potency, then if a very dark
+individual from one end of the series is crossed to a wild rat and the
+second generation raised we should expect that the hooded F_2 rats would
+all be dark like their dark grandparent. When Castle made this test he
+found that there were many grades of hooded rats in the F_2 progeny. They
+were darker, it is true, as a group than were the original hooded group at
+the beginning of the selection experiment, but they gave many intermediate
+grades. Castle attempts to explain this by the assumption that the factor
+made pure by selection became contaminated by its normal allelomorph in the
+F_1 parent, but not only does this assumption appear to beg the whole
+question, but it is in flat contradiction with what we have observed in
+hundreds of Mendelian cases where no evidence for such a contamination
+exists.
+
+Later Castle crossed some of the extracted rats of average grade (3.01)
+from the plus series to the same wild race and got F_2 hooded rats from
+this cross. These F_2 hooded rats did not further approach the ordinary
+range but were nearer the extreme selected plus hooded rats (3.33) than
+were the F_2's extracted from the first cross (2.59). Castle concludes from
+this that multiple factors can not account for the result. As a matter of
+fact, Castle's evidence _as published_ does not establish his conclusion
+because the wild rats used in the second experiment may have carried plus
+modifiers. This could only be determined by suitable tests which Castle
+does not furnish. This is the crucial point, without which the evidence
+carries no conviction.
+
+Furthermore, from Castle's point of view, these latest results would seem
+to increase the difficulty of interpretation of his first F_2 extracted
+cross, and it is now the first result that calls for explanation if one
+accepts his later conclusion.
+
+These and other objections that might be taken up show, I think, that
+Castle's experiment with hooded rats fails entirely to establish his
+contention of change in potency of the germ or of contamination of factors,
+while on the contrary they are in entire accord with the view that he is
+dealing with a case of modifying factors.
+
+[Illustration: FIG. 89. Races of Paramecium. (After Jennings.)]
+
+Equally important are the results that Jennings has obtained with certain
+protozoa. Paramecium multiplies by dividing across in the middle, each half
+replacing its lacking part. Both the small nucleus (micronucleus) and the
+large nucleus (macronucleus) divide at each division of the body. Jennings
+found that while individuals descended from a single paramecium vary in
+size (fig. 89), yet the population from a large individual is the same as
+the population derived from a small individual. In other words, selection
+produces no result and the probable explanation is, of course, that the
+different sizes of individuals are due to the environment, while the
+constancy of the type is genetic. Jennings found a number of races of
+paramecium of different sizes living under natural conditions. The largest
+individual of a small race might overlap the smallest individual of other
+larger races (fig. 89); nevertheless each kind reproduced its particular
+race. The results are like those of Johannsen in a general way, but differ
+in that reproduction takes place in paramecium by direct division instead
+of through self-fertilization as in beans, and also in that the paramecia
+were probably not homozygous. Since, however, so far as known no
+"reduction" takes place in paramecium at each division, the genetic
+composition of parent and offspring should be the same. Whether
+pseudo-parthenogenesis that Woodruff and Erdmann have found occurring in
+paramecium at intervals involves a redistribution of the hereditary factors
+is not clear. Jennings's evidence seems incompatible with such a view.
+
+[Illustration: FIG. 90. Stylonychia showing division into two. (After
+Stein.)]
+
+More recently one of Jennings's students, Middleton, has made a careful
+series of selection experiments with Stylonychia (fig. 90) in which he
+selected for lines showing more rapid or slower rates of division. His
+observations seem to show that his selection separated two such lines that
+came from the same original stock. The rapidity of the effects of selection
+seems to preclude the explanation that pseudo-parthenogenesis has
+complicated the results. Nevertheless, the results are of such a kind as to
+suggest that they were due to selection of vegetative (somatic) differences
+and that no genetic change of factors was involved, for his conclusion that
+the rapidity with which the effects gained by long selection might be
+suddenly reversed when selection was reversed is hardly consistent with an
+interpretation of the results based on changes in the "potencies" of the
+factors present.
+
+Equally striking are the interesting experiments that Jennings has recently
+carried out with Difflugia (fig. 91). This protozoon secretes a shell about
+itself which has a characteristic shape, and often carries spines. The
+opening at one end of the shell through which the protoplasm protrudes to
+make the pseudopodia is surrounded by a rim having a characteristic
+pattern. The protoplasm contains several nuclei and in addition there is
+scattered material or particles called chromidia that are supposed to be
+chromatic in nature and related to the material of the nuclei, possibly by
+direct interchange.
+
+[Illustration: FIG. 91. Difflugia Corona. (After Cash.)]
+
+When Difflugia divides, part of the protoplasm protrudes from the opening
+and a new shell is secreted about this mass which becomes a daughter
+individual. The behavior of the nucleus and of the chromidia at this time
+is obscure, but there is some evidence that their materials may be
+irregularly distributed between parent and offspring. If this is correct,
+and if in the protozoa the chromatin has the same influence that it seems
+to have in higher animals, the mode of reproduction in Difflugia would be
+expected to give little more than random sampling of the germ plasm.
+
+[Illustration: FIG. 92. Races of Difflugia. (After Leidy.)]
+
+Jennings was able by means of selection to get from the descendants of one
+original individual a number of different types that themselves bred true,
+except in so far as selection could affect another change in them. In this
+connection it is interesting to note that Leidy has published figures of
+Difflugia (fig. 92) that show that a great many "types" exist. If through
+sexual union (a process that occurs in Difflugia) the germ plasm
+(chromatin) of these wild types has in times past been recombined, then
+selection would be expected to separate certain types again, if, at
+division, irregular sampling of the germ plasm takes place. Until these
+points are settled the bearing of these important experiments of Jennings
+on the general problem of selection is uncertain.
+
+HOW DOES NATURAL SELECTION INFLUENCE THE COURSE OF EVOLUTION?
+
+The question still remains: Does selection play any rôle in evolution, and,
+if so, in what sense? Does the elimination of the unfit influence the
+course of evolution, except in the negative sense of leaving more room for
+the fit? There is something further to be said in this connection, although
+opinions may differ as to whether the following interpretation of the term
+"natural selection" is the only possible one.
+
+[Illustration: FIG. 93. Evolution of elephant's skulls. (After Dendy.)]
+
+If through a mutation a character appears that is neither advantageous nor
+disadvantageous, but indifferent, the chance that it may become established
+in the race is extremely small, although by good luck such a thing may
+occur rarely. It makes no difference whether the character in question is a
+dominant or a recessive one, the chance of its becoming established is
+exactly the same. If through a mutation a character appears that has an
+_injurious_ effect, however slight this may be, it has practically no
+chance of becoming established.
+
+[Illustration: FIG. 94. Evolution of elephant's trunk. (After Lull.)]
+
+If through a mutation a character appears that has a _beneficial_ influence
+on the individual, the chance that the individual will survive is
+increased, not only for itself, but for all of its descendants that come to
+inherit this character. It is this increase in the number of individuals
+possessing a particular character, that might have an influence on the
+course of evolution. This gives a better chance for improvement by several
+successive steps; but not because the species is more likely to mutate
+again in the same direction. An imaginary example will illustrate how this
+happens: When elephants had trunks less than a foot long, the chance of
+getting trunks more than one foot long was in proportion to the length of
+trunks already present and to the number of individuals; but increment in
+trunk length is no more likely to occur from an animal having a trunk more
+than one foot long than from an animal with a shorter trunk.
+
+The case is analogous to tossing pennies. At any stage in the game the
+chance of accumulating a hundred heads is in proportion to the number of
+heads already obtained, and to the number of throws still to be made. But
+the number of heads obtained has no influence on the number of heads that
+will appear in the next throw.
+
+[Illustration: FIG. 95. Evolution of elephant's trunk: above Maeritherium,
+in the middle Tetrabelodon (After Lancaster); below African elephants
+(After Gambier Bolton).]
+
+Owing then to this property of the germ plasm to duplicate itself in a
+large number of samples not only is an opportunity furnished to an
+advantageous variation to become extensively multiplied, but the presence
+of a large number of individuals of a given sort prejudices the probable
+future result.
+
+The question may be raised as to whether it is desirable to call selection
+a _creative_ process. There are so many supernatural and mystical
+implications that hang around the term creative that one can not be too
+careful in stating in what sense the term is to be used. If by creative is
+meant that something is made out of nothing, then of course there is no
+need for the scientist to try to answer such a question. But if by a
+creative process is meant that something is made out of something else,
+then there are two alternatives to be reckoned with.
+
+First, if it were true that selection of an individual of a certain kind
+determines that new variations in the same direction occur as a consequence
+of the selection, then selection would certainly be creative. How this
+could occur might be quite unintelligible, but of course it might be
+claimed that the point is not whether we can explain how creation takes
+place, but whether we can get verifiable evidence that such a kind of thing
+happens. This possibility is disposed of by the fact that there is no
+evidence that selection determines the direction in which variation occurs.
+
+Second, if you mean by a creative process that by picking out a certain
+kind of individual and multiplying its numbers a better chance is furnished
+that a certain end result will be obtained, such a process may be said to
+be creative. This is, I think, the proper use of the term creative in a
+mechanistic sense.
+
+CONCLUSIONS
+
+In reviewing the evidence relating to selection I have tried to handle the
+problem as objectively as I could.
+
+The evidence shows clearly that the characters of wild animals and plants,
+as well as those of domesticated races, are inherited both in the wild and
+in the domesticated forms according to Mendel's Law.
+
+The causes of the mutations that give rise to new characters we do not
+know, although we have no reason for supposing that they are due to other
+than natural processes.
+
+Evolution has taken place by the incorporation into the race of those
+mutations that are beneficial to the life and reproduction of the organism.
+Natural selection as here defined means both the increase in the number of
+individuals that results after a beneficial mutation has occurred (owing to
+the ability of living matter to propagate) and also that this preponderance
+of certain kinds of individuals in a population makes some further results
+more probable than others. More than this, natural selection can not mean,
+if factors are fixed and are not changed by selection.
+
+       *       *       *       *       *
+
+
+        INDEX
+
+  Abnormal abdomen 109
+  Abraxas 78-81
+  Allantois 17
+  Allelomorphs 83-84
+  Altenburg 112
+  Amnion 16-17
+  Andalusian fowl 45, 46
+  Annelids 22
+  Antlered wing 111
+  Apterous wing 11
+  Arc wing 111
+  Aristae 104
+
+  Bar eye 67, 108, 169
+  Bateson 18, 34, 36
+  Beaded wing 11, 115
+  Beans 147-149, 157
+  Belgian hare 171
+  Bent wing 116
+  Bergson 30, 31
+  Bildungstrieb 34
+  Biogenetic law 15, 18, 19, 21
+  Biometricians 156
+  Bird 21, 23
+  Bithorax 65, 112, 113
+  Black body color 111, 133
+  Blakeslee 152
+  Bridges 114, 143, 163
+  British Association 36
+  Brünn 40
+  Buff eye color 109
+  Bufon 27
+
+  Castle 176-180
+  Cat 33
+  Cell 90, 91
+  Chance variations 37
+  Chick 16, 17, 20
+  Chromatin 184
+  Chromosome group of Drosophila 102
+  Chromosomes 91, 95, 96, 98, 130, 131, 132
+  Cleavage 21, 22, 94
+  Clover butterfly 62
+  Club wing 69, 70, 108
+  Colias philodice 62
+  Color blindness 77, 125
+  Comb of Drosophila 103
+  Combs of fowls 33, 54
+  Comparative anatomy 7, 8, 9, 14
+  Corn 150, 153, 172
+  Correns 41
+  Cosmogonies 27
+  Cream eye color 163, 164
+  Crepidula 22
+  Criss-cross inheritance 78
+  Crossing over 131-133
+  Cuénot 178
+  Curled wing 115
+  Curved wing 111
+  Curve of probability 149
+  Cut wing 11, 104
+
+  Dachs legs 112
+  Dahlgren 62
+  Darwin 15, 24, 28, 32, 35-37, 64, 145, 146, 152, 154-156
+  Dendy 188
+  De Vries 18, 147, 156
+  Dexter 170
+  Dichaete 114
+  Difflugia 184-187
+  Discontinuous variation 13
+  Disuse 31
+  Drosophila ampelophila 10, 12, 13, 48-50, 60, 75, 84, 85, 93, 100, 103,
+      119, 155, 162, 169
+  Drosophila repleta 76
+  Duplication of legs 109
+  Dwarf 114
+
+  East 170, 172
+  Ebony 50, 55, 56, 115
+  Egg 91, 94
+  Elephant 191
+  Elephants' skulls 188
+  Elephants' trunks 190
+  Embryology 13-23
+  Emerson 172
+  Environment 27
+  Eosin eye color 61, 107, 163
+  Erdmann 183
+  Evolution Creatrice 30
+  Evolution--three kinds of 1, 2, 4
+  Eye color 13
+  Eyeless 66, 115
+
+  Factorial theory 89
+  Factors of Drosophila 143
+  Fantails 172, 175
+  Fertilization 91
+  Fish 16, 20, 21
+  Flatworms 22
+  Fluctuations 12
+  Forked bristles 106
+  Fowl 77
+  Fused veins 107, 108
+
+  Galton 154
+  Geneticist 26
+  Germ-plasm 142
+  Geoffroy St. Hilaire 27
+  Giant 114
+  Gill-slits 20, 21, 23
+  Groups I, II, III, IV 100-118
+
+  Haeckel 15
+  Haemophilia 77
+  Heliotropism 106, 107
+  Himalyan rabbits 83
+  History 1, 6
+  Hoge 66
+  Horse, evolution of 6
+
+  Indian corn 172, 173
+  Interference 137, 138
+
+  Janssens 132
+  Jaunty wing 111
+  Jennings 161, 181-184, 186
+  Johannsen 156, 157, 159-161, 166, 182
+
+  Lamarck 31-34
+  Langshan 77
+  Leaves 147
+  Leidy 186
+  Lethal 105
+  Linkage groups 103
+  Lizard 23
+  Localization of factors 118
+
+  MacDowell 155, 170, 171
+  Macritherium 191
+  Mammal 16, 21, 23
+  Man 20, 77, 125, 126
+  Map of Chromosomes 136
+  Maroon eye color 114
+  Mendel 40, 41, 52, 89
+  Mendelian heredity 39
+  Mendel's law 41-59, 64, 124
+  Mendel's second law 52
+  Mesenchyme cells 22
+  Mesoderm cells 22
+  Metaphysician 30
+  Mice 33, 178
+  Middleton 183
+  Miniature wing 108
+  Mirabilis 42
+  Modifiers 163, 164, 170, 171
+  Molluscs 22
+  Mouse 83
+  Muller 112, 167
+  Mutations 35, 39, 84
+
+  Nägeli 34, 35
+  Natural Selection 36, 145, 146, 187-194
+  Nisus formativus 34
+  Non-disjunction 139-142
+  Notch wing 104-106
+  Nucleus 91
+
+  Origin of Species 35, 145
+  Orthogenesis 34
+
+  Paleontology 24-27
+  Papilio polytes 63
+  Papilio turnus 63
+  Paramecium 181, 182
+  Paratettix 81
+  Peach eye color 114
+  Pea comb 54
+  Pearl 161
+  Peas 47
+  Pigeons 172, 174, 175
+  Pink eye color 114, 115
+  Planarian 22
+  Plymouth Rock 77
+  Podarke 22
+  Polar bodies 126
+  Pole arms 5
+  Protozoa 181
+  Pseudo-parthenogenesis 183
+  Purple eye color 109
+  Purpose 4
+
+  Rabbits 83, 170
+  Rats 176-180
+  Reduction division 182
+  Reproductive cells 96
+  Ruby eye color 106
+  Rudimentary organ 116
+  Rudimentary wing 70, 71, 107
+
+  Sable body color 107
+  Science definition of 6
+  Segregation 41
+  Selenka 94
+  Sepia eye color 13, 114
+  Sex chromosomes 118
+  Sex linked inheritance 75, 118-130
+  Sexual dimorphism 62
+  Sheep 33
+  Single comb 54
+  Sooty body color 50, 114, 115
+  Speck 68, 69, 111
+  Spencer 145
+  Spermatozoön 91, 98
+  Stars, evolution of 6
+  St. Hilaire 27-30
+  Strap wing 110, 111
+  Stumpy wing 11
+  Sturtevant 76, 143
+  Stylonychia 183
+  Survival of the fittest 146
+  Systematist 85
+
+  Tails 33
+  Tan flies 106, 107
+  Tetrabelodon 191
+  Trefoil 111
+  Truncate wing 111, 112, 167, 168
+
+  Unfolding principle 34
+  Unio 22
+  Unit character 74, 75
+  Use 31
+
+  Variation discontinuous 13
+  Vermilion eye color 108, 163
+  Vestigial wing 11, 55, 56, 109, 133
+  Vital force 34
+
+  Wallace 36
+  Walnut comb 54
+  Weismann 17, 31-33
+  Wilson, E. B. 125
+  Wingless 67
+  Winiwarter 126
+  White eye color 13, 75, 119-130
+  Whiting eye color 163, 164
+  Woodruff 183
+
+  Yellow body color 108, 133
+  Yolk sac 16, 17
+
+  Zeleny 169
+
+       *       *       *       *       *
+
+
+Corrections made to printed original.
+
+page 104, "shown in figures 53, 54, 55, 56": '52, 53, 54, 55' in original.
+
+
+
+
+
+
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+<pre>
+
+The Project Gutenberg EBook of A Critique of the Theory of Evolution, by 
+Thomas Hunt Morgan
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Critique of the Theory of Evolution
+
+Author: Thomas Hunt Morgan
+
+Release Date: December 17, 2009 [EBook #30701]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CRITIQUE OF THEORY OF EVOLUTION ***
+
+
+
+
+Produced by Bryan Ness, Keith Edkins, and the Online
+Distributed Proofreading Team at https://www.pgdp.net. Pages
+scanned by Bryan Ness.
+
+
+
+
+
+
+</pre>
+
+
+<table border="0" cellpadding="10" style="background-color: #ccccff;">
+<tr>
+<td style="width:25%; vertical-align:top">
+Transcriber's note:
+</td>
+<td>
+A few typographical errors have been corrected. They
+appear in the text <span class="correction" title="explanation will pop up">like this</span>, and the
+explanation will appear when the mouse pointer is moved over the marked
+passage.
+Figures 41 and 42 have been interchanged from the printed copy in order to match the text.
+</td>
+</tr>
+</table>
+
+<h3>Princeton University</h3>
+
+<h3>THE LOUIS CLARK VANUXEM FOUNDATION<br />
+LECTURES FOR 1915-1916</h3>
+
+  <p><br style="clear:both" /></p>
+<hr class="short" />
+
+<h2>The Louis Clark Vanuxem Foundation
+of Princeton University</h2>
+
+  <p>was established in 1912 with a bequest of $25,000 under the will of
+  Louis Clark Vanuxem, of the Class of 1879. By direction of the executors
+  of Mr. Vanuxem's estate, the income of the foundation is to be used for a
+  series of public lectures delivered in Princeton annually, at least one
+  half of which shall be on subjects of current scientific interest. The
+  lectures are to be published and distributed among schools and libraries
+  generally.</p>
+
+  <p>The following lectures have already been published or are in
+  press:</p>
+
+<blockquote class="b1n">
+
+  <p>1912-13 The Theory of Permutable Functions, by Vito Volterra</p>
+
+  <p>1913-14 Lectures delivered in connection with the dedication of the
+  Graduate College of Princeton University by Emile Boutroux, Alois Riehl,
+  A. D. Godley, and Arthur Shipley</p>
+
+  <p>1914-15 Romance, by Sir Walter Raleigh</p>
+
+  <p>1915-16 A Critique of the Theory of Evolution, by Thomas Hunt
+  Morgan</p>
+
+</blockquote>
+
+  <p><br style="clear:both" /></p>
+<hr class="short" />
+
+<h3>LOUIS CLARK VANUXEM FOUNDATION</h3>
+
+<h1>A CRITIQUE</h1>
+
+<p class="cenhead">OF THE</p>
+
+<h1>THEORY OF EVOLUTION</h1>
+
+<p class="cenhead">BY</p>
+
+<h2>THOMAS HUNT MORGAN</h2>
+
+<p class="cenhead">PROFESSOR OF EXPERIMENTAL ZOOLOGY IN<br />
+COLUMBIA UNIVERSITY</p>
+
+<p class="cenhead">LECTURES DELIVERED AT PRINCETON UNIVERSITY<br />
+FEBRUARY 24, MARCH 1, 8, 15, 1916</p>
+
+<p class="cenhead">PRINCETON UNIVERSITY PRESS<br />
+PRINCETON<br />
+LONDON: HUMPHREY MILFORD<br />
+OXFORD UNIVERSITY PRESS<br />
+1916</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="short" />
+
+<p class="cenhead">Copyright, 1916, by<br />
+<span class="sc">Princeton University Press</span><br />
+Published October, 1916</p>
+
+  <div class="figright" style="width:10%;">
+      <a href="images/copyright_page.png"><img style="width:100%" src="images/copyright_page.png"
+      alt="Publisher's Mark" title="Publisher's Mark" /></a>
+  </div>
+<div style="clear: both"></div>
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>{v}</span></p>
+
+<h3>PREFACE</h3>
+
+  <p>Occasionally one hears today the statement that we have come to
+  realize that we know nothing about evolution. This point of view is a
+  healthy reaction to the over-confident belief that we knew everything
+  about evolution. There are even those rash enough to think that in the
+  last few years we have learned more about evolution than we might have
+  hoped to know a few years ago. A <i>critique</i> therefore not only
+  becomes a criticism of the older evidence but an appreciation of the new
+  evidence.</p>
+
+  <p>In the first lecture an attempt is made to put a new valuation on the
+  traditional evidence for evolution. In the second lecture the most recent
+  work on heredity is dealt with, for only characters that are inherited
+  can become a part <!-- Page vi --><span class="pagenum"><a
+  name="pagevi"></a>{vi}</span>of the evolutionary process. In the third
+  lecture the physical basis of heredity and the composition of the germ
+  plasm stream are examined in the light of new observations; while in the
+  fourth lecture the thesis is developed that chance variation combined
+  with a property of living things to manifold themselves is the key note
+  of modern evolutionary thought.</p>
+
+  <p class="address"><span class="sc">T. H. Morgan</span></p>
+
+  <p class="author"><i>July, 1916</i></p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page vii --><span class="pagenum"><a name="pagevii"></a>{vii}</span></p>
+
+<h3>TABLE OF CONTENTS</h3>
+
+<table class="nobctr" summary="Contents." title="Contents." style="width:50%">
+
+<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER I</td></tr>
+
+<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> A REVALUATION OF THE EVIDENCE ON
+WHICH THE THEORY OF EVOLUTION
+WAS BASED</td></tr>
+
+<tr><td class="qspcsingle" colspan="3" style="vertical-align:top;"> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <span class="scac">PAGE</span></td></tr>
+
+<tr><td class="qspcsingle" colspan="3" style="vertical-align:top;"> <span class="sc">Preface</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#pagev">v</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 1. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Three Kinds of Evolution</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page1">1</a>-<a href="#page7">7</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 2. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Evidence for Organic Evolution</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page7">7</a>-<a href="#page27">27</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Comparative Anatomy </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page7">7</a>-<a href="#page14">14</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> b. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Embryology </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page14">14</a>-<a href="#page23">23</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Paleontology </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page24">24</a>-<a href="#page27">27</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 3. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Four Great Historical Speculations</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page27">27</a>-<a href="#page39">39</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> The Environment </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page27">27</a>-<a href="#page31">31</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Geoffroy St. Hilaire</td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> b. </td><td class="qspcsingle" style="vertical-align:top;"> Use and Disuse </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page31">31</a>-<a href="#page34">34</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> From Lamarck to Weismann</td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> The Unfolding Principle </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page34">34</a>-<a href="#page36">36</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Nägeli and Bateson</td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> d. </td><td class="qspcsingle" style="vertical-align:top;"> Natural Selection </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page36">36</a>-<a href="#page39">39</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Darwin</td></tr>
+
+<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;">
+<!-- Page viii --><span class="pagenum"><a name="pageviii"></a>{viii}</span>
+CHAPTER II</td></tr>
+
+<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> THE BEARING OF MENDEL'S DISCOVERY ON
+THE ORIGIN OF HEREDITY CHARACTERS</td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 1. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> Mendel's First Discovery&mdash;Segregation </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page41">41</a>-<a href="#page52">52</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 2. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> Mendel's Second Discovery&mdash;Independent Assortment</td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page52">52</a>-<a href="#page59">59</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 3. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> The Characters of Wild Animals and Plants
+Follow the Same Laws of Inheritance as do
+the Characters of Domesticated Animals and
+Plants </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page59">59</a>-<a href="#page84">84</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> Sexual Dimorphism </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page61">61</a>-<a href="#page64">64</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Eosin eye color of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page61">61</a>-<a href="#page62">62</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Color of the Clover Butterfly, Colias philodice</td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page62">62</a>-<a href="#page63">63</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Color of Papilio turnus </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page63">63</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Color pattern of Papilio polytes </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page63">63</a>-<a href="#page64">64</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> b. </td><td class="qspcsingle" style="vertical-align:top;"> Duplication of parts </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page65">65</a>-<a href="#page66">66</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Thorax of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page65">65</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Legs of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page65">65</a>-<a href="#page66">66</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> Loss of characters </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page66">66</a>-<a href="#page68">68</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> "Eyeless" of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page66">66</a>-<a href="#page67">67</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Vestigial wings of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page67">67</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Bar eye of Drosophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page67">67</a>-<a href="#page68">68</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> d. </td><td class="qspcsingle" style="vertical-align:top;"> Small changes of characters </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page68">68</a>-<a href="#page70">70</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> "Speck" </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page68">68</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> Bristles of "club" </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page70">70</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> e. </td><td class="qspcsingle" style="vertical-align:top;"> Manifold effects of same factor </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page71">71</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> f. </td><td class="qspcsingle" style="vertical-align:top;"> Constant but trivial effects may be the
+product of factors having other vital
+aspects </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page73">73</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;">
+<!-- Page ix --><span class="pagenum"><a name="pageix"></a>{ix}</span>
+g. </td><td class="qspcsingle" style="vertical-align:top;"> Sex-linked inheritance </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page75">75</a>-<a href="#page80">80</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in Drosophila ampelophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page75">75</a>-<a href="#page76">76</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in the wild species D. repleta </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page76">76</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in man </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page77">77</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in domesticated Fowls </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page77">77</a>-<a href="#page78">78</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in the wild moth, Abraxas </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page78">78</a>-<a href="#page80">80</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> h. </td><td class="qspcsingle" style="vertical-align:top;"> Multiple allelomorphs </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page81">81</a>-<a href="#page84">84</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in the wild Grouse Locust </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page81">81</a>-<a href="#page83">83</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in domesticated mice and rabbits </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page83">83</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> in Drosophila ampelophila </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page84">84</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 4. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Mutation and Evolution</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page84">84</a>-<a href="#page88">88</a></td></tr>
+
+<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER III</td></tr>
+
+<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> THE FACTORIAL THEORY OF HEREDITY
+AND THE COMPOSITION OF THE
+GERM PLASM</td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 1. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Cellular Basis of Organic Evolution and Heredity</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page89">89</a>-<a href="#page98">98</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 2. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Mechanism of Mendelian Heredity Discovered in the Behavior of the Chromosomes</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page98">98</a>-<a href="#page102">102</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 3. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Four Great Linkage Groups of Drosophila Ampelophila</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page103">103</a>-<a href="#page118">118</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> Group I. </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page104">104</a>-<a href="#page109">109</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> b. </td><td class="qspcsingle" style="vertical-align:top;"> Group II. </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page109">109</a>-<a href="#page112">112</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> Group III. </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page112">112</a>-<a href="#page115">115</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> d. </td><td class="qspcsingle" style="vertical-align:top;"> Group IV. </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page115">115</a>-<a href="#page118">118</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 4. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Localization of Factors in the Chromosomes</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page118">118</a>-<a href="#page142">142</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> a. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Sex Linked Inheritance </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page118">118</a>-<a href="#page137">137</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;">
+<!-- Page x --><span class="pagenum"><a name="pagex"></a>{x}</span>
+b. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Interference </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page137">137</a>-<a href="#page138">138</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> </td><td class="qspcsingle" style="vertical-align:top;"> c. </td><td class="qspcsingle" style="vertical-align:top;"> The Evidence from Non-Disjunction </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page139">139</a>-<a href="#page142">142</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 5. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">How Many Genetic Factors are there in the Germ-plasm of a Single Individual?</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page142">142</a>-<a href="#page143">143</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 6. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Conclusions </span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page144">144</a></td></tr>
+
+<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> CHAPTER IV</td></tr>
+
+<tr><td colspan="4" style="text-align:center; padding-top: 1.5ex; padding-bottom: 1.5ex;"> SELECTION AND EVOLUTION</td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 1. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">The Theory of Natural Selection</span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page145">145</a>-<a href="#page161">161</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 2. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">How has Selection in Domesticated Animals and Plants brought about its Results?</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page161">161</a>-<a href="#page165">165</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 3. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Are Factors Changed through Selection?</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page165">165</a>-<a href="#page187">187</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 4. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">How does Natural Selection Influence the course of Evolution?</span></td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page187">187</a>-<a href="#page193">193</a></td></tr>
+
+<tr><td class="qspcsingle" style="vertical-align:top;"> 5. </td><td class="qspcsingle" colspan="2" style="vertical-align:top;"> <span class="sc">Conclusions </span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page193">193</a>-<a href="#page194">194</a></td></tr>
+
+<tr><td class="qspcsingle" colspan="3" style="vertical-align:top;"> <span class="sc">Index </span> </td><td class="spacsingle" style="text-align:right; vertical-align:bottom;"> <a href="#page195">195</a>-<a href="#page197">197</a></td></tr>
+</table>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>{1}</span></p>
+
+<h3>CHAPTER I</h3>
+
+<p class="cenhead">A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED</p>
+
+  <p>We use the word evolution in many ways&mdash;to include many different
+  kinds of changes. There is hardly any other scientific term that is used
+  so carelessly&mdash;to imply so much, to mean so little.</p>
+
+<p class="cenhead"><span class="sc">Three Kinds of Evolution</span></p>
+
+  <p>We speak of the evolution of the stars, of the evolution of the horse,
+  of the evolution of the steam engine, as though they were all part of the
+  same process. What have they in common? Only this, that each concerns
+  itself with the <i>history</i> of something. When the astronomer thinks
+  of the <i>evolution</i> of the earth, the moon, the sun and the stars, he
+  has a picture of diffuse matter that has slowly condensed. With
+  condensation came heat; with heat, action and <!-- Page 2 --><span
+  class="pagenum"><a name="page2"></a>{2}</span>reaction within the mass
+  until the chemical substances that we know today were produced. This is
+  the nebular hypothesis of the astronomer. The astronomer explains, or
+  tries to explain, how this evolution took place, by an appeal to the
+  physical processes that have been worked out in the laboratory, processes
+  which he thinks have existed through all the eons during which this
+  evolution was going on and which were its immediate causes.</p>
+
+  <p>When the biologist thinks of the evolution of animals and plants, a
+  different picture presents itself. He thinks of series of animals that
+  have lived in the past, whose bones (fig. 1) and shells have been
+  preserved in the rocks. He thinks of these animals as having in the past
+  given birth, through an unbroken succession of individuals, to the living
+  inhabitants of the earth today. He thinks that the old, simpler types of
+  the past have in part changed over into the more complex forms of
+  today.</p>
+
+  <p>He is thinking as the historian thinks, but he sometimes gets confused
+  and thinks that he is explaining evolution when he is only describing it.
+  <!-- Page 3 --><span class="pagenum"><a name="page3"></a>{3}</span></p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_001.jpg"><img style="width:100%" src="images/fig_001.jpg"
+      alt="Fig. 1." title="Fig. 1." /></a>
+    <p class="poem"><span class="sc">Fig. 1.</span> A series of skulls and
+    feet. Eohippus, Mesohippus, Meryhippus, Hipparion and Equus. (American
+    Museum of Natural History. After Matthews.)</p>
+  </div>
+
+<p><!-- Page 4 --><span class="pagenum"><a name="page4"></a>{4}</span></p>
+
+  <p>A third kind of evolution is one for which man himself is responsible,
+  in the sense that he has brought it about, often with a definite end in
+  view.</p>
+
+  <p>His mind has worked slowly from stage to stage. We can often trace the
+  history of the stages through which his psychic processes have passed.
+  The evolution of the steam-boat, the steam engine, paintings, clothing,
+  instruments of agriculture, of manufacture, or of warfare (fig. 2)
+  illustrates the history of human progress. There is an obvious and
+  striking similarity between the evolution of man's inventions and the
+  evolution of the shells of molluscs and of the bones of mammals, yet in
+  neither case does a knowledge of the order in which these things arose
+  explain them. If we appeal to the psychologist he will probably tell us
+  that human inventions are either the result of happy accidents, that have
+  led to an unforeseen, but discovered use; or else the use of the
+  invention was foreseen. It is to the latter process more especially that
+  the idea of <i>purpose</i> is applied. When we come to review the four
+  great lines of evolutionary thought we <!-- Page 5 --><span
+  class="pagenum"><a name="page5"></a>{5}</span>shall see that this human
+  idea of purpose recurs in many forms, suggesting that man has often tried
+  to explain how organic evolution has taken place by an appeal to the
+  method which he believes he makes use of himself in the inorganic
+  world.</p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_002.jpg"><img style="width:100%" src="images/fig_002.jpg"
+      alt="Fig. 2." title="Fig. 2." /></a>
+    <p class="poem"><span class="sc">Fig. 2.</span> Evolution of pole arms.
+    (Metropolitan Museum. After Dean.)</p>
+  </div>
+
+<p><!-- Page 6 --><span class="pagenum"><a name="page6"></a>{6}</span></p>
+
+  <p>What has the evolution of the stars, of the horse and of human
+  inventions in common? Only this, that in each case from a simple
+  beginning through a series of changes something more complex, or at least
+  different, has come into being. To lump all these kinds of changes into
+  one and call them evolution is no more than asserting that you believe in
+  consecutive series of events (which is history) causally connected (which
+  is science); that is, that you believe in history and that you believe in
+  science. But let us not forget that we may have complete faith in both
+  without thereby offering any explanation of either. It is the business of
+  science to find out <i>specifically</i> what kinds of events were
+  involved when the stars evolved in the sky, when the horse evolved on the
+  earth, and the steam engine was evolved from the mind of man.</p>
+
+  <p>Is it not rather an empty generalization to say that any kind of
+  change is a process of evolution? At most it means little more than that
+  you want to intimate that miraculous <!-- Page 7 --><span
+  class="pagenum"><a name="page7"></a>{7}</span>intervention is not
+  necessary to account for such kinds of histories.</p>
+
+  <p>We are concerned here more particularly with the biologists' ideas of
+  evolution. My intention is to review the evidence on which the old theory
+  rested its case, in the light of some of the newer evidence of recent
+  years.</p>
+
+  <p>Four great branches of study have furnished the evidence of organic
+  evolution. They are:</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>Comparative anatomy.</p>
+      <p>Embryology.</p>
+      <p>Paleontology.</p>
+      <p>Experimental Breeding or Genetics.</p>
+    </div>
+  </div>
+
+<p class="cenhead"><i>The Evidence from Comparative Anatomy</i></p>
+
+  <p>When we study animals and plants we find that they can be arranged in
+  groups according to their resemblances. This is the basis of comparative
+  anatomy, which is only an accurate study of facts that are superficially
+  obvious to everyone.</p>
+
+  <p>The groups are based not on a single difference, but on a very large
+  number of resemblances. Let us take for example the group of vertebrates.
+  <!-- Page 8 --><span class="pagenum"><a name="page8"></a>{8}</span></p>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/fig_003.jpg"><img style="width:100%" src="images/fig_003.jpg"
+      alt="Fig. 3." title="Fig. 3." /></a>
+    <p class="poem"><span class="sc">Fig. 3.</span> Limb skeletons of
+    extinct and living animals, showing the homologous bones: 1,
+    salamander; 2, frog; 3, turtle; 4, Aetosaurus; 5, Pleisiosaurus; 6,
+    Ichthyosaurus; 7, Mesosaurus; 8, duck. (After Jordan and Kellogg.)</p>
+  </div>
+
+  <p>The hand and the arm of man are similar to the hand and arm of the
+  ape. We find the same plan in the forefoot of the rat, the elephant, the
+  horse and the opossum. We can identify the same parts in the forefoot of
+  the lizard, the frog (fig. 3), and even, though less certainly, in the
+  pectoral fins of fishes. Comparison does not end here. We find
+  similarities in the skull and back bones of these same animals; in the
+  brain; in the digestive system; in the heart and blood vessels; in the
+  muscles.</p>
+
+  <p>Each of these systems is very complex, but <!-- Page 9 --><span
+  class="pagenum"><a name="page9"></a>{9}</span>the same general
+  arrangement is found in all. Anyone familiar with the evidence will, I
+  think, probably reach the conclusion either that these animals have been
+  created on some preconceived plan, or else that they have some other bond
+  that unites them; for we find it difficult to believe that such complex,
+  yet similar things could have arisen independently. But we try to
+  convince our students of the truth of the theory of evolution not so much
+  by calling their attention to this relation as by tracing each organ from
+  a simple to a complex structure.</p>
+
+  <p>I have never known such a course to fail in its intention. In fact, I
+  know that the student often becomes so thoroughly convinced that he
+  resents any such attempt as that which I am about to make to point out
+  that the evidence for his conviction is not above criticism.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_004.jpg"><img style="width:100%" src="images/fig_004.jpg"
+      alt="Fig. 4." title="Fig. 4." /></a>
+    <p class="poem"><span class="sc">Fig. 4.</span> Drosophila ampelophila.
+    a, Female and b, male.</p>
+  </div>
+
+  <p>Because we can often arrange the series of structures in a line
+  extending from the very simple to the more complex, we are apt to become
+  unduly impressed by this fact and conclude that if we found the complete
+  series we should find all the intermediate steps and that they have
+  arisen in the order of their <!-- Page 10 --><span class="pagenum"><a
+  name="page10"></a>{10}</span>complexity. This conclusion is not
+  necessarily correct. Let me give some examples that have come under my
+  own observation. We have bred for five years the wild fruit fly
+  Drosophila ampelophila (fig. 4) and we have found over a hundred and
+  twenty-five new types that breed true. Each has arisen independently and
+  suddenly. Every part of the body has been affected by one or another of
+  these mutations. For instance many different kinds of changes have <!--
+  Page 11 --><span class="pagenum"><a name="page11"></a>{11}</span>taken
+  place in the wings and several of these involve the size of the wings. If
+  we arrange the latter arbitrarily in the order of their size there will
+  be an almost complete series beginning with the normal wings and ending
+  with those of apterous flies. Several of these types are represented in
+  figure 5. The order in which these mutations occurred bears no relation
+  to their size; each originated independently from the wild type.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_005.jpg"><img style="width:100%" src="images/fig_005.jpg"
+      alt="Fig. 5." title="Fig. 5." /></a>
+    <p class="poem"><span class="sc">Fig. 5.</span> Mutants of Drosophila
+    ampelophila arranged in order of size of wings: (a) cut; (b) beaded;
+    (c) stumpy; (d) another individual of stumpy; (f) vestigial (g)
+    apterous.</p>
+  </div>
+
+<p><!-- Page 12 --><span class="pagenum"><a name="page12"></a>{12}</span></p>
+
+  <p>The wings of the wild fly are straight (fig. 4). Several types have
+  arisen in which the wings are bent upwards and in the most extreme type
+  the wings are curled over the back, as seen in figure 54 (g), yet there
+  is no historical connection between these stages.</p>
+
+  <p>Mutations have occurred involving the pigmentation of the body and
+  wings. The head and thorax of the wild Drosophila ampelophila are grayish
+  yellow, the abdomen is banded with yellow and black, and the wings are
+  gray. There have appeared in our cultures several kinds of darker types
+  ranging to almost black flies (fig. 20) and to lighter types that are
+  quite yellow. If put in line a series may be made from the darkest flies
+  at one end to the light yellow flies at the other. These types, with the
+  fluctuations that occur within each type, furnish a complete series of
+  gradations; yet historically they have arisen independently of each
+  other.</p>
+
+  <p>Many changes in eye color have appeared. As many as thirty or more
+  races differing in eye <!-- Page 13 --><span class="pagenum"><a
+  name="page13"></a>{13}</span>color are now maintained in our cultures.
+  Some of them are so similar that they can scarcely be separated from each
+  other. It is easily possible beginning with the darkest eye color, sepia,
+  which is deep brown, to pick out a perfectly graded series ending with
+  pure white eyes. But such a serial arrangement would give a totally false
+  idea of the way the different types have arisen; and any conclusion based
+  on the existence of such a series might very well be entirely erroneous,
+  for the fact that such a series exists bears no relation to the order in
+  which its members have appeared.</p>
+
+  <p>Suppose that evolution "in the open" had taken place in the same way,
+  by means of <i>discontinuous</i> variation. What value then would the
+  evidence from comparative anatomy have in so far as it is based on a
+  continuous series of variants of any organ?</p>
+
+  <p>No one familiar with the entire evidence will doubt for a moment that
+  these 125 races of Drosophila ampelophila belong to the same species and
+  have had a common origin, for while they may differ mainly in one thing
+  they are extremely alike in a hundred other things, and <!-- Page 14
+  --><span class="pagenum"><a name="page14"></a>{14}</span>in the general
+  relation of the parts to each other.</p>
+
+  <p>It is in this sense that the evidence from comparative anatomy can be
+  used I think as an argument for evolution. It is the resemblances that
+  the animals or plants in any group have in common that is the basis for
+  such a conclusion; it is not because we can arrange in a continuous
+  series any particular variations. In other words, our inference
+  concerning the common descent of two or more species is based on the
+  totality of such resemblances that still remain in large part after each
+  change has taken place. In this sense the argument from comparative
+  anatomy, while not a demonstration, carries with it, I think, a high
+  degree of probability.</p>
+
+<p class="cenhead"><i>The Evidence from Embryology</i></p>
+
+  <p>In passing from the egg to the adult the individual goes through a
+  series of changes. In the course of this development we see not only the
+  beginnings of the organs that gradually enlarge and change into those of
+  the adult animal, but also see that organs appear and <!-- Page 15
+  --><span class="pagenum"><a name="page15"></a>{15}</span>later disappear
+  before the adult stage is reached. We find, moreover, that the young
+  sometimes resemble in a most striking way the adult stage of groups that
+  we place lower in the scale of evolution.</p>
+
+  <p>Many years before Darwin advanced his theory of evolution through
+  natural selection, the resemblance of the young of higher animals to the
+  adults of lower animals had attracted the attention of zoölogists and
+  various views, often very naïve, had been advanced to account for the
+  resemblance. Among these speculations there was one practically identical
+  with that adopted by Darwin and the post-Darwinians, namely that the
+  higher animals repeat in their development the <i>adult stages</i> of
+  lower animals. Later this view became one of the cornerstones of the
+  theory of organic evolution. It reached its climax in the writings of
+  Haeckel, and I think I may add without exaggeration that for twenty-five
+  years it furnished the chief inspiration of the school of descriptive
+  embryology. Today it is taught in practically all textbooks of biology.
+  Haeckel called this interpretation the Biogenetic Law. <!-- Page 16
+  --><span class="pagenum"><a name="page16"></a>{16}</span></p>
+
+  <div class="figcenter" style="width:27%;">
+      <a href="images/fig_006.jpg"><img style="width:100%" src="images/fig_006.jpg"
+      alt="Fig. 6." title="Fig. 6." /></a>
+    <p class="poem"><span class="sc">Fig. 6.</span> Young trout (Trutta
+    fario) six days after hatching. (After Ziegler.)</p>
+  </div>
+
+  <p>It was recognized, of course, that many embryonic stages could not
+  possibly represent ancestral animals. A young fish with a huge yolk sac
+  attached (fig. 6) could scarcely ever have led a happy, free life as an
+  adult individual. Such stages were interpreted, however, as
+  <i>embryonic</i> additions to the original ancestral type. The embryo had
+  done something on its own account.</p>
+
+  <p>In some animals the young have structures that attach them to the
+  mother, as does the placenta of the mammals. In other cases the young
+  develop membranes about themselves&mdash;like the amnion of the chick
+  (fig. 7) and mammal&mdash;that would have shut off an adult animal from
+  all intercourse with the outside <!-- Page 17 --><span class="pagenum"><a
+  name="page17"></a>{17}</span>world. Hundreds of such embryonic
+  adaptations are known to embryologists. These were explained as
+  adaptations and as falsifications of the ancestral records.</p>
+
+  <div class="figcenter" style="width:21%;">
+      <a href="images/fig_007.jpg"><img style="width:100%" src="images/fig_007.jpg"
+      alt="Fig. 7." title="Fig. 7." /></a>
+    <p class="poem"><span class="sc">Fig. 7.</span> Diagram of chick
+    showing relations of amnion, allantois and yolk. (After Lillie.)</p>
+  </div>
+
+  <p>At the end of the last century Weismann injected a new idea into our
+  views concerning the origin of variations. He urged that variations are
+  germinal, i.e. they first appear in the egg and the sperm as changes that
+  later bring about modifications in the individual. The idea has been
+  fruitful and is generally accepted by most biologists today. It means
+  that the <!-- Page 18 --><span class="pagenum"><a
+  name="page18"></a>{18}</span>offspring of a pair of animals are not
+  affected by the structure or the activities of their parents, but the
+  germ plasm is the unmodified stream from which both the parent and the
+  young have arisen. Hence their resemblance. Now, it has been found that a
+  variation arising in the germ plasm, no matter what its cause, may affect
+  any stage in the development of the next individuals that arise from it.
+  There is no reason to suppose that such a change produces a new character
+  that always sticks itself, as it were, on to the end of the old series.
+  This idea of germinal variation therefore carried with it the death of
+  the older conception of evolution by superposition.</p>
+
+  <p>In more recent times another idea has become current, mainly due to
+  the work of Bateson and of de Vries&mdash;the idea that variations are
+  discontinuous. Such a conception does not fall easily into line with the
+  statement of the biogenetic "law"; for actual experience with
+  discontinuous variation has taught us that new characters that arise do
+  not add themselves to the end of the line of already existing characters
+  but if they affect the adult characters <!-- Page 19 --><span
+  class="pagenum"><a name="page19"></a>{19}</span>they change them without,
+  as it were, passing through and beyond them.</p>
+
+<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
+
+  <div class="figright" style="width:59%;">
+      <a href="images/fig_008.jpg"><img style="width:100%" src="images/fig_008.jpg"
+      alt="Fig. 8." title="Fig. 8." /></a>
+    <p class="poem"><span class="sc">Fig. 8.</span> Diagram of head of
+    chick A and B, showing gill slits, and aortic arches; and head of fish
+    C showing aortic arches. (After Hesse.)</p>
+  </div>
+
+</td><td style="width:50%; vertical-align:top;">
+
+  <div class="figleft" style="width:59%;">
+      <a href="images/fig_009.jpg"><img style="width:100%" src="images/fig_009.jpg"
+      alt="Fig. 9." title="Fig. 9." /></a>
+    <p class="poem"><span class="sc">Fig. 9.</span> Human embryo showing
+    gill slits and aortic arches. (After His; from Marshall.)</p>
+  </div>
+
+</td></tr></table>
+
+  <p>I venture to think that these new ideas and this new evidence have
+  played havoc with the biogenetic "law". Nevertheless, there is an
+  interpretation of the facts that is entirely <!-- Page 20 --><span
+  class="pagenum"><a name="page20"></a>{20}</span>compatible with the
+  theory of evolution. Let me illustrate this by an example.</p>
+
+  <div class="figcenter" style="width:21%;">
+      <a href="images/fig_010.jpg"><img style="width:100%" src="images/fig_010.jpg"
+      alt="Fig. 10." title="Fig. 10." /></a>
+    <p class="poem"><span class="sc">Fig. 10.</span> Young fish, dorsal
+    view, and side view, showing gill slits. (After Kopsch.)</p>
+  </div>
+
+  <p>The embryos of the chick (fig. 8) and of man (fig. 9) possess at an
+  early stage in their development gill-slits on the sides of the neck like
+  those of fishes. No one familiar with the relations of the parts will for
+  a moment doubt that the gill slits of these embryos and of the fish
+  represent the same structures. When we look further into the matter we
+  find that young fish also possess gill slits (fig. 10 and 11)&mdash;even
+  in young stages in their development. Is it not <!-- Page 21 --><span
+  class="pagenum"><a name="page21"></a>{21}</span>then more probable that
+  the mammal and bird possess this stage in their development simply
+  because it has never been lost? Is not this a more reasonable view than
+  to suppose that the gill slits of the embryos of the higher forms
+  represent the adult gill slits of the fish that in some mysterious way
+  have been pushed back into the embryo of the bird?</p>
+
+  <div class="figcenter" style="width:24%;">
+      <a href="images/fig_011.jpg"><img style="width:100%" src="images/fig_011.jpg"
+      alt="Fig. 11." title="Fig. 11." /></a>
+    <p class="poem"><span class="sc">Fig. 11.</span> Side views of head of
+    embryo sharks, showing gill slits.</p>
+  </div>
+
+  <p>I could give many similar examples. All can be interpreted as
+  embryonic survivals rather than as phyletic contractions. Not one of them
+  calls for the latter interpretation.</p>
+
+  <p>The study of the cleavage pattern of the segmenting egg furnishes the
+  most convincing evidence that a different explanation from the one stated
+  in the biogenetic law is the more probable explanation. <!-- Page 22
+  --><span class="pagenum"><a name="page22"></a>{22}</span></p>
+
+  <div class="figcenter" style="width:36%;">
+      <a href="images/fig_012.jpg"><img style="width:100%" src="images/fig_012.jpg"
+      alt="Fig. 12." title="Fig. 12." /></a>
+    <p class="poem"><span class="sc">Fig. 12.</span> Cleavage stages of
+    four types of eggs, showing the origin of the mesenchyme cells
+    (stippled) and mesoderm cells (darker); a, Planarian; b, Annelid
+    (Podarke); c, Mollusc (Crepidula), d, Mollusc (Unio).</p>
+  </div>
+
+  <p>It has been found that the cleavage pattern has the same general
+  arrangement in the early stages of flat worms, annelids and molluscs
+  (fig. 12). Obviously these stages have never been adult ancestors, and
+  obviously if their resemblance has any meaning at all, it is that each
+  group has retained the same general plan <!-- Page 23 --><span
+  class="pagenum"><a name="page23"></a>{23}</span>of cleavage, possessed by
+  their common ancestor.</p>
+
+  <p>Accepting this view, let us ask, does the evidence from embryology
+  favor the theory of evolution? I think that it does very strongly. The
+  embryos of the mammal, bird, and lizard have gill slits today because
+  gill slits were present in the embryos of their ancestors. There is no
+  other view that explains so well their presence in the higher forms.</p>
+
+  <p>Perhaps someone will say, Well! is not this all that we have contended
+  for! Have you not reached the old conclusion in a roundabout way? I think
+  not. To my mind there is a wide difference between the old statement that
+  the higher animals living today have the original adult stages telescoped
+  into their embryos, and the statement that the resemblance between
+  certain characters in the embryos of higher animals and corresponding
+  stages in the embryos of lower animals is most plausibly explained by the
+  assumption that they have descended from the same ancestors, and that
+  their common structures are embryonic survivals. <!-- Page 24 --><span
+  class="pagenum"><a name="page24"></a>{24}</span></p>
+
+<p class="cenhead"><i>The Evidence from Paleontology</i></p>
+
+  <p>The direct evidence furnished by fossil remains is by all odds the
+  strongest evidence that we have in favor of organic evolution.
+  Paleontology holds the incomparable position of being able to point
+  directly to the evidence showing that the animals and plants living in
+  past times are connected with those living at the present time, often
+  through an unbroken series of stages. Paleontology has triumphed over the
+  weakness of the evidence, which Darwin admitted was serious, by filling
+  in many of the missing links.</p>
+
+  <p>Paleontology has been criticised on the ground that she cannot pretend
+  to show the actual ancestors of living forms because, if in the past
+  genera and species were as abundant and as diverse as we find them at
+  present, it is very improbable that the bones of any individual that
+  happened to be preserved are the bones of just that species that took
+  part in the evolution. Paleontologists will freely admit that in many
+  cases this is probably true, but even then the evidence is, I think,
+  still just as valuable and <!-- Page 25 --><span class="pagenum"><a
+  name="page25"></a>{25}</span>in exactly the same sense as is the evidence
+  from comparative anatomy. It suffices to know that there lived in the
+  past a particular "group" of animals that had many points in common with
+  those that preceded them and with those that came later. Whether these
+  are the actual ancestors or not does not so much matter, for the view
+  that from such a group of species the later species have been derived is
+  far more probable than any other view that has been proposed.</p>
+
+  <p>With this unrivalled material and splendid series of gradations,
+  paleontology has constructed many stages in the past history of the
+  globe. But paleontologists have sometimes gone beyond this descriptive
+  phase of the subject and have attempted to formulate the "causes", "laws"
+  and "principles" that have led to the development of their series. It has
+  even been claimed that paleontologists are in an incomparably better
+  position than zoölogists to discover such principles, because they know
+  both the beginning and the end of the evolutionary series. The retort is
+  obvious. In his sweeping and poetic vision the paleontologist may fail
+  completely to find out the nature of <!-- Page 26 --><span
+  class="pagenum"><a name="page26"></a>{26}</span>the pigments that have
+  gone into the painting of his picture, and he may confuse a familiarity
+  with the different views he has enjoyed of the canvas with a knowledge of
+  how the painting is being done.</p>
+
+  <p>My good friend the paleontologist is in greater danger than he
+  realizes, when he leaves descriptions and attempts explanation. He has no
+  way to check up his speculations and it is notorious that the human mind
+  without control has a bad habit of wandering.</p>
+
+  <p>When the modern student of variation and heredity&mdash;the
+  geneticist&mdash;looks over the different "continuous" series, from which
+  certain "laws" and "principles" have been deduced, he is struck by two
+  facts: that the gaps, in some cases, are enormous as compared with the
+  single changes with which he is familiar, and (what is more important)
+  that they involve numerous parts in many ways. The geneticist says to the
+  paleontologist, since you do not know, and from the nature of your case
+  can never know, whether your differences are due to one change or to a
+  thousand, you can not with certainty tell us anything about the
+  hereditary units <!-- Page 27 --><span class="pagenum"><a
+  name="page27"></a>{27}</span>which have made the process of evolution
+  possible. And without this knowledge there can be no understanding of the
+  causes of evolution.</p>
+
+<p class="cenhead">THE FOUR GREAT HISTORICAL SPECULATIONS</p>
+
+  <p>Looking backward over the history of the evolution theory we recognize
+  that during the hundred and odd years that have elapsed since Buffon,
+  there have been four main lines of <i>speculation</i> concerning
+  evolution. We might call them the four great cosmogonies or the four
+  modern epics of evolution.</p>
+
+<p class="cenhead"><span class="sc">The Environment</span></p>
+
+<p class="cenhead"><i>Geoffroy St. Hilaire</i></p>
+
+  <p>About the beginning of the last century Geoffroy St. Hilaire, protégé,
+  and in some respects a disciple of Buffon, was interested as to how
+  living species are related to the animals and plants that had preceded
+  them. He was familiar with the kind of change that takes place in the
+  embryo if it is put into new or changed surroundings, and from this
+  knowledge he concluded that as the surface of the <!-- Page 28 --><span
+  class="pagenum"><a name="page28"></a>{28}</span>earth slowly
+  changed&mdash;as the carbon dioxide contents in the air altered&mdash;as
+  land appeared&mdash;and as marine animals left the water to inhabit it,
+  they or their embryos responded to the new conditions and those that
+  responded favorably gave rise to new creations. As the environment
+  changed the fauna and flora changed&mdash;change for change. Here we have
+  a picture of progressive evolution that carries with it an idea of
+  mechanical necessity. If there is anything mystical or even improbable in
+  St. Hilaire's argument it does not appear on the surface; for he did not
+  assume that the response to the new environment was always a favorable
+  one or, as we say, an adaptation. He expressly stated that <i>if</i> the
+  response was unfavorable the individual or the race died out. He assumed
+  that <i>sometimes</i> the change might be favorable, i.e., that certain
+  species, entire groups, would respond in a direction favorable to their
+  existence in a new environment and these would come to inherit the earth.
+  In this sense he anticipated certain phases of the natural selection
+  theory of Darwin, but only in part; for his picture is not one of strife
+  within and without <!-- Page 29 --><span class="pagenum"><a
+  name="page29"></a>{29}</span>the species, but rather the escape of the
+  species from the old into a new world.</p>
+
+  <p>If then we recognize the intimate bond in chemical constitution of
+  living things and of the world in which they develop, what is there
+  improbable in St. Hilaire's hypothesis? Why, in a word is not more credit
+  given to St. Hilaire in modern evolutionary thought? The reasons are to
+  be found, I think, first, in that the evidence to which he appealed was
+  meagre and inconclusive; and, second, in that much of his special
+  evidence does not seem to us to be applicable. For example the monstrous
+  forms that development often assumes in a strange environment, and with
+  which every embryologist is only too familiar, rarely if ever furnish
+  combinations, as he supposed, that are capable of living. On the
+  contrary, they lead rather to the final catastrophe of the organism. And
+  lastly, St. Hilaire's appeal to sudden and great transformations, such as
+  a crocodile's egg hatching into a bird, has exposed his view to too easy
+  ridicule.</p>
+
+  <p>But when all is said, St. Hilaire's conception of evolution contains
+  elements that form the <!-- Page 30 --><span class="pagenum"><a
+  name="page30"></a>{30}</span>background of our thinking to-day, for taken
+  broadly, the interaction between the organism and its environment was a
+  mechanistic conception of evolution even though the details of the theory
+  were inadequate to establish his contention.</p>
+
+  <p>In our own time the French metaphysician Bergson in his <i>Evolution
+  Creatrice</i> has proposed in mystical form a thought that has at least a
+  superficial resemblance to St. Hilaire's conception. The response of
+  living things is no longer hit in one species and miss in another; it is
+  precise, exact; yet not mechanical in the sense at least in which we
+  usually employ the word mechanical. For Bergson claims that the one chief
+  feature of living material is that it responds favorably to the situation
+  in which it finds itself; at least so far as lies within the possible
+  physical limitations of its organization. Evolution has followed no
+  preordained plan; it has had no creator; it has brought about its own
+  creation by responding adaptively to each situation as it arose.</p>
+
+  <p>But note: the man of science believes that the organism responds today
+  as it does, because at <!-- Page 31 --><span class="pagenum"><a
+  name="page31"></a>{31}</span>present it has a chemical and physical
+  constitution that gives this response. We find a specific chemical
+  composition and generally a specific physical structure already existing.
+  We have no reason to suppose that such particular reactions would take
+  place until a specific chemical configuration had been acquired. Where
+  did this constitution come from? This is the question that the scientist
+  asks himself. I suppose Bergson would have to reply that it came into
+  existence at the moment that the first specific stimulus was applied. But
+  if this is the answer we have passed at once from the realm of
+  observation to the realm of fancy&mdash;to a realm that is foreign to our
+  experience; for such a view assumes that chemical and physical reactions
+  are guided by the needs of the organism when the reactions take place
+  inside living beings.</p>
+
+<p class="cenhead"><span class="sc">Use and Disuse</span></p>
+
+<p class="cenhead"><i>From Lamarck to Weismann</i></p>
+
+  <p>The second of the four great historical explanations appeals to a
+  change not immediately connected with the outer world, but to one within
+  the organism itself. <!-- Page 32 --><span class="pagenum"><a
+  name="page32"></a>{32}</span></p>
+
+  <p>Practice makes perfect is a familiar adage. Not only in human affairs
+  do we find that a part through use becomes a better tool for performing
+  its task, and through disuse degenerates; but in the field of animal
+  behavior we find that many of the most essential types of behavior have
+  been learned through repeated associations formed by contact with the
+  outside.</p>
+
+  <p>It was not so long ago that we were taught that the instincts of
+  animals are the inherited experience of their ancestors&mdash;lapsed
+  intelligence was the current phrase.</p>
+
+  <p>Lamarck's name is always associated with the application of the theory
+  of the inheritance of acquired characters. Darwin fully endorsed this
+  view and made use of it as an explanation in all of his writings about
+  animals. Today the theory has few followers amongst trained
+  investigators, but it still has a popular vogue that is widespread and
+  vociferous.</p>
+
+  <p>To Weismann more than to any other single individual should be
+  ascribed the disfavor into which this view has fallen. In a series of
+  brilliant essays he laid bare the inadequacy of the supposed evidence on
+  which the inheritance of <!-- Page 33 --><span class="pagenum"><a
+  name="page33"></a>{33}</span>acquired characters rested. Your neighbor's
+  cat, for instance, has a short tail, and it is said that it had its tail
+  pinched off by a closing door. In its litter of kittens one or more is
+  found without a tail. Your neighbor believes that here is a case of cause
+  and effect. He may even have known that the mother and grandmother of the
+  cat had natural tails. But it has been found that short tail is a
+  dominant character; therefore, until we know who was the father of the
+  short-tailed kittens the accident to its mother and the normal condition
+  of her maternal ancestry is not to the point.</p>
+
+  <p>Weismann appealed to common sense. He made few experiments to disprove
+  Lamarck's hypothesis. True, he cut off the tails of some mice for a few
+  generations but got no tailless offspring and while he gives no exact
+  measurements with coefficients of error he did not observe that the tails
+  of the descendants had shortened one whit. The combs of fighting cocks
+  and the tails of certain breeds of sheep have been cropped for many
+  generations and the practice continues today, because their tails are
+  still long. While in Lamarck's time there <!-- Page 34 --><span
+  class="pagenum"><a name="page34"></a>{34}</span>was no evidence opposed
+  to his ingenious theory, based as it was on an appeal to the acknowledged
+  facts of improvement that take place in the organs of an individual
+  through their own functioning (a fact that is as obvious and remarkable
+  today as in the time of Lamarck), yet now there is evidence as to whether
+  the effects of use and disuse are inherited, and this evidence is not in
+  accord with Lamarck's doctrine.</p>
+
+<p class="cenhead">THE UNFOLDING PRINCIPLE</p>
+
+<p class="cenhead"><i>Nägeli and Bateson</i></p>
+
+  <p>I have ventured to put down as one of the four great historical
+  explanations, under the heading of the unfolding principle, a conception
+  that has taken protean forms. At one extreme it is little more than a
+  mystic sentiment to the effect that evolution is the result of an inner
+  driving force or principle which goes under many names such as
+  Bildungstrieb, nisus formativus, vital force, and orthogenesis.
+  Evolutionary thought is replete with variants of this idea, often naïvely
+  expressed, sometimes unconsciously implied. Evolution once meant, in <!--
+  Page 35 --><span class="pagenum"><a name="page35"></a>{35}</span>fact, an
+  unfolding of what pre-existed in the egg, and the term still carries with
+  it something of its original significance.</p>
+
+  <p>Nägeli's speculation written several years after Darwin's "Origin of
+  Species" may be taken as a typical case. Nägeli thought that there exists
+  in living material an innate power to grow and expand. He vehemently
+  protested that he meant only a mechanical principle but as he failed to
+  refer such a principle to any properties of matter known to physicists
+  and chemists his view seems still a mysterious affirmation, as difficult
+  to understand as the facts themselves which it purports to explain.</p>
+
+  <p>Nägeli compared the process of evolution to the growth of a tree,
+  whose ultimate twigs represent the living world of species. Natural
+  selection plays only the rôle of the gardener who prunes the tree into
+  this or that shape but who has himself <i>produced</i> nothing. As an
+  imaginative figure of speech Nägeli's comparison of the tree might even
+  today seem to hold if we substituted "mutations" for "growth", but
+  although we know so little about what causes mutations there is no reason
+  for <!-- Page 36 --><span class="pagenum"><a
+  name="page36"></a>{36}</span>supposing them to be due to an inner
+  impulse, and hence they furnish no justification for such a
+  hypothesis.</p>
+
+  <p>In his recent presidential address before the British Association
+  Bateson has inverted this idea. I suspect that his effort was intended as
+  little more than a <i>tour de force</i>. He claims for it no more than
+  that it is a possible line of speculation. Perhaps he thought the time
+  had come to give a shock to our too confident views concerning evolution.
+  Be this as it may, he has invented a striking paradox. Evolution has
+  taken place through the steady loss of inhibiting factors. Living matter
+  was stopped down, so to speak, at the beginning of the world. As the
+  stops are lost, new things emerge. Living matter has changed only in that
+  it has become simpler.</p>
+
+<p class="cenhead"><span class="sc">Natural Selection</span></p>
+
+<p class="cenhead"><i>Darwin</i></p>
+
+  <p>Of the four great historical speculations about evolution, the
+  doctrine of Natural Selection of Darwin and Wallace has met with the most
+  widespread acceptance. In the last <!-- Page 37 --><span
+  class="pagenum"><a name="page37"></a>{37}</span>lecture I intend to
+  examine this theory critically. Here we are concerned only with its
+  broadest aspects.</p>
+
+  <p>Darwin appealed to <i>chance variations</i> as supplying evolution
+  with the material on which natural selection works. If we accept, for the
+  moment, this statement as the cardinal doctrine of natural selection it
+  may appear that evolution is due, (1) <i>not</i> to an <i>orderly</i>
+  response of the organism to its environment, (2) <i>not</i> in the main
+  to the activities of the animal through the use or disuse of its parts,
+  (3) <i>not</i> to any innate principle of living material itself, and (4)
+  above all <i>not</i> to purpose either from within or from without.
+  Darwin made quite clear what he meant by chance. By chance he did not
+  mean that the variations were not causal. On the contrary he taught that
+  in Science we mean by chance only that the particular combination of
+  causes that bring about a variation are not known. They are accidents, it
+  is true, but they are causal accidents.</p>
+
+  <p>In his famous book on "Animals and Plants under Domestication", Darwin
+  dwells at great length on the nature of the conditions that <!-- Page 38
+  --><span class="pagenum"><a name="page38"></a>{38}</span>bring about
+  variations. If his views seem to us today at times vague, at times
+  problematical, and often without a secure basis, nevertheless we find in
+  every instance, that Darwin was searching for the <i>physical causes of
+  variation</i>. He brought, in consequence, conviction to many minds that
+  there are abundant indications, even if certain proof is lacking, that
+  the causes of variation are to be found in natural processes.</p>
+
+  <p>Today the belief that evolution takes place by means of natural
+  processes is generally accepted. It does not seem probable that we shall
+  ever again have to renew the old contest between evolution and special
+  creation.</p>
+
+  <p>But this is not enough. We can never remain satisfied with a negative
+  conclusion of this kind. We must find out what natural causes bring about
+  variations in animals and plants; and we must also find out what kinds of
+  variations are inherited, and how they are inherited. If the
+  circumstantial evidence for organic evolution, furnished by comparative
+  anatomy, embryology and paleontology is cogent, we should be able to
+  observe evolution going on at <!-- Page 39 --><span class="pagenum"><a
+  name="page39"></a>{39}</span>the present time, i.e. we should be able to
+  observe the occurrence of variations and their transmission. This has
+  actually been done by the geneticist in the study of mutations and
+  Mendelian heredity, as the succeeding lectures will show.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 40 --><span class="pagenum"><a name="page40"></a>{40}</span></p>
+
+<h3>CHAPTER II</h3>
+
+<p class="cenhead">THE BEARING OF MENDEL'S DISCOVERY
+ON THE ORIGIN OF HEREDITARY
+CHARACTERS</p>
+
+  <p>Between the years 1857 and 1868 Gregor Mendel, Augustinian monk,
+  studied the heredity of certain characters of the common edible pea, in
+  the garden of the monastery at Brünn.</p>
+
+  <p>In his account of his work written in 1868, he said:</p>
+
+<blockquote class="b1n">
+
+  <p>"It requires indeed some courage to undertake a labor of such a
+  far-reaching extent; it appears, however, to be the only right way by
+  which we can finally reach the solution of a question the importance of
+  which cannot be over-estimated in connection with the history of the
+  evolution of organic forms."</p>
+
+</blockquote>
+
+  <p>He tells us also why he selected peas for his work:</p>
+
+<blockquote class="b1n">
+
+  <p>"The selection of the plant group which shall serve for experiments of
+  this kind must be made with all possible care if it be desired to avoid
+  from the outset every risk of questionable results."</p>
+
+  <p>"The experimental plants must necessarily <!-- Page 41 --><span
+  class="pagenum"><a name="page41"></a>{41}</span></p>
+
+  <p>1. Possess constant differentiating characters.</p>
+
+  <p>2. The hybrids of such plants must, during the flowering period, be
+  protected from the influence of all foreign pollen, or be easily capable
+  of such protection."</p>
+
+</blockquote>
+
+  <p>Why do biologists throughout the world to-day agree that Mendel's
+  discovery is one of first rank?</p>
+
+  <p>A great deal might be said in this connection. What is essential may
+  be said in a few words. Biology had been, and is still, largely a
+  descriptive and speculative science. <i>Mendel showed by experimental
+  proof that heredity could be explained by a simple mechanism. His
+  discovery has been exceedingly fruitful.</i></p>
+
+  <p>Science begins with naïve, often mystic conceptions of its problems.
+  It reaches its goal whenever it can replace its early guessing by
+  verifiable hypotheses and predictable results. This is what Mendel's law
+  did for heredity.</p>
+
+<p class="cenhead"><span class="sc">Mendel's First Discovery&mdash;Segregation</span></p>
+
+<p><!-- Page 42 --><span class="pagenum"><a name="page42"></a>{42}</span></p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_013.jpg"><img style="width:100%" src="images/fig_013.jpg"
+      alt="Fig. 13." title="Fig. 13." /></a>
+    <p class="poem"><span class="sc">Fig. 13.</span> Diagram illustrating a
+    cross between a red (dark) and a white variety of four o'clock
+    (Mirabilis jalapa).</p>
+  </div>
+
+  <p>Let us turn to the demonstration of his first law&mdash;the law of
+  segregation. The first case I choose is not the one given by Mendel but
+  one worked out later by Correns. If the common garden plant called four
+  o'clock (Mirabilis jalapa) with red flowers is crossed to one having
+  white flowers, the offspring are pink (fig. 13). The hybrid, then, is
+  intermediate in the color of its flowers between the two parents. If
+  these hybrids are inbred the offspring are white, pink and red, in the
+  proportion of 1:2:1. All of these had the same ancestry, yet they are of
+  three different kinds. If we did not know their <!-- Page 43 --><span
+  class="pagenum"><a name="page43"></a>{43}</span>history it would be quite
+  impossible to state what the ancestry of the white or of the red had
+  been, for they might just as well have come from pure white and pure red
+  ancestors respectively as to have emerged from the pink hybrids.
+  Moreover, when we test them we find that they are as pure as are white or
+  red flowering plants that have had all white or all red flowering
+  ancestors.</p>
+
+  <p>Mendel's Law explains the results of this cross as shown in figure
+  14.</p>
+
+  <p>The egg cell from the white parent carries the factor for white, the
+  pollen cell from the red parent carries the factor for red. The hybrid
+  formed by their union carries both factors. The result of their combined
+  action is to produce flowers intermediate in color.</p>
+
+  <p>When the hybrids mature and their germ cells (eggs or pollen) ripen,
+  each carries only one of these factors, either the red or the white, but
+  not both. In other words, the two factors that have been brought together
+  in the hybrid separate in its germ cells. Half of the egg cells are white
+  bearing, half red bearing. Half of the pollen cells are white bearing,
+  half red <!-- Page 44 --><span class="pagenum"><a
+  name="page44"></a>{44}</span>bearing. Chance combinations at
+  fertilization give the three classes of individuals of the second
+  generation.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_014.jpg"><img style="width:100%" src="images/fig_014.jpg"
+      alt="Fig. 14." title="Fig. 14." /></a>
+    <p class="poem"><span class="sc">Fig. 14.</span> Diagram illustrating
+    the history of the factors in the germ cells of the cross shown in Fig.
+    13.</p>
+  </div>
+
+  <p>The white flowering plants should forever breed true, as in fact they
+  do. The red flowering plants also breed true. The pink flowering plants,
+  having the same composition as the hybrids of the first generation,
+  should give the same kind of result. They do, indeed, give this result
+  i.e. one white to two pink to one red flowered offspring. <!-- Page 45
+  --><span class="pagenum"><a name="page45"></a>{45}</span></p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_015.jpg"><img style="width:100%" src="images/fig_015.jpg"
+      alt="Fig. 15." title="Fig. 15." /></a>
+    <p class="poem"><span class="sc">Fig. 15.</span> Diagram illustrating a
+    cross between special races of white and black fowls, producing the
+    blue (here gray) Andalusian.</p>
+  </div>
+
+  <p>Another case of the same kind is known to breeders of poultry. One of
+  the most beautiful of the domesticated breeds is known as the Andalusian.
+  It is a slate blue bird shading into blue-black on the neck and back.
+  Breeders know that these blue birds do not breed true but produce white,
+  black, and blue offspring. <!-- Page 46 --><span class="pagenum"><a
+  name="page46"></a>{46}</span></p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_016.jpg"><img style="width:100%" src="images/fig_016.jpg"
+      alt="Fig. 16." title="Fig. 16." /></a>
+    <p class="poem"><span class="sc">Fig. 16.</span> Diagram showing
+    history of germ cells of cross of Fig. 15. The larger circles indicate
+    the color of the birds; their enclosed small circles the nature of the
+    factors in the germ cells of such birds.</p>
+  </div>
+
+  <p>The explanation of the failure to produce a pure race of Andalusians
+  is that they are like the pink flowers of the four o'clock, i.e., they
+  are a hybrid type formed by the meeting of the white and the black germ
+  cells. If the whites produced by the Andalusians are bred to the blacks
+  (both being pure strains), all the offspring will be blue (fig. 15); if
+  these blues are inbred they will give 1 white, to 2 blues, to 1 <!-- Page
+  47 --><span class="pagenum"><a name="page47"></a>{47}</span>black. In
+  other words, the factor for white and the factor for black separate in
+  the germ cells of the hybrid Andalusian birds (fig. 16).</p>
+
+  <div class="figcenter" style="width:20%;">
+      <a href="images/fig_017.jpg"><img style="width:100%" src="images/fig_017.jpg"
+      alt="Fig. 17." title="Fig. 17." /></a>
+    <p class="poem"><span class="sc">Fig. 17.</span> Diagram of Mendel's
+    cross between yellow (dominant) and green (recessive) peas.</p>
+  </div>
+
+  <p>The third case is Mendel's classical case of yellow and green peas
+  (fig. 17). He crossed a plant belonging to a race having yellow peas with
+  one having green peas. The hybrid plants had yellow seeds. These hybrids
+  inbred gave three yellows to one green. The explanation <!-- Page 48
+  --><span class="pagenum"><a name="page48"></a>{48}</span>(fig. 18) is the
+  same in principle as in the preceding cases. The only difference between
+  them is that the hybrid which contains both the yellow and the green
+  factors is in appearance not intermediate, but like the yellow parent
+  stock. Yellow is said therefore to be dominant and green to be
+  recessive.</p>
+
+  <div class="figcenter" style="width:32%;">
+      <a href="images/fig_018.jpg"><img style="width:100%" src="images/fig_018.jpg"
+      alt="Fig. 18." title="Fig. 18." /></a>
+    <p class="poem"><span class="sc">Fig. 18.</span> Diagram illustrating
+    the history of the factors in the cross shown in Fig. 17.</p>
+  </div>
+
+  <p>Another example where one of the contrasted characters is dominant is
+  shown by the cross of Drosophila with vestigial wings to the wild type
+  with long wings (fig. 19). The F<sub>1</sub> flies have long wings not
+  differing from those of the wild fly, so far as can be observed. When two
+  such flies are inbred there result three long to one vestigial. <!-- Page
+  49 --><span class="pagenum"><a name="page49"></a>{49}</span></p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_019.jpg"><img style="width:100%" src="images/fig_019.jpg"
+      alt="Fig. 19." title="Fig. 19." /></a>
+    <p class="poem"><span class="sc">Fig. 19.</span> Diagram illustrating a
+    cross between a fly (Drosophila ampelophila) with long wings and a
+    mutant fly with vestigial wings.</p>
+  </div>
+
+<p><!-- Page 50 --><span class="pagenum"><a name="page50"></a>{50}</span></p>
+
+  <p>The question as to whether a given character is dominant or recessive
+  is a matter of no theoretical importance for the principle of
+  segregation, although from the notoriety given to it one might easily be
+  misled into the erroneous supposition that it was the discovery of this
+  relation that is Mendel's crowning achievement.</p>
+
+  <p>Let me illustrate by an example in which the hybrid standing between
+  two types overlaps them both. There are two mutant races in our cultures
+  of the fruit fly Drosophila that have dark body color, one called sooty,
+  another which is even blacker, called ebony (fig. 20). Sooty crossed to
+  ebony gives offspring that are intermediate in color. Some of them are so
+  much like sooty that they cannot be distinguished from sooty. At the
+  other extreme some of the hybrids are as dark as the lightest of the
+  ebony flies. If these hybrids are inbred there is a continuous series of
+  individuals, sooties, intermediates and ebonies. Which color here shall
+  we call the dominant? If the ebony, then in the second generation we
+  count three ebonies to one sooty, putting the hybrids with the ebonies.
+  If the dominant is the sooty then we count three <!-- Page 51 --><span
+  class="pagenum"><a name="page51"></a>{51}</span>sooties to one ebony,
+  putting the hybrids with the sooties. The important fact to find out is
+  whether there actually exist three classes in the second generation. This
+  can be ascertained even when, as in this case, there is a perfectly
+  graded series from one end to the other, by testing out individually
+  enough of the flies to show that one-fourth of them never produce any
+  descendants but ebonies, one-fourth never any but sooties, and one-half
+  of them give rise to both ebony and sooty.</p>
+
+  <div class="figcenter" style="width:35%;">
+      <a href="images/fig_020.jpg"><img style="width:100%" src="images/fig_020.jpg"
+      alt="Fig. 20." title="Fig. 20." /></a>
+    <p class="poem"><span class="sc">Fig. 20.</span> Cross between two
+    allelomorphic races of Drosophila, sooty and ebony, that give a
+    completely graded series in F<sub>2</sub>.</p>
+  </div>
+
+<p><!-- Page 52 --><span class="pagenum"><a name="page52"></a>{52}</span></p>
+
+<p class="cenhead"><span class="sc">Mendel's Second Discovery&mdash;Independent Assortment</span></p>
+
+  <p>Besides his discovery that there are pairs of characters that disjoin,
+  as it were, in the germ cells of the hybrid (law of segregation) Mendel
+  made a second discovery which also has far-reaching consequences. The
+  following case illustrates Mendel's second law.</p>
+
+  <p>If a pea that is yellow and round is crossed to one that is green and
+  wrinkled (fig. 21), all of the offspring are yellow and round. Inbred,
+  these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green
+  wrinkled. All the yellows taken together are to the green as 3:1. All the
+  round taken together are to the wrinkled as three to one; but some of the
+  yellows are now wrinkled and some of the green are now <!-- Page 53
+  --><span class="pagenum"><a name="page53"></a>{53}</span>round. There has
+  been a recombination of characters, while at the same time the results,
+  for each pair of characters taken separately, are in accord with Mendel's
+  Law of Segregation, (fig. 22). The second law of Mendel may be called the
+  law of independent assortment of different character pairs.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_021.jpg"><img style="width:100%" src="images/fig_021.jpg"
+      alt="Fig. 21." title="Fig. 21." /></a>
+    <p class="poem"><span class="sc">Fig. 21.</span> Cross between
+    yellow-round and green-wrinkled peas, giving the 9: 3: 3: 1 ratio in
+    F<sub>2</sub>.</p>
+  </div>
+
+  <p>We can, as it were, take the characters of one organism and recombine
+  them with those <!-- Page 54 --><span class="pagenum"><a
+  name="page54"></a>{54}</span>of a different organism. We can explain this
+  result as due to the assortment of factors for these characters in the
+  germ cells according to a definite law.</p>
+
+  <div class="figcenter" style="width:26%;">
+      <a href="images/fig_022.jpg"><img style="width:100%" src="images/fig_022.jpg"
+      alt="Fig. 22." title="Fig. 22." /></a>
+    <p class="poem"><span class="sc">Fig. 22.</span> Diagram to show the
+    history of the factor pairs yellow-green and round-wrinkled of the
+    cross in Fig. 21.</p>
+  </div>
+
+  <p>As a second illustration let me take the <!-- Page 55 --><span
+  class="pagenum"><a name="page55"></a>{55}</span>classic case of the combs
+  of fowls. If a bird with a rose comb is bred to one with a pea comb (fig.
+  23), the offspring have a comb different from either. It is called a
+  walnut comb. If two such individuals are bred they give 9 walnut, 3 rose,
+  3 pea, 1 single. This proportion shows that the grandparental types
+  differed in respect to two pairs of characters.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_023.jpg"><img style="width:100%" src="images/fig_023.jpg"
+      alt="Fig. 23." title="Fig. 23." /></a>
+    <p class="poem"><span class="sc">Fig. 23.</span> Cross between pea and
+    rose combed fowls. (Charts of Baur and Goldschmidt.)</p>
+  </div>
+
+  <p>A fourth case is shown in the fruit fly, where an ebony fly with long
+  wings is mated to a grey fly with vestigial wings (fig. 24). The <!--
+  Page 56 --><span class="pagenum"><a
+  name="page56"></a>{56}</span>offspring are gray with long wings. If these
+  are inbred they give 9 gray long, 3 gray vestigial, 3 ebony long, 1 ebony
+  vestigial (figs. 24 and 25).</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_024.jpg"><img style="width:100%" src="images/fig_024.jpg"
+      alt="Fig. 24." title="Fig. 24." /></a>
+    <p class="poem"><span class="sc">Fig. 24.</span> Cross between long
+    ebony and gray vestigial flies.</p>
+  </div>
+
+<p><!-- Page 57 --><span class="pagenum"><a name="page57"></a>{57}</span></p>
+
+  <p>The possibility of interchanging characters might be illustrated over
+  and over again. It is true not only when two pairs of characters are
+  involved, but when three, four, or more enter the cross.</p>
+
+  <div class="figcenter" style="width:32%;">
+      <a href="images/fig_025.jpg"><img style="width:100%" src="images/fig_025.jpg"
+      alt="Fig. 25." title="Fig. 25." /></a>
+    <p class="poem"><span class="sc">Fig. 25.</span> Diagram to show the
+    history of the factors in the cross shown in Fig. 24.</p>
+  </div>
+
+  <p>It is as though we took individuals apart and put together parts of
+  two, three or more individuals by substituting one part for another. <!--
+  Page 58 --><span class="pagenum"><a name="page58"></a>{58}</span></p>
+
+  <p>Not only has this power to make whatever combinations we choose great
+  practical importance, it has even greater theoretical significance; for,
+  it follows that the individual is not in itself the unit in heredity, but
+  that within the germ-cells there exist smaller units concerned with the
+  transmission of characters.</p>
+
+  <p>The older mystical statement of the individual as a unit in heredity
+  has no longer any interest in the light of these discoveries, except as a
+  past phase of biological history. We see, too, more clearly that the
+  sorting out of factors in the germ plasm is a very different process from
+  the influence of these factors on the development of the organism. There
+  is today no excuse for confusing these two problems.</p>
+
+  <p>If mechanistic principles apply also to embryonic development then the
+  course of development is capable of being stated as a series of
+  chemico-physical reactions and the "<i>individual</i>" is merely a term
+  to express the sum total of such reactions and should not be interpreted
+  as something different from or more than these reactions. So long as so
+  little is known of the actual processes involved in <!-- Page 59 --><span
+  class="pagenum"><a name="page59"></a>{59}</span>development the use of
+  the term "individuality", while giving the appearance of profundity, in
+  reality often serves merely to cover ignorance and to make a mystery out
+  of a mechanism.</p>
+
+<p class="cenhead"><span class="sc">The Characters of Wild Animals and Plants Follow the Same Laws of Inheritance as do the Characters of Domesticated Animals and Plants.</span></p>
+
+  <p>Darwin based many of his conclusions concerning variation and heredity
+  on the evidence derived from the garden and from the stock farm. Here he
+  was handicapped to some extent, for he had at times to rely on
+  information much of which was uncritical, and some of which was
+  worthless.</p>
+
+  <p>Today we are at least better informed on <i>two</i> important points;
+  one concerning the <i>kinds</i> of variations that furnish to the
+  cultivator the materials for his selection; the other concerning the
+  modes of inheritance of these variations. We know now that new characters
+  are continually appearing in domesticated as well as in wild animals and
+  plants, that these characters are often sharply marked <!-- Page 60
+  --><span class="pagenum"><a name="page60"></a>{60}</span>off from the
+  original characters, and whether the differences are great or whether
+  they are small they are transmitted alike according to Mendel's law.</p>
+
+  <p>Many of the characteristics of our domesticated animals and cultivated
+  plants originated long ago, and only here and there have the records of
+  their first appearance been preserved. In only a few instances are these
+  records clear and definite, while the complete history of any large group
+  of our domesticated products is unknown to us.</p>
+
+  <p>Within the last five or six years, however, from a common wild species
+  of fly, the fruit fly, Drosophila ampelophila, which we have brought into
+  the laboratory, have arisen over a hundred and twenty-five new types
+  whose origin is completely known. Let me call attention to a few of the
+  more interesting of these types and their modes of inheritance, comparing
+  them with wild types in order to show that the kinds of inheritance found
+  in domesticated races occur also in wild types. The results will show
+  beyond dispute that the characters of wild types are inherited in
+  precisely <!-- Page 61 --><span class="pagenum"><a
+  name="page61"></a>{61}</span>the same way as are the characters of the
+  mutant types&mdash;a fact that is not generally appreciated except by
+  students of genetics, although it is of the most far-reaching
+  significance for the theory of evolution.</p>
+
+  <p>A mutant appeared in which the eye color of the female was different
+  from that of the male. The eye color of the mutant female is a dark eosin
+  color, that of the male yellowish eosin. From the beginning this
+  difference was as marked as it is to-day. Breeding experiments show that
+  eosin eye color differs from the red color of the eye of the wild fly by
+  a single mutant factor. Here then at a single step a type appeared that
+  was sexually dimorphic.</p>
+
+  <p>Zoölogists know that sexual dimorphism is not uncommon in wild species
+  of animals, and Darwin proposed the theory of sexual selection to account
+  for the difference between the sexes. He assumed that the male preferred
+  certain kinds of females differing from himself in a particular
+  character, and thus in time through sexual selection, the sexes came to
+  differ from each other. <!-- Page 62 --><span class="pagenum"><a
+  name="page62"></a>{62}</span></p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_026.jpg"><img style="width:100%" src="images/fig_026.jpg"
+      alt="Fig. 26." title="Fig. 26." /></a>
+    <p class="poem"><span class="sc">Fig. 26.</span> Clover butterfly
+    (Colias philodice) with two types of females, above; and one type of
+    male, below.</p>
+  </div>
+
+  <p>In the case of eosin eye color no such process as that postulated by
+  Darwin to account for the differences between the sexes was involved; for
+  the single mutation that brought about the change also brought in the
+  dimorphism with it.</p>
+
+  <p>In recent years zoölogists have carefully studied several cases in
+  which two types of female are found in the same species. In the common
+  clover butterfly, there is a yellow and a white type of female, while the
+  male is yellow (fig. 26). It has been shown that a single factor
+  difference determines whether the female <!-- Page 63 --><span
+  class="pagenum"><a name="page63"></a>{63}</span>is yellow or white. The
+  inheritance is, according to Gerould, strictly Mendelian.</p>
+
+  <div class="figcenter" style="width:26%;">
+      <a href="images/fig_027.jpg"><img style="width:100%" src="images/fig_027.jpg"
+      alt="Fig. 27." title="Fig. 27." /></a>
+    <p class="poem"><span class="sc">Fig. 27.</span> Papilio turnus with
+    two types of females above and one type of male below.</p>
+  </div>
+
+  <p>In Papilio turnus there exist, in the southern states, two kinds of
+  females, one yellow like the male, one black (fig. 27). The evidence here
+  is not so certain, but it seems probable that a single factor difference
+  determines whether the female shall be yellow or black.</p>
+
+  <p>Finally in Papilio polytes of Ceylon and India three different types
+  of females appear, <!-- Page 64 --><span class="pagenum"><a
+  name="page64"></a>{64}</span>(fig. 28 to right) only one of which is like
+  the male. Here the analysis of the breeding data shows the possibility of
+  explaining this case as due to two pairs Mendelian factors which give in
+  combination the three types of female.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_028.jpg"><img style="width:100%" src="images/fig_028.jpg"
+      alt="Fig. 28." title="Fig. 28." /></a>
+    <p class="poem"><span class="sc">Fig. 28.</span> Papilio polytes, with
+    three types of female to right and one type of male above to left.</p>
+  </div>
+
+  <p>Taking these cases together, they furnish a much simpler explanation
+  than the one proposed by Darwin. They show also that characters like
+  these shown by wild species may follow Mendel's law. <!-- Page 65
+  --><span class="pagenum"><a name="page65"></a>{65}</span></p>
+
+  <div class="figcenter" style="width:25%;">
+      <a href="images/fig_029.jpg"><img style="width:100%" src="images/fig_029.jpg"
+      alt="Fig. 29." title="Fig. 29." /></a>
+    <p class="poem"><span class="sc">Fig. 29.</span> Mutant race of fruit
+    fly with intercalated duplicate mesothorax on dorsal side.</p>
+  </div>
+
+  <p>There has appeared in our cultures a fly in which the third division
+  of the thorax with its appendages has changed into a segment like the
+  second (fig. 29). It is smaller than the normal mesothorax and its wings
+  are imperfectly developed, but the bristles on the upper surface may have
+  the typical arrangement of the normal mesothorax. The mutant shows how
+  great a change may result from a single factor difference.</p>
+
+  <p>A factor that causes duplication in the legs <!-- Page 66 --><span
+  class="pagenum"><a name="page66"></a>{66}</span>has also been found. Here
+  the interesting fact was discovered (Hoge) that duplication takes place
+  only in the cold. At ordinary temperatures the legs are normal.</p>
+
+  <div class="figcenter" style="width:21%;">
+      <a href="images/fig_030.jpg"><img style="width:100%" src="images/fig_030.jpg"
+      alt="Fig. 30." title="Fig. 30." /></a>
+    <p class="poem"><span class="sc">Fig. 30.</span> Mutant race of fruit
+    fly, called eyeless; a, a' normal eye.</p>
+  </div>
+
+  <p>In contrast to the last case, where a character is doubled, is the
+  next one in which the eyes are lost (fig. 30). This change also took
+  place at a single step. All the flies of this stock however, cannot be
+  said to be eyeless, since many of them show pieces of the
+  eye&mdash;indeed the variation is so wide that the eye may even appear
+  like a normal eye unless carefully <!-- Page 67 --><span
+  class="pagenum"><a name="page67"></a>{67}</span>examined. Formerly we
+  were taught that eyeless animals arose in caves. This case shows that
+  they may also arise suddenly in glass milk bottles, by a change in a
+  single factor.</p>
+
+  <p>I may recall in this connection that wingless flies (fig. 5 f) also
+  arose in our cultures by a single mutation. We used to be told that
+  wingless insects occurred on desert islands because those insects that
+  had the best developed wings had been blown out to sea. Whether this is
+  true or not, I will not pretend to say, but at any rate wingless insects
+  may also arise, not through a slow process of elimination, but at a
+  single step.</p>
+
+  <p>The preceding examples have all related to recessive characters. The
+  next one is dominant.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_031.jpg"><img style="width:100%" src="images/fig_031.jpg"
+      alt="Fig. 31." title="Fig. 31." /></a>
+    <p class="poem"><span class="sc">Fig. 31.</span> Mutant race of fruit
+    fly called bar to the right (normal to the left). The eye is a narrow
+    vertical bar, the outline of the original eye is indicated.</p>
+  </div>
+
+<p><!-- Page 68 --><span class="pagenum"><a name="page68"></a>{68}</span></p>
+
+  <p>A single male appeared with a narrow vertical red bar (fig. 31)
+  instead of the broad red oval eye. Bred to wild females the new character
+  was found to dominate, at least to the extent that the eyes of all its
+  offspring were narrower than the normal eye, although not so narrow as
+  the eye of the pure stock. Around the bar there is a wide border that
+  corresponds to the region occupied by the rest of the eye of the wild
+  fly. It lacks however the elements of the eye. It is therefore to be
+  looked upon as a rudimentary organ, which is, so to speak, a by-product
+  of the dominant mutation.</p>
+
+  <p>The preceding cases have all involved rather great changes in some one
+  organ of the body. The following three cases involve slight changes, and
+  yet follow the same laws of inheritance as do the larger changes.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_032.jpg"><img style="width:100%" src="images/fig_032.jpg"
+      alt="Fig. 32." title="Fig. 32." /></a>
+    <p class="poem"><span class="sc">Fig. 32.</span> Mutant race of fruit
+    fly, called speck. There is a minute black speck at base of wing.</p>
+  </div>
+
+<p><!-- Page 69 --><span class="pagenum"><a name="page69"></a>{69}</span></p>
+
+  <p>At the base of the wings a minute black speck appeared (fig. 32). It
+  was found to be a Mendelian character. In another case the spines on the
+  thorax became forked or kinky (fig. 52b). This stock breeds true, and the
+  character is inherited in strictly Mendelian fashion.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_033.jpg"><img style="width:100%" src="images/fig_033.jpg"
+      alt="Fig. 33." title="Fig. 33." /></a>
+    <p class="poem"><span class="sc">Fig. 33.</span> Mutant race of fruit
+    fly called club. The wings often remain unexpanded and two bristles
+    present in wild fly (b) are absent on side of thorax (c).</p>
+  </div>
+
+  <p>In a certain stock a number of flies appeared <!-- Page 70 --><span
+  class="pagenum"><a name="page70"></a>{70}</span>in which the wing pads
+  did not expand (fig. 33). It was found that this peculiarity is shown in
+  only about twenty per cent of the individuals supposed to inherit it.
+  Later it was found that this stock lacked two bristles on the sides of
+  the thorax. By means of this knowledge the heredity of the character was
+  easily determined. It appears that while the expansion of the wing pads
+  fails to occur once in five times&mdash;probably because it is an
+  environmental effect peculiar to this stock,&mdash;yet the minute
+  difference of the presence or absence of the two lateral bristles is a
+  constant feature of the flies that carry this particular factor.</p>
+
+  <p>In the preceding cases I have spoken as though a factor influenced
+  only one part of the body. It would have been more accurate to have
+  stated that the <i>chief</i> effect of the factor was observed in a
+  particular part of the body. Most students of genetics realize that a
+  factor difference usually affects more than a single character. For
+  example, a mutant stock called rudimentary wings has as its principle
+  characteristic very short wings (fig. 34). But the factor for rudimentary
+  wings also produces other <!-- Page 71 --><span class="pagenum"><a
+  name="page71"></a>{71}</span>effects as well. The females are almost
+  completely sterile, while the males are fertile. The viability of the
+  stock is poor. When flies with rudimentary wings are put into competition
+  with wild flies relatively few of the rudimentary flies come through,
+  especially if the culture is crowded. The hind legs are also shortened.
+  All of these effects are the results of a single factor-difference.</p>
+
+  <div class="figcenter" style="width:23%;">
+      <a href="images/fig_034.jpg"><img style="width:100%" src="images/fig_034.jpg"
+      alt="Fig. 34." title="Fig. 34." /></a>
+    <p class="poem"><span class="sc">Fig. 34.</span> Mutant race of fruit
+    fly, called rudimentary.</p>
+  </div>
+
+  <p>One may venture the guess that some of the specific and varietal
+  differences that are <!-- Page 72 --><span class="pagenum"><a
+  name="page72"></a>{72}</span>characteristic of wild types and which at
+  the same time appear to have no survival value, are only by-products of
+  factors whose most important effect is on another part of the organism
+  where their influence is of vital importance.</p>
+
+  <p>It is well known that systematists make use of characters that are
+  constant for groups of species, but which do not appear in themselves to
+  have an adaptive significance. If we may suppose that the constancy of
+  such characters may be only an index of the presence of a factor whose
+  <i>chief</i> influence is in some other direction or directions, some
+  physiological influence, for example, we can give at least a reasonable
+  explanation of the constancy of such characters.</p>
+
+  <p>I am inclined to think that an overstatement to the effect that each
+  factor may affect the entire body, is less likely to do harm than to
+  state that each factor affects only a particular character. The reckless
+  use of the phrase "unit character" has done much to mislead the
+  uninitiated as to the effects that a single change in the germ plasm may
+  produce on the organism. Fortunately, the expression "unit character"
+  <!-- Page 73 --><span class="pagenum"><a name="page73"></a>{73}</span>is
+  being less used by those students of genetics who are more careful in
+  regard to the implications of their terminology.</p>
+
+  <p>There is a class of cases of inheritance, due to the XY chromosomes,
+  that is called sex linked inheritance. It is shown both by mutant
+  characters and characters of wild species.</p>
+
+  <p>For instance, white eye color in Drosophila shows sex linked
+  inheritance. If a white eyed male is mated to a wild red eyed female
+  (fig. 35) all the offspring have red eyes. If these are inbred, there are
+  three red to one white eyed offspring, but white eyes occur only in the
+  males. The grandfather has transmitted his peculiarity to half of his
+  grandsons, but to none of his granddaughters.</p>
+
+<p><!-- Page 74 --><span class="pagenum"><a name="page74"></a>{74}</span></p>
+
+  <div class="figcenter" style="width:26%;">
+      <a href="images/fig_035.jpg"><img style="width:100%" src="images/fig_035.jpg"
+      alt="Fig. 35." title="Fig. 35." /></a>
+    <p class="poem"><span class="sc">Fig. 35.</span> Diagram showing a
+    cross between a white eyed male and a red eyed female of the fruit fly.
+    Sex linked inheritance.</p>
+  </div>
+
+  <p>The reciprocal cross (fig. 36) is also interesting. If a white eyed
+  female is bred to a red eyed male, all of the daughters have red eyes and
+  all of the sons have white eyes. We call this criss-cross inheritance. If
+  these offspring are inbred, they produce equal numbers of red eyed and
+  white eyed females and equal numbers of red eyed and white eyed males.
+  The ratio is 1: 1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the
+  usual 3:1 Mendelian ratio. Yet, as will be shown later, the result is in
+  entire accord with Mendel's principle of segregation.</p>
+
+<p><!-- Page 75 --><span class="pagenum"><a name="page75"></a>{75}</span></p>
+
+  <div class="figcenter" style="width:27%;">
+      <a href="images/fig_036.jpg"><img style="width:100%" src="images/fig_036.jpg"
+      alt="Fig. 36." title="Fig. 36." /></a>
+    <p class="poem"><span class="sc">Fig. 36.</span> Diagram illustrating a
+    cross between a red eyed male and white eyed female of the fruit fly
+    (reciprocal cross of that shown in Fig. 35).</p>
+  </div>
+
+  <p>It has been shown by Sturtevant that in a wild species of Drosophila,
+  viz., D. repleta, two varieties of individuals exist, in one of which the
+  thorax has large splotches and in the <!-- Page 76 --><span
+  class="pagenum"><a name="page76"></a>{76}</span>other type smaller
+  splotches (fig. 37). The factors that differentiate these varieties are
+  sex linked.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_037.jpg"><img style="width:100%" src="images/fig_037.jpg"
+      alt="Fig. 37." title="Fig. 37." /></a>
+    <p class="poem"><span class="sc">Fig. 37.</span> Two types of markings
+    on thorax of Drosophila repleta, both found "wild". They show sex
+    linked inheritance.</p>
+  </div>
+
+  <p>Certain types of color blindness (fig. 38) and certain other abnormal
+  conditions in man such as haemophilia, are transmitted as sex linked
+  characters.</p>
+
+<p><!-- Page 77 --><span class="pagenum"><a name="page77"></a>{77}</span></p>
+
+<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
+
+  <div class="figright" style="width:52%;">
+      <a href="images/fig_038a.jpg"><img style="width:100%" src="images/fig_038a.jpg"
+      alt="Fig. 38A." title="Fig. 38A." /></a>
+    <p class="poem"><span class="sc">Fig. 38, A.</span> Diagram
+    illustrating inheritance of color blindness in man; the iris of the
+    color-blind eye is here black.</p>
+  </div>
+
+</td><td style="width:50%; vertical-align:top;">
+
+  <div class="figleft" style="width:52%;">
+      <a href="images/fig_038b.jpg"><img style="width:100%" src="images/fig_038b.jpg"
+      alt="Fig. 38B." title="Fig. 38B." /></a>
+    <p class="poem"><span class="sc">Fig. 38, B.</span> Reciprocal of cross
+    in Fig. 38 a.</p>
+  </div>
+
+</td></tr></table>
+
+  <p>In domestic fowls sex linked inheritance has been found as the
+  characteristic method of transmission for at least as many as six
+  characters, but here the relation of the sexes is in a sense reversed.
+  For instance, if a black Langshan hen is crossed to a barred Plymouth
+  Rock cock (fig. 39), the offspring are all barred. If these are inbred
+  half of the daughters are black and half are barred; all of the sons are
+  barred. The grandmother has transmitted her color to half of her
+  granddaughters but to none of her grandsons.</p>
+
+<p><!-- Page 78 --><span class="pagenum"><a name="page78"></a>{78}</span></p>
+
+<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
+
+  <div class="figright" style="width:61%;">
+      <a href="images/fig_039.jpg"><img style="width:100%" src="images/fig_039.jpg"
+      alt="Fig. 39." title="Fig. 39." /></a>
+    <p class="poem"><span class="sc">Fig. 39.</span> Sex-linked inheritance
+    in domesticated birds shown here in a cross between barred Plymouth
+    Rock male and black Langshan female.</p>
+  </div>
+
+</td><td style="width:50%; vertical-align:top;">
+
+  <div class="figleft" style="width:61%;">
+      <a href="images/fig_040.jpg"><img style="width:100%" src="images/fig_040.jpg"
+      alt="Fig. 40." title="Fig. 40." /></a>
+    <span class="sc">Fig. 40.</span> Reciprocal of Fig. 39.
+  </div>
+
+</td></tr></table>
+
+  <p>In the reciprocal cross (fig. 40) black cock by barred hen, the
+  daughters are black and the sons barred&mdash;criss-cross inheritance.
+  These inbred give black hens and black cocks, barred hens and barred
+  cocks.</p>
+
+<p><!-- Page 79 --><span class="pagenum"><a name="page79"></a>{79}</span></p>
+
+  <p>There is a case comparable to this found in a wild species of moth,
+  Abraxas grossulariata. A wild variation of this type is lighter in color
+  and is known as A. lacticolor. When these two types are crossed they
+  exhibit exactly the same type of heredity as does the black-barred
+  combination in the domestic fowl. As shown in figure 41, lacticolor
+  female bred to grossulariata male gives grossulariata sons and daughters.
+  These inbred give grossulariata males and females and lacticolor females.
+  Reciprocally lacticolor male by grossulariata female, <!-- Page 80
+  --><span class="pagenum"><a name="page80"></a>{80}</span>(fig. 42) gives
+  lacticolor daughters and grossulariata sons and these inbred give
+  grossulariata males and females and lacticolor males and females.</p>
+
+<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
+
+  <div class="figright" style="width:68%;">
+      <a href="images/fig_041.jpg"><img style="width:100%" src="images/fig_041.jpg"
+      alt="Fig. 41." title="Fig. 41." /></a>
+    <p class="poem"><span class="sc">Fig. 41.</span> Sex-linked inheritance
+    in the wild moth, Abraxas grossulariata (darker) and A. lacticolor.</p>
+  </div>
+
+</td><td style="width:50%; vertical-align:top;">
+<!-- Page 81 --><span class="pagenum"><a name="page81"></a>{81}</span>
+
+  <div class="figleft" style="width:68%;">
+      <a href="images/fig_042.jpg"><img style="width:100%" src="images/fig_042.jpg"
+      alt="Fig. 42." title="Fig. 42." /></a>
+    <span class="sc">Fig. 42.</span> Reciprocal of Fig. 41.
+  </div>
+
+</td></tr></table>
+
+<p><!-- Page 82 --><span class="pagenum"><a name="page82"></a>{82}</span></p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_043.jpg"><img style="width:100%" src="images/fig_043.jpg"
+      alt="Fig. 43." title="Fig. 43." /></a>
+    <p class="poem"><span class="sc">Fig. 43.</span> Four wild types of
+    Paratettix in upper line with three hybrids below.</p>
+  </div>
+
+  <p>It has been found that there may be even more than two factors that
+  show Mendelian segregation when brought together in pairs. For example,
+  in the southern States there are several races of the grouse locust
+  (Paratettix) that differ from each other markedly in color patterns (fig.
+  43). When any two individuals of these races are crossed they give, as
+  Nabours has shown, in F<sub>2</sub> a Mendelian ratio of 1: 2: 1. It is
+  obvious, therefore, that there are here at least nine characters, any two
+  of which behave as a Mendelian pair. These races have <!-- Page 83
+  --><span class="pagenum"><a name="page83"></a>{83}</span>arisen in nature
+  and differ definitely and strikingly from each other, yet any two differ
+  by only one factor difference.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_044.jpg"><img style="width:100%" src="images/fig_044.jpg"
+      alt="Fig. 44." title="Fig. 44." /></a>
+    <p class="poem"><span class="sc">Fig. 44.</span> Diagram illustrating
+    four allelomorphs in mice, viz. gray bellied gray (wild type) (above,
+    to left); white bellied gray (above, to right); yellow (below, to
+    right); and black (below, to left).</p>
+  </div>
+
+  <p>Similar relations have been found in a number of domesticated races.
+  In mice there is a quadruple system represented by the gray house mouse,
+  the white bellied, the yellow and the black mouse (fig. 44). In rabbits
+  there is probably a triple system, that includes the albino, the
+  Himalayan, and the black races. In <!-- Page 84 --><span
+  class="pagenum"><a name="page84"></a>{84}</span>the silkworm moth there
+  have been described four types of larvae, distinguished by different
+  color markings, that form a system of quadruple allelomorphs. In
+  Drosophila there is a quintuple system of factors in the sex chromosome
+  represented by eye colors, a triple system of body colors, and a triple
+  system of factors for eye colors in the third chromosome.</p>
+
+<p class="cenhead"><span class="sc">Mutation and Evolution</span></p>
+
+  <p>What bearing has the appearance of these new types of Drosophila on
+  the theory of evolution may be asked. The objection has been raised in
+  fact that in the breeding work with Drosophila we are dealing with
+  artificial and unnatural conditions. It has been more than implied that
+  results obtained from the breeding pen, the seed pan, the flower pot and
+  the milk bottle do not apply to evolution in the "open", nature "at
+  large" or to "wild" types. To be consistent, this same objection should
+  be extended to the use of the spectroscope in the study of the evolution
+  of the stars, to the use of the test tube and the balance by the chemist,
+  of the galvanometer by the physicist. All these <!-- Page 85 --><span
+  class="pagenum"><a name="page85"></a>{85}</span>are unnatural instruments
+  used to torture Nature's secrets from her. I venture to think that the
+  real antithesis is not between unnatural and natural treatment of Nature,
+  but rather between controlled or verifiable data on the one hand, and
+  unrestrained generalization on the other.</p>
+
+  <p>If a systematist were asked whether these new races of Drosophila are
+  comparable to wild species, he would not hesitate for a moment. He would
+  call them all one species. If he were asked why, he would say, I think,
+  "These races differ only in one or two striking points, while in a
+  hundred other respects they are identical even to the minutest details."
+  He would add, that as large a group of wild species of flies would show
+  on the whole the reverse relations, <i>viz.</i>, they would differ in
+  nearly every detail and be identical in only a few points. In all this I
+  entirely agree with the systematist, for I do not think such a group of
+  types differing by one character each, is comparable to most wild groups
+  of species because the difference between wild species is due to a large
+  number of such single differences. The characters <!-- Page 86 --><span
+  class="pagenum"><a name="page86"></a>{86}</span>that have been
+  accumulated in wild species are of significance in the maintenance of the
+  species, or at least we are led to infer that even though the visible
+  character that we attend to may not itself be important, one at least of
+  the other effects of the factors that represent these characters is
+  significant. It is, of course, hardly to be expected that <i>any</i>
+  random change in as complex a mechanism as an insect would improve the
+  mechanism, and as a matter of fact it is doubtful whether any of the
+  mutant types so far discovered are better adapted to those conditions to
+  which a fly of this structure and habits is already adjusted. But this is
+  beside the mark, for modern genetics shows very positively that adaptive
+  characters are inherited in exactly the same way as are those that are
+  not adaptive; and I have already pointed out that we cannot study a
+  single mutant factor without at the same time studying one of the factors
+  responsible for normal characters, for the two together constitute the
+  Mendelian pair.</p>
+
+  <p>And, finally, I want to urge on your attention a question that we are
+  to consider in more detail in the last lecture. Evolution of wild <!--
+  Page 87 --><span class="pagenum"><a name="page87"></a>{87}</span>species
+  appears to have taken place by modifying and improving bit by bit the
+  structures and habits that the animal or plant already possessed. We have
+  seen that there are thirty mutant factors at least that have an influence
+  on eye color, and it is probable that there are at least as many normal
+  factors that are involved in the production of the red eye of the wild
+  fly.</p>
+
+  <p>Evolution from this point of view has consisted largely in introducing
+  new factors that influence characters already present in the animal or
+  plant.</p>
+
+  <p>Such a view gives us a somewhat different picture of the process of
+  evolution from the old idea of a ferocious struggle between the
+  individuals of a species with the survival of the fittest and the
+  annihilation of the less fit. Evolution assumes a more peaceful aspect.
+  New and advantageous characters survive by incorporating themselves into
+  the race, improving it and opening to it new opportunities. In other
+  words, the emphasis may be placed less on the competition between the
+  individuals of a species (because the destruction of the less fit does
+  <!-- Page 88 --><span class="pagenum"><a name="page88"></a>{88}</span>not
+  <i>in itself</i> lead to anything that is new) than on the appearance of
+  new characters and modifications of old characters that become
+  incorporated in the species, for on these depends the evolution of the
+  race.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 89 --><span class="pagenum"><a name="page89"></a>{89}</span></p>
+
+<h3>CHAPTER III</h3>
+
+<p class="cenhead">THE FACTORIAL THEORY OF HEREDITY AND
+THE COMPOSITION OF THE GERM PLASM</p>
+
+  <p>The discovery that Mendel made with edible peas concerning heredity
+  has been found to apply everywhere throughout the plant and animal
+  kingdoms&mdash;to flowering plants, to insects, snails, crustacea,
+  fishes, amphibians, birds, and mammals (including man).</p>
+
+  <p>There must be something that these widely separated groups of plants
+  and animals have in common&mdash;some simple mechanism perhaps&mdash;to
+  give such definite and orderly series of results. There is, in fact, a
+  mechanism, possessed alike by animals and plants, that fulfills every
+  requirement of Mendel's principles.</p>
+
+<p class="cenhead"><span class="sc">The Cellular Basis of Organic Evolution and Heredity</span></p>
+
+  <p>In order to appreciate the full force of the evidence, let me first
+  pass rapidly in review a <!-- Page 90 --><span class="pagenum"><a
+  name="page90"></a>{90}</span>few familiar, historical facts, that
+  preceded the discovery of the mechanism in question.</p>
+
+  <div class="figcenter" style="width:38%;">
+      <a href="images/fig_045.jpg"><img style="width:100%" src="images/fig_045.jpg"
+      alt="Fig. 45." title="Fig. 45." /></a>
+    <p class="poem"><span class="sc">Fig. 45.</span> Typical cell showing
+    the cell wall, the protoplasm (with its contained materials); the
+    nucleus with its contained chromatin and nuclear sap. (After
+    Dahlgren.)</p>
+  </div>
+
+  <p>Throughout the greater part of the last century, while students of
+  evolution and of heredity were engaged in what I may call the more
+  general, or, shall I say, the <i>grosser</i> aspects of the subject,
+  there existed another group of students who were engaged in working out
+  the minute structure of the material basis of the living organism. They
+  found that organs such as the brain, the heart, the liver, the lungs, the
+  kidneys, etc., are not themselves the units of structure, but that all
+  these organs can be reduced to a simpler unit that repeats itself a <!--
+  Page 91 --><span class="pagenum"><a
+  name="page91"></a>{91}</span>thousand-fold in every organ. We call this
+  unit a cell (fig. 45).</p>
+
+  <p>The egg is a cell, and the spermatozoon is a cell. The act of
+  fertilization is the union of two cells (fig. 47, upper figure). Simple
+  as the process of fertilization appears to us today, its discovery swept
+  aside a vast amount of mystical speculation concerning the rôle of the
+  male and of the female in the act of procreation.</p>
+
+  <p>Within the cell a new microcosm was revealed. Every cell was found to
+  contain a spherical body called the nucleus (fig. 46a). Within the
+  nucleus is a network of fibres, a sap fills the interstices of the
+  network. The network resolves itself into a definite number of threads at
+  each division of the cell (fig. 46 b-e). These threads we call
+  chromosomes. Each species of animals and plants possesses a
+  characteristic number of these threads which have a definite size and
+  sometimes a specific shape and even characteristic granules at different
+  levels. Beyond this point our strongest microscopes fail to penetrate.
+  Observation has reached, for the time being, its limit.</p>
+
+<p><!-- Page 92 --><span class="pagenum"><a name="page92"></a>{92}</span></p>
+
+  <div class="figcenter" style="width:26%;">
+      <a href="images/fig_046.jpg"><img style="width:100%" src="images/fig_046.jpg"
+      alt="Fig. 46." title="Fig. 46." /></a>
+    <p class="poem"><span class="sc">Fig. 46.</span> A series of cells in
+    process of cell division. The chromosomes are the black threads and
+    rods. (After Dahlgren.)</p>
+  </div>
+
+  <p>The story is taken up at this point by a new set of students who have
+  worked in an entirely different field. Certain observations and
+  experiments that we have not time to consider <!-- Page 93 --><span
+  class="pagenum"><a name="page93"></a>{93}</span>now, led a number of
+  biologists to conclude that the chromosomes are the bearers of the
+  hereditary units. If so, there should be many such units carried by
+  <i>each</i> chromosome, for the number of chromosomes is limited while
+  the number of independently inherited characters is large. In Drosophila
+  it has been demonstrated not only that there are exactly as many groups
+  of characters that are inherited together as there are pairs of
+  chromosomes, but even that it is possible to locate one of these groups
+  in a particular chromosome and to state the <i>relative position</i>
+  there of the factors for the characters. If the validity of this evidence
+  is accepted, the study of the cell leads us finally in a mechanical, but
+  not in a chemical sense, to the ultimate units about which the whole
+  process of the transmission of the hereditary factors centers.</p>
+
+  <p>But before plunging into this somewhat technical matter (that is
+  difficult only because it is unfamiliar), certain facts which are
+  familiar for the most part should be recalled, because on these turns the
+  whole of the subsequent story.</p>
+
+<p><!-- Page 94 --><span class="pagenum"><a name="page94"></a>{94}</span></p>
+
+  <div class="figcenter" style="width:26%;">
+      <a href="images/fig_047.jpg"><img style="width:100%" src="images/fig_047.jpg"
+      alt="Fig. 47." title="Fig. 47." /></a>
+    <p class="poem"><span class="sc">Fig. 47.</span> An egg, and the
+    division of the egg&mdash;the so-called process of cleavage. (After
+    Selenka.)</p>
+  </div>
+
+  <p>The thousands of cells that make up the cell-state that we call an
+  animal or plant come from the fertilized egg. An hour or two after
+  fertilization the egg divides into two cells (fig. 47). Then each half
+  divides again. Each <!-- Page 95 --><span class="pagenum"><a
+  name="page95"></a>{95}</span>quarter next divides. The process continues
+  until a large number of cells is formed and out of these organs mould
+  themselves.</p>
+
+  <div class="figcenter" style="width:27%;">
+      <a href="images/fig_048.jpg"><img style="width:100%" src="images/fig_048.jpg"
+      alt="Fig. 48." title="Fig. 48." /></a>
+    <p class="poem"><span class="sc">Fig. 48.</span> Section of the egg of
+    the beetle, Calligrapha, showing the pigment at one end where the germ
+    cells will later develop as shown in the other two figures. (After
+    Hegner.)</p>
+  </div>
+
+  <p>At every division of the cell the chromosomes also divide. Half of
+  these have come from the mother, half from the father. Every cell
+  contains, therefore, the sum total of all the chromosomes, and if these
+  are the bearers of the hereditary qualities, every cell in the body, <!--
+  Page 96 --><span class="pagenum"><a name="page96"></a>{96}</span>whatever
+  its function, has a common inheritance.</p>
+
+  <p>At an early stage in the development of the animal certain cells are
+  set apart to form the organs of reproduction. In some animals these cells
+  can be identified early in the cleavage (fig. 48).</p>
+
+  <p>The reproductive cells are at first like all the other cells in the
+  body in that they contain a full complement of chromosomes, half paternal
+  and half maternal in origin (fig. 49). They divide as do the other cells
+  of the body for a long time (fig. 49, upper row). At each division each
+  chromosome splits lengthwise and its halves migrate to opposite poles of
+  the spindle (fig. 49 c).</p>
+
+  <p>But there comes a time when a new process appears in the germ cells
+  (fig 49 e-h). It is essentially the same in the egg and in the sperm
+  cells. The discovery of this process we owe to the laborious researches
+  of many workers in many countries. The list of their names is long, and I
+  shall not even attempt to repeat it. The chromosomes come together in
+  pairs (fig. 49 a). Each maternal chromosome mates with a paternal
+  chromosome of the same kind. <!-- Page 97 --><span class="pagenum"><a
+  name="page97"></a>{97}</span></p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_049.jpg"><img style="width:100%" src="images/fig_049.jpg"
+      alt="Fig. 49." title="Fig. 49." /></a>
+    <p class="poem"><span class="sc">Fig. 49.</span> In the upper row of
+    the diagram a typical process of nuclear division, such as takes place
+    in the early germ cells or in the body cells. In the lower row the
+    separation of the chromosomes that have paired. This sort of separation
+    takes place at one of the two reduction divisions.</p>
+  </div>
+
+  <p>Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one
+  of the divisions the double chromosomes separate so that each resulting
+  cell comes to contain some maternal and <!-- Page 98 --><span
+  class="pagenum"><a name="page98"></a>{98}</span>some paternal
+  chromosomes, i.e. one or the other member of each pair. At the other
+  division each chromosome simply splits as in ordinary cell division.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_050.jpg"><img style="width:100%" src="images/fig_050.jpg"
+      alt="Fig. 50." title="Fig. 50." /></a>
+    <p class="poem"><span class="sc">Fig. 50.</span> The two maturation
+    divisions of the sperm cell. Four sperms result, each with half
+    (haploid) the full number (diploid) of chromosomes.</p>
+  </div>
+
+  <p>The upshot of the process is that the ripe eggs (fig. 51) and the ripe
+  spermatozoa (fig. <!-- Page 99 --><span class="pagenum"><a
+  name="page99"></a>{99}</span>50) come to contain only half the total
+  number of chromosomes.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_051.jpg"><img style="width:100%" src="images/fig_051.jpg"
+      alt="Fig. 51." title="Fig. 51." /></a>
+    <p class="poem"><span class="sc">Fig. 51.</span> The two maturation
+    divisions of the egg. The divisions are unequal, so that two small
+    polar bodies are formed one of these subsequently divides. The three
+    polar bodies and the egg are comparable to the four sperms.</p>
+  </div>
+
+  <p>When the eggs are fertilized the whole number of chromosomes is
+  restored again.</p>
+
+<p class="cenhead"><span class="sc">The Mechanism of Mendelian Heredity Discovered in the Behavior of the Chromosomes</span></p>
+
+  <p>If the factors in heredity are carried in the chromosomes and if the
+  chromosomes are definite structures, we should anticipate that there
+  should be as many <i>groups</i> of characters as there are kinds of
+  chromosomes. In only one <!-- Page 100 --><span class="pagenum"><a
+  name="page100"></a>{100}</span>case has a sufficient number of characters
+  been studied to show whether there is any correspondence between the
+  number of hereditary groups of characters and the number of chromosomes.
+  In the fruit fly, Drosophila ampelophila, we have found about 125
+  characters that are inherited in a perfectly definite way. On the
+  opposite page is a list of some of them.</p>
+
+  <p>It will be observed in this list that the characters are arranged in
+  four groups, Groups I, II, III and IV. Three of these groups are equally
+  large or nearly so; Group IV contains only two characters. The characters
+  are put into these groups because in heredity the members of each group
+  tend to be inherited together, i.e., if two or more enter the cross
+  together they tend to remain together through subsequent generations. On
+  the other hand, any member of one group is inherited entirely
+  independently of any member of the other groups; in the same way as
+  Mendel's yellow-green pair of characters is inherited independently of
+  the round-wrinkled pair.</p>
+
+<p><!-- Page 101 --><span class="pagenum"><a name="page101"></a>{101}</span></p>
+
+<table class="nobctr" summary="Groups I-IV." title="Groups I-IV.">
+<tr><td class="spacsingle" style="width:25%; vertical-align:top;"><i>Group I</i><br />
+Abnormal<br />
+Bar<br />
+Bifid<br />
+Bow<br />
+Cherry<br />
+Chrome<br />
+Cleft<br />
+Club<br />
+Depressed<br />
+Dot<br />
+Eosin<br />
+Facet<br />
+Forked<br />
+Furrowed<br />
+Fused<br />
+Green<br />
+Jaunty<br />
+Lemon<br />
+Lethals, 13<br />
+Miniature<br />
+Notch<br />
+Reduplicated<br />
+Ruby<br />
+Rudimentary<br />
+Sable<br />
+Shifted<br />
+Short<br />
+Skee<br />
+Spoon<br />
+Spot<br />
+Tan<br />
+Truncate intensifier<br />
+Vermilion<br />
+White<br />
+Yellow
+</td><td class="spacsingle" style="width:25%; vertical-align:top;"><i>Group II</i><br />
+Antlered<br />
+Apterous<br />
+Arc<br />
+Balloon<br />
+Black<br />
+Blistered<br />
+Comma<br />
+Confluent<br />
+Cream II<br />
+Curved<br />
+Dachs<br />
+Extra vein<br />
+Fringed<br />
+Jaunty<br />
+Limited<br />
+Little crossover<br />
+Morula<br />
+Olive<br />
+Plexus<br />
+Purple<br />
+Speck<br />
+Strap<br />
+Streak<br />
+Trefoil<br />
+Truncate<br />
+Vestigial
+</td><td class="spacsingle" style="width:25%; vertical-align:top;"><i>Group III</i><br />
+Band<br />
+Beaded<br />
+Cream III<br />
+Deformed<br />
+Dwarf<br />
+Ebony<br />
+Giant<br />
+Kidney<br />
+Low crossing over<br />
+Maroon<br />
+Peach<br />
+Pink<br />
+Rough<br />
+Safranin<br />
+Sepia<br />
+Sooty<br />
+Spineless<br />
+Spread<br />
+Trident<br />
+Truncate intensifier<br />
+Whitehead<br />
+White ocelli
+</td><td class="spacsingle" style="width:25%; vertical-align:top;"><i>Group IV</i><br />
+Bent<br />
+Eyeless
+</td></tr></table>
+
+<p><!-- Page 102 --><span class="pagenum"><a name="page102"></a>{102}</span></p>
+
+  <p>If the factors for these characters are carried by the chromosomes,
+  then we should expect that those factors that are carried by the same
+  chromosome would be inherited together, provided the chromosomes are
+  definite structures in the cell.</p>
+
+  <div class="figcenter" style="width:27%;">
+      <a href="images/fig_052.jpg"><img style="width:100%" src="images/fig_052.jpg"
+      alt="Fig. 52." title="Fig. 52." /></a>
+    <p class="poem"><span class="sc">Fig. 52.</span> Chromosomes (diploid)
+    of D. ampelophila. The sex chromosomes are XX in the female and XY in
+    the male. There are three other pairs of chromosomes.</p>
+  </div>
+
+  <p>In the chromosome group of Drosophila, (fig. 52) there are <i>four</i>
+  pairs of chromosomes, three of nearly the same size and one much smaller.
+  Not only is there agreement between the number of hereditary groups and
+  the number of the chromosomes, but even the size relations are the same,
+  for there are three great groups of characters and three pairs of large
+  chromosomes, and one small group of characters and one pair of small
+  chromosomes. <!-- Page 103 --><span class="pagenum"><a
+  name="page103"></a>{103}</span></p>
+
+<p class="cenhead"><span class="sc">The Four Great Linkage Groups of Drosophila Ampelophila</span></p>
+
+  <p>The following description of the characters of the wild fly may be
+  useful in connection with the account of the modifications of these
+  characters that appear in the mutants.</p>
+
+  <p>The head and thorax of the wild fly are grayish-yellow, the abdomen is
+  banded with alternate stripes of yellow and black. In the male, (fig. 4
+  to right), there are three narrow bands and a black tip. In the female
+  there are five black bands (fig. 4 to left). The wings are gray with a
+  surface texture of such a kind that at certain angles they are
+  iridescent. The eyes are a deep, solid, brick-red. The minute hairs that
+  cover the body have a very definite arrangement that is most obvious on
+  the head and thorax. There is a definite number of larger hairs called
+  bristles or chaetae which have a characteristic position and are used for
+  diagnostic purposes in classifying the species. On the foreleg of the
+  male there is a comb-like organ formed by a row of bristles; it is absent
+  in the female. The comb is a secondary sexual character, and it is, so
+  far as known, functionless. <!-- Page 104 --><span class="pagenum"><a
+  name="page104"></a>{104}</span></p>
+
+  <p>Some of the characters of the mutant types are shown in figures <span
+  class="correction" title="Original reads `52, 53, 54, 55'.">53, 54, 55,
+  56</span>. The drawing of a single fly is often used here to illustrate
+  more than one character. This is done to economize space, but of course
+  there would be no difficulty in actually bringing together in the same
+  individual any two or more characters belonging to the same group (or to
+  different groups). Without colored figures it is not possible to show
+  many of the most striking differences of these mutant races; at most dark
+  and light coloring can be indicated by the shading of the body, wings, or
+  eyes.</p>
+
+<p class="cenhead"><i>Group I</i></p>
+
+  <p>In the six flies drawn in figure 53 there are shown five different
+  wing characters. The first of these types (a) is called cut, because the
+  ends of the wings look as though they had been cut to a point. The
+  antennae are displaced downward and appressed and their bristle-like
+  aristae are crumpled.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_053.jpg"><img style="width:100%" src="images/fig_053.jpg"
+      alt="Fig. 53." title="Fig. 53." /></a>
+    <span class="sc">Fig. 53.</span> Group I. (See text)
+  </div>
+
+  <p>The second figure (b) represents a fly with a notch in the ends of the
+  wings. This character is dominant, but the same factor that <!-- Page 105
+  --><span class="pagenum"><a name="page105"></a>{105}</span>produces the
+  notch in the wings is also a recessive lethal factor; because of this
+  latter effect of the character no males of this race exist, and the
+  females of the race are never pure but hybrid. Every female with notch
+  wings bred to a wild male, will produce in equal numbers notch winged
+  daughters and daughters with normal wings. There will be half as many
+  sons as daughters. The explanation of <!-- Page 106 --><span
+  class="pagenum"><a name="page106"></a>{106}</span>this peculiar result is
+  quite simple. Every notch winged female has one X chromosome that carries
+  the factor for notch and one X chromosome that is "normal". Daughters
+  receiving the former chromosomes are notched because the factor for notch
+  is dominant, but they are not killed since the lethal effect of the notch
+  factor is recessive to the normal allelomorph carried by the other
+  chromosome that the daughters get from their father. This normal factor
+  is recessive for notch but dominant for life. This same figure (b) is
+  used here to show three other sex linked characters. The spines on the
+  thorax are twisted or kinky, which is due to a factor called "forked".
+  The effect is best seen on the thorax, but all spines on the body are
+  similarly modified; even the minute hairs are also affected. Ruby eye
+  color might be here represented&mdash;if the eyes in the figure were
+  colored. The lighter color of the body and antennae is intended to
+  indicate that the character tan is also present. The light color of the
+  antennae is the most certain way of identifying tan. The tan flies are
+  interesting because they have lost the positive heliotropism <!-- Page
+  107 --><span class="pagenum"><a name="page107"></a>{107}</span>that is so
+  marked a feature in the behavior of D. ampelophila. As this peculiarity
+  of the tan flies is inherited like all the other sex linked characters,
+  it follows that when a tan female is bred to a wild male all the sons
+  inherit the recessive tan color and indifference to light, while the
+  daughters show the dominant sex linked character of their father, i.e.,
+  they are "gray", and go to the light. Hence when such a brood is
+  disturbed the females fly to the light, but the males remain behind.</p>
+
+  <p>One of the first mutants that appeared in D. ampelophila was called
+  rudimentary on account of the condition of the wings (c). The same
+  mutation has appeared independently several times. In the drawing (c) the
+  dark body color is intended to indicate "sable" and the lighter color of
+  the eyes is intended to indicate eosin. This eye color, which is an
+  allelomorph of white, is also interesting because in the female the color
+  is deeper than in the male. In other cases of sex linked factors the
+  character is the same in the two sexes.</p>
+
+  <p>In the fourth figure (d) the third and fourth longitudinal veins of
+  the wing are <i>fused</i> into <!-- Page 108 --><span class="pagenum"><a
+  name="page108"></a>{108}</span>one vein from the base of the wing to the
+  level of the first cross-vein and in addition converge and meet near
+  their outer ends. The shape of the eye is represented in the figure as
+  different from the normal, due to another factor called "bar". This is a
+  dominant character, the hybrid condition being also narrow, but not so
+  narrow as the pure type. Vermilion eye color might also be here
+  represented&mdash;due to a factor that has appeared independently on
+  several occasions.</p>
+
+  <p>In the fifth figure (e) the wings are shorter and more pointed than in
+  the wild fly. This character is called miniature. The light color of the
+  drawing may be taken to represent yellow body color, and the light color
+  of the eye white eye color.</p>
+
+  <p>In the last figure (f) the wings are represented as pads, essentially
+  in the same condition that they are in when the fly emerges from the pupa
+  case. Not all the flies of this stock have the wings in this condition;
+  some have fully expanded wings that appear normal in all respects.
+  Nevertheless, about the same percentage of offspring show the pads
+  irrespective of <!-- Page 109 --><span class="pagenum"><a
+  name="page109"></a>{109}</span>whether the parents had pads or expanded
+  wings.</p>
+
+  <p>The flies of this stock show, however, another character, which is a
+  product of the same factor, and which is constant, i.e., repeated in all
+  individuals. The two bristles on the sides of the thorax are constantly
+  absent in this race. The lighter color of the eye in the figure may be
+  taken to indicate buff&mdash;a faint yellowish color. The factor for this
+  eye color is another allelomorph of white.</p>
+
+  <p>There are many other interesting characters that belong to the first
+  group, such as abnormal abdomen, short legs, duplication of the legs,
+  etc. In fact, any part of the body may be affected by a sex-linked
+  factor.</p>
+
+<p class="cenhead"><i>Group II</i></p>
+
+  <p>In the first figure (a) of figure 54 that contains members of Group II
+  the wings are almost entirely absent or "vestigial". This condition arose
+  at a single step and breeds true, although it appears to be influenced to
+  some extent by temperature, also by modifiers that sometimes appear in
+  the stock. Purple <!-- Page 110 --><span class="pagenum"><a
+  name="page110"></a>{110}</span>eye color belongs in Group II; it
+  resembles the color of the eye of the wild fly but is darker and more
+  translucent.</p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_054.jpg"><img style="width:100%" src="images/fig_054.jpg"
+      alt="Fig. 54." title="Fig. 54." /></a>
+    <span class="sc">Fig. 54.</span> Group II. (See text.)
+  </div>
+
+  <p>In the second figure (b) the wing is again long and narrow and
+  sometimes bent back on itself, as shown here. In several respects the
+  wing resembles strap (d) but seems to be due <!-- Page 111 --><span
+  class="pagenum"><a name="page111"></a>{111}</span>to another factor,
+  called antler, insufficiently studied as yet.</p>
+
+  <p>In the third figure (c) the wings turn up at the end. This is brought
+  about by the presence of the factor called jaunty.</p>
+
+  <p>In the fourth figure the wings are long and narrow and several of the
+  veins are unrepresented. This character, "strap", is very variable and
+  has not yet been thoroughly studied. On the thorax there is a deep black
+  mark called trefoil. Even in the wild fly there is a three pronged mark
+  on the thorax present in many individuals. Trefoil is a further
+  development and modification of this mark and is due to a special
+  factor.</p>
+
+  <p>In the fifth figure (e) the wings are arched. The factor is called
+  arc. The dark color of the body, and especially of the wings, indicates
+  the factor for black.</p>
+
+  <p>The sixth figure (f) shows the wings "curved" downwards. In addition
+  there is present a minute black speck at the base of each wing, due to
+  another factor called speck.</p>
+
+  <p>In the seventh figure (g) the wing is truncate. Its end is obliquely
+  squared instead of <!-- Page 112 --><span class="pagenum"><a
+  name="page112"></a>{112}</span>rounded; it may be longer than the body,
+  or shorter when other modifying factors are present. The mutation that
+  produces this type of wing is of not infrequent occurrence. It has been
+  shown by Muller and Altenburg that there are at least two factors that
+  modify this character&mdash;the chief factor is present in the second
+  chromosome; alone it produces the truncate wing in only a certain
+  percentage of cases, but when the modifiers are also present about ninety
+  percent of the individuals may show the truncate condition of the wing.
+  But the presence of these factors makes the stock very infertile, so that
+  it is difficult to maintain.</p>
+
+  <p>In the eighth figure (h) the legs are shortened owing to the absence
+  of a segment of the tarsus. The stock is called dachs&mdash;a nickname
+  given to it because the short legs suggested the dachshund.</p>
+
+<p class="cenhead"><i>Group III</i></p>
+
+  <p>In figure 55, (a), a mutant type called bithorax is shown. The old
+  metathorax is replaced by another mesothorax thrust in between the normal
+  mesothorax and the abdomen. It <!-- Page 113 --><span class="pagenum"><a
+  name="page113"></a>{113}</span>carries a pair of wings that do not
+  completely unfold. On this new mesothorax the characteristic arrangement
+  of the bristles is shown. Thus at a single step a typical region of the
+  body has doubled. The character is recessive.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_055.jpg"><img style="width:100%" src="images/fig_055.jpg"
+      alt="Fig. 55." title="Fig. 55." /></a>
+    <span class="sc">Fig. 55.</span> Group III. (See text.)
+  </div>
+
+  <p>The size of the adult fly of D. ampelophila varies greatly according
+  to the amount of nourishment obtained by the larva. After the fly emerges
+  its size remains nearly constant, as in many insects. Two races have,
+  <!-- Page 114 --><span class="pagenum"><a
+  name="page114"></a>{114}</span>however, been separated by Bridges that
+  are different in size as a result of a genetic factor. The first of
+  these, called dwarf, is represented by figure 55, (b).</p>
+
+  <p>The race is minute, although of course its size is variable, depending
+  on food and other conditions. The same figure shows the presence of
+  another factor, "sooty", that makes the fly very dark. Maroon eye color
+  might be here represented, due to still another factor.</p>
+
+  <p>In the third figure (c) the other mutation in size is shown. It is
+  called "giant". The flies are twice the size of wild flies. An eye color,
+  called peach, might here be represented. It is an allelomorph of
+  pink.</p>
+
+  <p>In the fourth figure (d) the mutant called dichaete is shown. It is
+  characterized by the absence of two of the bristles on the thorax. Other
+  bristles may also be absent, but not so constantly as the two just
+  mentioned. Another effect of the same factor is the spread-out condition
+  of the wings. The very dark eye color in this figure may be taken to
+  indicate the presence of another factor, "sepia", which causes the eyes
+  to assume a brown color that <!-- Page 115 --><span class="pagenum"><a
+  name="page115"></a>{115}</span>becomes black with age. Most of the other
+  mutations in eye color that have occurred tend to give a lighter color:
+  this one, which is also recessive, makes the eye darker.</p>
+
+  <p>In the fifth figure (e) the color of the darkest fly is due to a
+  factor called ebony, which is an allelomorph of sooty.</p>
+
+  <p>In the sixth figure (f) the wings are beaded, i.e., the margin is
+  defective at intervals, giving a beaded-like outline to the wings. This
+  condition is very variable and much affected by other factors that
+  influence the shape of the wings. The lighter eye color of the drawing
+  may be taken to represent pink.</p>
+
+  <p>In the seventh figure (g) the wings are curled up over the back. This
+  is a recessive character.</p>
+
+<p class="cenhead"><i>Group IV</i></p>
+
+  <p>Only two mutants have been obtained that do not belong to any of the
+  preceding groups; these are put together in Group IV. It has been shown
+  that they are linked to each other and the linkage is so close that it
+  has thus far been impossible to obtain the dominant recessive. <!-- Page
+  116 --><span class="pagenum"><a name="page116"></a>{116}</span>One of
+  these mutants, called "eyeless" (fig. 56, a, a<sup>1</sup>), is
+  variable&mdash;the eyes are often entirely absent or represented by one
+  or more groups of ommatidia. The outline of the original eye, so to
+  speak, is strongly marked out and its area might be called a rudimentary
+  organ, if such a statement has any meaning here.</p>
+
+  <div class="figcenter" style="width:21%;">
+      <a href="images/fig_056.jpg"><img style="width:100%" src="images/fig_056.jpg"
+      alt="Fig. 56." title="Fig. 56." /></a>
+    <span class="sc">Fig. 56.</span> Group IV. (See text.)
+  </div>
+
+  <p>The other figure (b) represents "bent", so called from the shape of
+  the wings. This <!-- Page 117 --><span class="pagenum"><a
+  name="page117"></a>{117}</span>mutant is likewise very variable, often
+  indistinguishable from the wild type, yet when well developed strikingly
+  different from any other mutant.</p>
+
+  <p>This brief account of a few of the mutant races that can be most
+  easily represented by uncolored figures will serve to show how all parts
+  of the body may change, some of the changes being so slight that they
+  would be overlooked except by an expert, others so great that in the
+  character affected the flies depart far from the original species.</p>
+
+  <p><i>It is important to note that mutations in the first chromosome are
+  not limited to any part of the body nor do they affect more frequently a
+  particular part. The same statement holds equally for all of the other
+  chromosomes. In fact, since each factor may affect visibly several parts
+  of the body at the same time there are no grounds for expecting any
+  special relation between a given chromosome and special regions of the
+  body. It can not too insistently be urged that when we say a character is
+  the product of a particular factor we mean no more than that it is the
+  most conspicuous effect of the factor.</i> <!-- Page 118 --><span
+  class="pagenum"><a name="page118"></a>{118}</span></p>
+
+  <p>If, then, as these and other results to be described point to the
+  chromosomes as the bearers of the Mendelian factors, and if, as will be
+  shown presently, these factors have a definite location in the
+  chromosomes it is clear that the location of the factors in the
+  chromosomes bears no spatial relation to the location of the parts of the
+  body to each other.</p>
+
+<p class="cenhead"><span class="sc">Localization of Factors in the Chromosomes</span></p>
+
+<p class="cenhead"><i>The Evidence from Sex Linked Inheritance</i></p>
+
+  <p>When we follow the history of pairs of chromosomes we find that their
+  distribution in successive generations is paralleled by the inheritance
+  of Mendelian characters. This is best shown in the sex chromosomes (fig.
+  57). In the female there are two of these chromosomes that we call the X
+  chromosomes; in the male there are also two but one differs from those of
+  the female in its shape, and in the fact that it carries none of the
+  normal allelomorphs of the mutant factors. It is called the Y
+  chromosome.</p>
+
+  <p>The course followed by the sex chromosomes and that by the characters
+  in the case of sex <!-- Page 119 --><span class="pagenum"><a
+  name="page119"></a>{119}</span>linked inheritance are shown in the next
+  diagram of Drosophila illustrating a cross between a white eyed male and
+  a red eyed female.</p>
+
+  <div class="figcenter" style="width:36%;">
+      <a href="images/fig_057.jpg"><img style="width:100%" src="images/fig_057.jpg"
+      alt="Fig. 57." title="Fig. 57." /></a>
+    <p class="poem"><span class="sc">Fig. 57.</span> Scheme of sex
+    determination in Drosophila type. Each <i>mature</i> egg contains one
+    X, each mature sperm contains one X, or a Y chromosome. Chance union of
+    any egg with any sperm will give either XX (female) or XY (male).</p>
+  </div>
+
+<p><!-- Page 120 --><span class="pagenum"><a name="page120"></a>{120}</span></p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/fig_058.jpg"><img style="width:100%" src="images/fig_058.jpg"
+      alt="Fig. 58." title="Fig. 58." /></a>
+    <p class="poem"><span class="sc">Fig. 58.</span> Cross between white
+    eyed male of D. ampelophila and red eyed female. The sex chromosomes
+    are indicated by the rods. A black rod indicates that the chromosome
+    carries the factor for red; the open chromosome the factor for white
+    eye color.</p>
+  </div>
+
+<p><!-- Page 121 --><span class="pagenum"><a name="page121"></a>{121}</span></p>
+
+  <p>The first of these represents a cross between a white eyed male and a
+  red eyed female (fig. 58, top row). The X chromosome in the male is
+  represented by an open bar, the Y chromosome is bent. In the female the
+  two X chromosomes are black. Each egg of such a female will contain one
+  "black" X after the polar bodies have been thrown off. In the male there
+  will be two classes of sperm&mdash;the female-producing, carrying the
+  (open) X, and the male-producing, carrying the Y chromosome. Any egg
+  fertilized by an X bearing sperm will produce a female that will have red
+  eyes because the X (black) chromosome it gets from the mother carries the
+  dominant factor for red. Any egg fertilized by a Y-bearing sperm will
+  produce a male that will also have red eyes because he gets his (black) X
+  chromosome from his mother.</p>
+
+<p><!-- Page 122 --><span class="pagenum"><a name="page122"></a>{122}</span></p>
+
+  <p>When, then, these two F<sub>1</sub> flies (second row) are inbred the
+  following combinations are expected. Each egg will contain a black X (red
+  eye producing) or a white X (white eye producing) after the polar bodies
+  have been extruded. The male will produce two kinds of sperms, of which
+  the female producing will contain a black X (red eye producing). Since
+  any egg may by chance be fertilized by any sperm there will result the
+  four classes of individuals shown on the bottom row of the diagram. All
+  the females will have red eyes, because irrespective of the two kinds of
+  eggs involved all the female-producing sperm carry a black X. Half of the
+  males have red eyes because half of the eggs have had each a
+  red-producing X chromosome. The other half of the males have white eyes,
+  because the other half of the eggs had each a white-producing X
+  chromosome. Other evidence has shown that the Y chromosome of the male is
+  indifferent, so far as these Mendelian factors are concerned.</p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_059.jpg"><img style="width:100%" src="images/fig_059.jpg"
+      alt="Fig. 59." title="Fig. 59." /></a>
+    <p class="poem"><span class="sc">Fig. 59.</span> Cross between red eyed
+    male and white eyed female; reciprocal cross of Fig. 58.</p>
+  </div>
+
+<p><!-- Page 123 --><span class="pagenum"><a name="page123"></a>{123}</span></p>
+
+  <p>The reciprocal experiment is illustrated in figure 59. A white eyed
+  female is mated to a red eyed male (top row). All the mature eggs of such
+  a female contain one white-producing X chromosome represented by the open
+  bar in the diagram. The red eyed male contains female-producing X-bearing
+  sperm that carry the factor for red eye color, and male-producing Y
+  chromosomes. Any egg fertilized by an X-bearing sperm will become a red
+  eyed female because the X chromosome that comes from the father carries
+  the dominant factor for red eye color. Any egg fertilized by a Y-bearing
+  sperm will become a male with white eyes because the only X chromosome
+  that the male contains comes from his mother and is white producing. <!--
+  Page 124 --><span class="pagenum"><a name="page124"></a>{124}</span></p>
+
+  <p>When these two F<sub>1</sub> flies are inbred (middle row) the
+  following combinations are expected. Half the eggs will contain each a
+  white producing X chromosome and half red producing. The female-producing
+  sperms will each contain a white X and the male-producing sperms will
+  each contain an indifferent Y chromosome. Chance meetings of egg and
+  sperm will give the four F<sub>2</sub> classes (bottom row). These
+  consist of white eyed and red eyed females and white eyed and red eyed
+  males. The ratio here is 1:1 and not three to one (3:1) as in other
+  Mendelian cases. But Mendel's law of segregation is not transgressed, as
+  the preceding analysis has shown; for, the chromosomes have followed
+  strictly the course laid down on Mendel's principle for the distribution
+  of factors. The peculiar result in this case is due to the fact that the
+  F<sub>1</sub> male gets his single factor for eye color from his mother
+  only and it is linked to or contained in a body (the X chromosome) that
+  is involved in producing the females, while the mate of this
+  body&mdash;the Y chromosome&mdash;is indifferent with regard to these
+  factors, yet active as a mate to X in synapsis. <!-- Page 125 --><span
+  class="pagenum"><a name="page125"></a>{125}</span></p>
+
+  <div class="figcenter" style="width:22%;">
+      <a href="images/fig_060.jpg"><img style="width:100%" src="images/fig_060.jpg"
+      alt="Fig. 60." title="Fig. 60." /></a>
+    <p class="poem"><span class="sc">Fig. 60.</span> Diagram of sex
+    determination in type with XX female and XO male (after Wilson).</p>
+  </div>
+
+  <p>In man there are several characters that show exactly this same kind
+  of inheritance. Color blindness, or at least certain kinds of color
+  blindness, appear to follow the same scheme. A color blind father
+  transmits through his daughters his peculiarity to half of his grandsons,
+  but to none of his grand-daughters (fig. 38A). The result is the same as
+  in the case of the white eyed male of Drosophila. Color blind women are
+  rather unusual, which is expected from the method of inheritance of this
+  character, but in the few known cases where such color blind women have
+  married normal husbands the sons have inherited the peculiarity from the
+  mother (fig. 38B). Here again the result is the same as for the similar
+  combination in Drosophila. <!-- Page 126 --><span class="pagenum"><a
+  name="page126"></a>{126}</span></p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_061.jpg"><img style="width:100%" src="images/fig_061.jpg"
+      alt="Fig. 61." title="Fig. 61." /></a>
+    <p class="poem"><span class="sc">Fig. 61.</span> Spermatogenesis in
+    man. There are 47 chromosomes (diploid) in the male. After reduction
+    half of the sperm carry 24 chromosomes (one of which is X) and half
+    carry 23 chromosomes (no X).</p>
+  </div>
+
+  <p>In man the sex formula appears to be XX for the female and XO for the
+  male (fig. 60), and since the relation is essentially the same as that in
+  Drosophila the chromosome explanation is the same. According to von
+  Winiwarter there are 48 chromosomes in the female and 47 in the male
+  (fig. 61). After the extrusion of the polar bodies there are 24
+  chromosomes in the egg. In the male at one of the two maturation <!--
+  Page 127 --><span class="pagenum"><a
+  name="page127"></a>{127}</span>divisions the X chromosome passes to one
+  pole undivided (fig. 61, C). In consequence there are two classes of
+  sperms in man; female producing containing 24 chromosomes, and male
+  producing containing 23 chromosomes. If the factor for color blindness is
+  carried by the X chromosome its inheritance in man works out on the same
+  chromosome scheme and in the same way as does white eye color (or any
+  other sex linked character) in the fly, for the O sperm in man is
+  equivalent to the Y sperm in the fly.</p>
+
+  <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;</p>
+
+  <p>In these cases we have been dealing with a single pair of characters.
+  Let us now take a case where two pairs of sex linked characters enter the
+  cross at the same time, and preferably a case where the two recessives
+  enter the cross from the same parent.</p>
+
+  <p>If a female with white eyes and yellow wings is crossed to a wild male
+  with red eyes and gray wings (fig. 62), the sons are yellow and have
+  white eyes and the daughters are gray and have red eyes. If two
+  F<sub>1</sub> flies are mated they will produce the following classes.
+  <!-- Page 128 --><span class="pagenum"><a
+  name="page128"></a>{128}</span></p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/fig_062.jpg"><img style="width:100%" src="images/fig_062.jpg"
+      alt="Fig. 62." title="Fig. 62." /></a>
+    <p class="poem"><span class="sc">Fig. 62.</span> Cross between a white
+    eyed, yellow winged female of D. ampelophila and a red eyed, gray
+    winged male. Two pairs of sex linked characters, viz., white-red and
+    yellow-gray are involved. (See text.)</p>
+  </div>
+
+<p><!-- Page 129 --><span class="pagenum"><a name="page129"></a>{129}</span></p>
+
+<table class="nobctr" summary="Groups I-IV." title="Groups I-IV.">
+<tr><td class="spacsingle" style="width:25%; text-align:center;"> Yellow<br />White</td><td class="spacsingle" style="width:25%; text-align:center;"> Gray<br />Red</td><td class="spacsingle" style="width:25%; text-align:center;"> Yellow<br />Red</td><td class="spacsingle" style="width:25%; text-align:center;"> Gray<br />White</td></tr>
+<tr><td class="spacsingle" colspan="2" style="text-align:center;"> <a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:3ex; width:6em" alt="brace" /></a><br />99.%</td><td class="spacsingle" colspan="2" style="text-align:center;"> <a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:3ex; width:6em" alt="brace" /></a><br />1.%</td></tr>
+</table>
+
+  <p>Not only have the two grandparental combinations reappeared, but in
+  addition two new combinations, viz., grey white and yellow red. The two
+  original combinations far exceed in numbers the new or exchange
+  combinations. If we follow the history of the X chromosomes we discover
+  that the <i>larger classes</i> of grandchildren appear in accord with the
+  way in which the X chromosomes are transmitted from one generation to the
+  next.</p>
+
+  <p>The <i>smaller classes</i> of grandchildren, the exchange combinations
+  or cross-overs, as we call them, can be explained by the assumption that
+  at some stage in their history an interchange of parts has taken place
+  between the chromosomes. This is indicated in the diagrams.</p>
+
+  <p>The most important fact brought out by the experiment is that the
+  factors that went in together tend to stick together. It makes no
+  difference in what combination the members of <!-- Page 130 --><span
+  class="pagenum"><a name="page130"></a>{130}</span>the two pairs of
+  characters enter, they tend to remain in that combination.</p>
+
+  <p>If one admits that the sex chromosomes carry these factors for the
+  sex-linked characters&mdash;and the evidence is certainly very strong in
+  favor of this view&mdash;it follows necessarily from these facts that at
+  some time in their history there has been an interchange between the two
+  sex chromosomes in the female.</p>
+
+  <p>There are several stages in the conjugation of the chromosomes at
+  which such an interchange between the members of a pair might occur.
+  There is further a small amount of direct evidence, unfortunately very
+  meagre at present, showing that an interchange does actually occur.</p>
+
+  <p>At the ripening period of the germ cell the members of each pair of
+  chromosomes come together (fig. 49, e). In several forms they have been
+  described as meeting at one end and then progressively coming to lie side
+  by side as shown in fig. 63, e, f, g, h, i. At the end of the process
+  they appear to have completely united along their length (fig. 63, j, k,
+  l). It is always a maternal and a paternal <!-- Page 131 --><span
+  class="pagenum"><a name="page131"></a>{131}</span>chromosome that meet in
+  this way and always two of the same kind. It has been observed that as
+  the members of a pair come together they occasionally twist around each
+  other (fig. 63, g, l, and 64, and 65). In consequence a part of one
+  chromosome comes to be now on one side and now on the other side of its
+  mate.</p>
+
+  <div class="figcenter" style="width:32%;">
+      <a href="images/fig_063.jpg"><img style="width:100%" src="images/fig_063.jpg"
+      alt="Fig. 63." title="Fig. 63." /></a>
+    <p class="poem"><span class="sc">Fig. 63.</span> Conjugation of
+    chromosomes (side to side union) in the spermatogenesis of Batracoseps.
+    (After Janssens.)</p>
+  </div>
+
+<p><!-- Page 132 --><span class="pagenum"><a name="page132"></a>{132}</span></p>
+
+  <p>When the chromosomes separate at the next division of the germ cell
+  the part on one side passes to one pole, the part on the other to the
+  opposite pole, (figs. 64 and 65). Whenever the chromosomes do not untwist
+  at this time there must result an interchange of pieces where they were
+  crossed over each other.</p>
+
+  <div class="figcenter" style="width:26%;">
+      <a href="images/fig_064.jpg"><img style="width:100%" src="images/fig_064.jpg"
+      alt="Fig. 64." title="Fig. 64." /></a>
+    <p class="poem"><span class="sc">Fig. 64.</span> Scheme to illustrate a
+    method of crossing over of the chromosomes.</p>
+  </div>
+
+  <p>Janssens has found at the time of separation <!-- Page 133 --><span
+  class="pagenum"><a name="page133"></a>{133}</span>evidence in favor of
+  the view that some such interchange probably takes place.</p>
+
+  <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;</p>
+
+  <p>We find this same process of interchange of characters taking place in
+  each of the other three groups of Drosophila. An example will show this
+  for the Group II.</p>
+
+  <div class="figcenter" style="width:16%;">
+      <a href="images/fig_065.jpg"><img style="width:100%" src="images/fig_065.jpg"
+      alt="Fig. 65." title="Fig. 65." /></a>
+    <p class="poem"><span class="sc">Fig. 65.</span> Scheme to illustrate
+    double crossing over.</p>
+  </div>
+
+  <p>If a black vestigial male is crossed to a gray long-winged female
+  (fig. 66) the offspring are gray long. If an F<sub>1</sub> female is
+  back-crossed to a black vestigial male the following kinds of flies are
+  produced:</p>
+
+<p><!-- Page 134 --><span class="pagenum"><a name="page134"></a>{134}</span></p>
+
+<table class="nobctr" summary="Groups I-IV." title="Groups I-IV.">
+<tr><td class="spacsingle" style="width:25%; text-align:center;"> Black<br />vestigial</td><td class="spacsingle" style="width:25%; text-align:center;"> Gray<br />long</td><td class="spacsingle" style="width:25%; text-align:center;"> Black<br />long</td><td class="spacsingle" style="width:25%; text-align:center;"> Gray<br />vestigial</td></tr>
+<tr><td class="spacsingle" colspan="2" style="text-align:center;"> <a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:3ex; width:6em" alt="brace" /></a><br />83%</td><td class="spacsingle" colspan="2" style="text-align:center;"> <a href="images/$ubrace.png"><img src="images/$ubrace.png" class="middle" style="height:3ex; width:6em" alt="brace" /></a><br />17%</td></tr>
+</table>
+
+  <p>The combinations that entered are more common in the F<sub>2</sub>
+  generations than the cross-over classes, showing that there is linkage of
+  the factors that entered together.</p>
+
+  <p>Another curious fact is brought out if instead of back-crossing the
+  F<sub>1</sub> female we back-cross the F<sub>1</sub> male to a black
+  vestigial female. Their offspring are now of only two kinds, black
+  vestigial and gray long. This means that in the male there is no
+  crossing-over or interchange of pieces. This relation holds not only for
+  the Group II but for all the other groups as well.</p>
+
+  <p>Why interchange takes place in the female of Drosophila and not in the
+  male we do not know at present. We might surmise that when in the male
+  the members of a pair come together they do not twist around each other,
+  hence no crossing-over results.</p>
+
+<p><!-- Page 135 --><span class="pagenum"><a name="page135"></a>{135}</span></p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/fig_066.jpg"><img style="width:100%" src="images/fig_066.jpg"
+      alt="Fig. 66." title="Fig. 66." /></a>
+    <p class="poem"><span class="sc">Fig. 66.</span> Cross between black
+    vestigial and gray long flies. Two pairs of factors involved in the
+    second group. The F<sub>1</sub> female is back crossed (to right) to
+    black vestigial male; and the F<sub>1</sub> male is back crossed to
+    black vestigial female (to left). Crossing over takes place in the
+    F<sub>1</sub> female but not in the F<sub>1</sub> male.</p>
+  </div>
+
+  <p>Crossing-over took place between white and yellow only once in a
+  hundred times. Other characters show different values, but the same value
+  under the same conditions is obtained from the same pair of characters.
+  <!-- Page 136 --><span class="pagenum"><a
+  name="page136"></a>{136}</span></p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/fig_067.jpg"><img style="width:100%" src="images/fig_067.jpg"
+      alt="Fig. 67." title="Fig. 67." /></a>
+    <p class="poem"><span class="sc">Fig. 67.</span> Map of four
+    chromosomes of D. ampelophila locating those factors in each group that
+    have been most fully studied.</p>
+  </div>
+
+<p><!-- Page 137 --><span class="pagenum"><a name="page137"></a>{137}</span></p>
+
+  <p>If we assume that the nearer together the factors lie in the
+  chromosome the less likely is a twist to occur between them, and
+  conversely the farther apart they lie the more likely is a twist to occur
+  between them, we can understand how the linkage is different for
+  different pairs of factors.</p>
+
+  <p>On this basis we have made out chromosomal maps for each chromosome
+  (fig. 67). The diagram indicates those loci that have been most
+  accurately placed.</p>
+
+<p class="cenhead"><i>The Evidence from Interference</i></p>
+
+  <p>There is a considerable body of information that we have obtained that
+  corroborates the location of the factors in the chromosome. This evidence
+  is too technical to take up in any detail, but there is one result that
+  is so important that I must attempt to explain it. If, as I assume,
+  crossing over is brought about by twisting of the chromosomes, and if
+  owing to the material of the chromosomes there is a most frequent
+  distance of internode, then, when crossing over between nodes takes place
+  at same level at a-b in figure 68, the region on <!-- Page 138 --><span
+  class="pagenum"><a name="page138"></a>{138}</span>each side of that
+  point, a to A and b to B, should be protected, so to speak, from further
+  crossing over. This in fact we have found to be the case. No other
+  explanation so far proposed will account for this extraordinary
+  relation.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_068.jpg"><img style="width:100%" src="images/fig_068.jpg"
+      alt="Fig. 68." title="Fig. 68." /></a>
+    <p class="poem"><span class="sc">Fig. 68.</span> Scheme to indicate
+    that when the members of a pair of chromosomes cross (at a-b) the
+    region on each side is protected inversely to the distance from
+    a-b.</p>
+  </div>
+
+  <p>What advantage, may be asked, is there in obtaining numerical data of
+  this kind? It is this:&mdash;whenever a new character appears we need
+  only determine in which of the four groups it lies and its distance from
+  two members within that group. With this information we can predict with
+  a high degree of probability what results it will give with any other
+  member of any group. Thus we can do on paper what would require many
+  months of labor by making the actual experiment. In a word we can predict
+  what will happen in a situation where prediction is impossible without
+  this numerical information. <!-- Page 139 --><span class="pagenum"><a
+  name="page139"></a>{139}</span></p>
+
+<p class="cenhead"><i>The Evidence from Non-Disjunction</i></p>
+
+  <p>In the course of the work on Drosophila exceptions appeared in one
+  strain where certain individuals did not conform to the scheme of sex
+  linked inheritance. For a moment the hypothesis seemed to fail, but a
+  careful examination led to the suspicion that in this strain something
+  had happened to the sex chromosomes. It was seen that if in some way the
+  X chromosomes failed to disjoin in certain eggs, the exceptions could be
+  explained. The analysis led to the suggestion that if the Y chromosome
+  had got into the female line the results would be accounted for, since
+  its presence there would be expected to cause this peculiar
+  non-disjunction of the X chromosomes.</p>
+
+  <p>That this was the explanation was shown when the material was
+  examined. The females that gave these results were found by Bridges to
+  have two X's and a Y chromosome.</p>
+
+  <p>The normal chromosome group of the female is shown in figure 52 and
+  the chromosome group of one of the exceptional females is shown in figure
+  69. In a female of this kind <!-- Page 140 --><span class="pagenum"><a
+  name="page140"></a>{140}</span>there are three sex chromosomes X X Y
+  which are homologous in the sense that in normal individuals the two
+  present are mates and separate at the reduction division. If in the X X Y
+  individual X and X conjugate and separate at reduction and the unmated Y
+  is free to move to either pole of the spindle, two kinds of mature eggs
+  will result, viz., X and XY. If, on the other hand, X and Y conjugate and
+  separate at reduction and the remaining X is free to go to either pole,
+  four kinds of eggs will result&mdash;XY&mdash;X&mdash;XX&mdash;Y. As a
+  total result four kinds of eggs are expected: viz. many XY and X eggs and
+  a few XX and Y eggs.</p>
+
+  <div class="figcenter" style="width:26%;">
+      <a href="images/fig_069.jpg"><img style="width:100%" src="images/fig_069.jpg"
+      alt="Fig. 69." title="Fig. 69." /></a>
+    <p class="poem"><span class="sc">Fig. 69.</span> Figure of the
+    chromosome group of an XXY female, that gives non-disjunction.</p>
+  </div>
+
+  <p>These four kinds of eggs may be fertilized either by female-producing
+  sperms or <!-- Page 141 --><span class="pagenum"><a
+  name="page141"></a>{141}</span>male-producing sperms, as indicated in the
+  diagram (fig. 70).</p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/fig_070.jpg"><img style="width:100%" src="images/fig_070.jpg"
+      alt="Fig. 70." title="Fig. 70." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 70. Scheme showing the
+    results of fertilizing white bearing eggs (4 kinds) resulting from
+    non-disjunction. The upper half of the diagram gives the results when
+    these eggs are fertilized by normal red bearing, female producing
+    sperm, the lower half by normal, male producing sperm.</p>
+  </div>
+
+  <p>If such an XXY female carried white bearing Xs (open X in the
+  figures), and the male <!-- Page 142 --><span class="pagenum"><a
+  name="page142"></a>{142}</span>carried a red bearing X (black X in the
+  figures) it will be seen that there should result an exceptional class of
+  sons that are red, and an exceptional class of daughters that are white.
+  Tests of these exceptions show that they behave subsequently in heredity
+  as their composition requires. Other tests may also be made of the other
+  classes of offspring. Bridges has shown that they fulfill all the
+  requirements predicted. Thus a result that seemed in contradiction with
+  the chromosome hypothesis has turned out to give a brilliant confirmation
+  of that theory both genetically and cytologically.</p>
+
+<p class="cenhead"><span class="sc">How Many Genetic Factors are there in the Germ-plasm of a Single Individual</span></p>
+
+  <p>In passing I invite your attention to a speculation based on our maps
+  of the chromosomes&mdash;a speculation which I must insist does not
+  pretend to be more than a guess but has at least the interest of being
+  the first guess that we have ever been in position to make as to how many
+  factors go towards the makeup of the germ plasm. <!-- Page 143 --><span
+  class="pagenum"><a name="page143"></a>{143}</span></p>
+
+  <p>We have found practically no factors less than .04 of a unit apart. If
+  our map includes the entire length of the chromosomes and if we assume
+  factors are uniformly distributed along the chromosome at distances equal
+  to the shortest distance yet observed, viz. .04, then we can calculate
+  roughly how many hereditary factors there are in Drosophila. The
+  calculation gives about 7500 factors. The reader should be cautioned
+  against accepting the above assumptions as strictly true, for
+  crossing-over values are known to differ according to different
+  environmental conditions (as shown by Bridges for age), and to differ
+  even in different parts of the chromosome as a result of the presence of
+  specific genetic factors (as shown by Sturtevant). Since all the
+  chromosomes except the X chromosomes are double we must double our
+  estimate to give the <i>total</i> number of factors, but the half number
+  is the number of the different kinds of factors of Drosophila. <!-- Page
+  144 --><span class="pagenum"><a name="page144"></a>{144}</span></p>
+
+<p class="cenhead"><span class="sc">Conclusions</span></p>
+
+  <p>I have passed in review a long series of researches as to the nature
+  of the hereditary material. We have in consequence of this work arrived
+  within sight of a result that seemed a few years ago far beyond our
+  reach. The mechanism of heredity has, I think, been
+  discovered&mdash;discovered not by a flash of intuition but as the result
+  of patient and careful study of the evidence itself.</p>
+
+  <p>With the discovery of this mechanism I venture the opinion that the
+  problem of heredity has been solved. We know how the factors carried by
+  the parents are sorted out to the germ cells. The explanation does not
+  pretend to state how factors arise or how they influence the development
+  of the embryo. But these have never been an integral part of the doctrine
+  of heredity. The problems which they present must be worked out in their
+  own field. So, I repeat, the mechanism of the chromosomes offers a
+  satisfactory solution of the traditional problem of heredity.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 145 --><span class="pagenum"><a name="page145"></a>{145}</span></p>
+
+<h3>CHAPTER IV</h3>
+
+<p class="cenhead">SELECTION AND EVOLUTION</p>
+
+  <p>Darwin's Theory of Natural Selection still holds today first place in
+  every discussion of evolution, and for this very reason the theory calls
+  for careful scrutiny; for it is not difficult to show that the expression
+  "natural selection" is to many men a metaphor that carries many meanings,
+  and sometimes different meanings to different men. While I heartily agree
+  with my fellow biologists in ascribing to Darwin himself, and to his
+  work, the first place in biological philosophy, yet recognition of this
+  claim should not deter us from a careful analysis of the situation in the
+  light of work that has been done since Darwin's time.</p>
+
+<p class="cenhead"><span class="sc">The Theory of Natural Selection</span></p>
+
+  <p>In his great book on the <i>Origin of Species</i>, Darwin tried to do
+  two things: first, to show that the evidence bearing on evolution makes
+  <!-- Page 146 --><span class="pagenum"><a
+  name="page146"></a>{146}</span>that explanation probable. No such great
+  body of evidence had ever been brought together before, and it wrought,
+  as we all know, a revolution in our modes of thinking.</p>
+
+  <p>Darwin also set himself the task of showing <i>how</i> evolution might
+  have taken place. He pointed to the influence of the environment, to the
+  effects of use and disuse, and to natural selection. It is to the last
+  theory that his name is especially attached. He appealed to a fact
+  familiar to everyone, that no two individuals are identical and that some
+  of the differences that they show are inherited. He argued that those
+  individuals that are best suited to their environment are the most
+  probable ones to survive and to leave most offspring. In consequence
+  their descendants should in time replace through competition the less
+  well-adapted individuals of the species. This is the process Darwin
+  called natural selection, and Spencer the survival of the fittest.</p>
+
+  <p>Stated in these general terms there is nothing in the theory to which
+  anyone is likely to take exception. But let us examine the argument more
+  critically. <!-- Page 147 --><span class="pagenum"><a
+  name="page147"></a>{147}</span></p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/fig_071.jpg"><img style="width:100%" src="images/fig_071.jpg"
+      alt="Fig. 71." title="Fig. 71." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 71. Series of leaves of a
+    tree arranged according to size. (After de Vries.)</p>
+  </div>
+
+  <p>If we measure, or weigh, or classify any character shown by the
+  individuals of a population, we find differences. We recognize that some
+  of the differences are due to the varied experiences that the individuals
+  have encountered in the course of their lives, i.e. to their environment,
+  but we also recognize that some of the differences may be due to
+  individuals having different inheritances&mdash;different germ plasms.
+  Some familiar examples will help to bring home this relation.</p>
+
+  <p>If the leaves of a tree are arranged according to size (fig. 71), we
+  find a continuous series, but there are more leaves of medium size than
+  extremes. If a lot of beans be sorted out <!-- Page 148 --><span
+  class="pagenum"><a name="page148"></a>{148}</span>according to their
+  weights, and those between certain weights put into cylinders, the
+  cylinders, when arranged according to the size of the beans, will appear
+  as shown in figure 72. An imaginary line running over the tops of the
+  piles will give a curve (fig. 73) that corresponds to the curve of
+  probability (fig. 74).</p>
+
+<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
+
+  <div class="figright" style="width:61%;">
+      <a href="images/fig_072.jpg"><img style="width:100%" src="images/fig_072.jpg"
+      alt="Fig. 72." title="Fig. 72." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 72. Beans put into
+    cylindrical jars according to the sizes of the beans. The jars arranged
+    according to size of contained beans. (After de Vries.)</p>
+  </div>
+
+</td><td style="width:50%; vertical-align:top;">
+
+  <div class="figleft" style="width:61%;">
+      <a href="images/fig_073.jpg"><img style="width:100%" src="images/fig_073.jpg"
+      alt="Fig. 73." title="Fig. 73." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 73. A curve resulting from
+    arrangement of beans according to size. (After de Vries.)</p>
+  </div>
+
+</td></tr></table>
+
+  <p>If we stand men in lines according to their height (fig. 75) we get a
+  similar arrangement. <!-- Page 149 --><span class="pagenum"><a
+  name="page149"></a>{149}</span></p>
+
+<table class="nobctr"><tr><td style="width:50%; vertical-align:top;">
+
+  <div class="figright" style="width:65%;">
+      <a href="images/fig_074.jpg"><img style="width:100%" src="images/fig_074.jpg"
+      alt="Fig. 74." title="Fig. 74." /></a>
+    <span class="sc">Fig.</span> 74. Curve of probability.
+  </div>
+
+</td><td style="width:50%; vertical-align:top;">
+
+  <div class="figleft" style="width:76%;">
+      <a href="images/fig_075.jpg"><img style="width:100%" src="images/fig_075.jpg"
+      alt="Fig. 75." title="Fig. 75." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 75. Students arranged
+    according to size. (After Blakeslee.)</p>
+  </div>
+
+</td></tr></table>
+
+  <p>The differences in size shown by the individual beans or by the
+  individual men are due in part to heredity, in part to the environment
+  <!-- Page 150 --><span class="pagenum"><a
+  name="page150"></a>{150}</span>in which they have developed. This is a
+  familiar fact of almost every-day observation. It is well shown in the
+  following example. In figure 76 the two boys and the two varieties of
+  corn, which they are holding, differ in height. The pedigrees of the boys
+  (fig. 77) make it probable that their height is largely inherited and the
+  two races of corn are known to belong to a tall and a short race
+  respectively. Here, then, the chief effect or difference is due to
+  heredity. On the other hand, if individuals of the same race develop in a
+  favorable environment the result is different from the development in an
+  unfavorable environment, as shown in figure 78. Here to the right the
+  corn is crowded and in consequence dwarfed, while to the left the same
+  kind of corn has had more room to develop and is taller.</p>
+
+<p><!-- Page 151 --><span class="pagenum"><a name="page151"></a>{151}</span></p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_076.jpg"><img style="width:100%" src="images/fig_076.jpg"
+      alt="Fig. 76." title="Fig. 76." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 76. A short and a tall boy
+    each holding a stalk of corn&mdash;one stalk of a race of short corn,
+    the other of tall corn. (After Blakeslee.)</p>
+  </div>
+
+<p><!-- Page 152 --><span class="pagenum"><a name="page152"></a>{152}</span></p>
+
+  <div class="figcenter" style="width:49%;">
+      <a href="images/fig_077.png"><img style="width:100%" src="images/fig_077.png"
+      alt="Fig. 77." title="Fig. 77." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 77. Pedigree of boys shown
+    in Fig. 76. (After Blakeslee.)</p>
+  </div>
+
+  <p>Darwin knew that if selection of particular kinds of individuals of a
+  population takes place the next generation is affected. If the taller men
+  of a community are selected <i>the average</i> of their offspring will be
+  taller than the average of the former population. If selection for
+  tallness again takes place, still taller men will <i>on the average</i>
+  arise. If, amongst these, selection again makes a choice the process
+  would, he thought, continue (fig. 79).</p>
+
+<p><!-- Page 153 --><span class="pagenum"><a name="page153"></a>{153}</span></p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_078.jpg"><img style="width:100%" src="images/fig_078.jpg"
+      alt="Fig. 78." title="Fig. 78." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 78. A race of corn reared
+    under different conditions.</p>
+  </div>
+
+  <p>We now recognize that this statement contains an important truth, but
+  we have found that it contains only a part of the truth. Any one who
+  repeats for himself this kind of selection experiment will find that
+  while his average class will often change in the direction of his
+  selection, the process slows down as a rule rather suddenly (fig. 80). He
+  finds, moreover, that the limits of variability are not necessarily
+  transcended as the process continues even although the average may for a
+  while be increased. More tall men may be produced by selection of this
+  kind, but the tallest men are not necessarily any taller than the tallest
+  in the original population. <!-- Page 154 --><span class="pagenum"><a
+  name="page154"></a>{154}</span></p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_079.jpg"><img style="width:100%" src="images/fig_079.jpg"
+      alt="Fig. 79." title="Fig. 79." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 79. Curves showing how
+    (hypothetically) selection might be supposed to bring about progress in
+    direction of selection. (After Goldschmidt.)</p>
+  </div>
+
+  <p>Selection, then, has not produced anything new, but only more of
+  certain kinds of individuals. Evolution, however, means producing more
+  new things, not more of what already exists.</p>
+
+  <p>Darwin seems to have thought that the range of variation shown by the
+  offspring of a given individual about that type of individual would be as
+  wide as the range shown by the original population (fig. 79), but
+  Galton's work has made it clear that this is not the case in a general or
+  mixed population. If the offspring of individuals continued to show, as
+  Darwin seems to have thought, as wide a range on each side of their
+  parents' size, so to speak, as did the original population, then it would
+  follow that <!-- Page 155 --><span class="pagenum"><a
+  name="page155"></a>{155}</span>selection could slide successive
+  generations along in the direction of selection.</p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/fig_080.jpg"><img style="width:100%" src="images/fig_080.jpg"
+      alt="Fig. 80." title="Fig. 80." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 80. Diagram illustrating
+    the results of selection for extra bristles in D. ampelophila.
+    Selection at first produces decided effects which soon slow down and
+    then cease. (MacDowell.)</p>
+  </div>
+
+  <p>Darwin himself was extraordinarily careful, however, in the statements
+  he made in this connection and it is rather by implication than by actual
+  reference that one can ascribe this <!-- Page 156 --><span
+  class="pagenum"><a name="page156"></a>{156}</span>meaning to his views.
+  His contemporaries and many of his followers, however, appear to have
+  accepted this <i>sliding scale</i> interpretation as the cardinal
+  doctrine of evolution. If this is doubted or my statement is challenged
+  then one must explain why de Vries' mutation theory met with so little
+  enthusiasm amongst the older group of zoölogists and botanists; and one
+  must explain why Johannsen's splendid work met with such bitter
+  opposition from the English school&mdash;the biometricians&mdash;who
+  amongst the post-Darwinian school are assumed to be the lineal
+  descendants of Darwin.</p>
+
+  <p>And in this connection we should not forget that just this sort of
+  process was supposed to take place in the inheritance of use and disuse.
+  What is gained in one generation forms the basis for further gains in the
+  next generation. Now, Darwin not only believed that acquired characters
+  are inherited but turned more and more to this explanation in his later
+  writings. Let us, however, not make too much of the matter; for it is
+  much less important to find out whether Darwin's ideas were vague, than
+  it is to make sure that our own ideas are clear. <!-- Page 157 --><span
+  class="pagenum"><a name="page157"></a>{157}</span></p>
+
+  <p>If I have made several statements here that appear dogmatic let me now
+  attempt to justify them, or at least give the evidence which seems to me
+  to make them probable.</p>
+
+  <p>The work of the Danish botanist, Johannsen, has given us the most
+  carefully analyzed case of selection that has ever been obtained. There
+  are, moreover, special reasons why the material that he used is better
+  suited to give definite information than any other so far studied.
+  Johannsen worked with the common bean, weighing the seeds or else
+  measuring them. These beans if taken from many plants at random give the
+  typical curve of probability (fig. 74). The plant multiplies by
+  self-fertilization. Taking advantage of this fact Johannsen kept the
+  seeds of each plant separate from the others, and raised from them a new
+  generation. When curves were made from these new groups it was found that
+  some of them had different modes from that of the original general
+  population (fig. 81 A-E, bottom group). They are shown in the upper
+  groups (A, B, C, D, E). But do not understand me to say that the
+  offspring of each bean gave a different mode. <!-- Page 158 --><span
+  class="pagenum"><a name="page158"></a>{158}</span></p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_081.jpg"><img style="width:100%" src="images/fig_081.jpg"
+      alt="Fig. 81." title="Fig. 81." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 81. Pure lines of beans.
+    The lower figure gives the general population, the other figures give
+    the pure lines within the population. (After Johannsen.)</p>
+  </div>
+
+<p><!-- Page 159 --><span class="pagenum"><a name="page159"></a>{159}</span></p>
+
+  <p>On the contrary, some of the lines would be the same.</p>
+
+  <p>The result means that the general population is made up of definite
+  kinds of individuals that may have been sorted out.</p>
+
+  <p>That his conclusion is correct is shown by rearing a new generation
+  from any plant or indeed from several plants of any one of these lines.
+  Each line repeats the same modal class. There is no further breaking up
+  into groups. Within the line it does not matter at all whether one
+  chooses a big bean or a little one&mdash;they will give the same result.
+  In a word, the germ plasm in each of these lines is pure, or homozygous,
+  as we say. The differences that we find between the weights (or sizes) of
+  the individual beans are due to external conditions to which they have
+  been subjected.</p>
+
+  <p>In a word, Johannsen's work shows that the frequency distribution of a
+  pure line is due to factors that are extrinsic to the germ plasm. It does
+  not matter then which individuals in a pure line are used to breed from,
+  for they all carry the same germ plasm.</p>
+
+  <p>We can now understand more clearly how <!-- Page 160 --><span
+  class="pagenum"><a name="page160"></a>{160}</span>selection acting on a
+  general population brings about results in the direction of
+  selection.</p>
+
+  <p>An individual is picked out from the population in order to get a
+  particular kind of germ plasm. Although the different classes of
+  individuals may overlap, so that one can not always judge an individual
+  from its appearance, nevertheless on the whole chance favors the picking
+  out of the kind of germ plasm sought.</p>
+
+  <p>In species with separate sexes there is the further difficulty that
+  two individuals must be chosen for each mating, and superficial
+  examination of them does not insure that they belong to the same
+  group&mdash;their germ plasm cannot be inspected. Hence selection of
+  biparental forms is a precarious process, now going forward, now
+  backwards, now standing still. In time, however, the process forward is
+  almost certain to take place if the selection is from a heterogeneous
+  population. Johannsen's work was simplified because he started with pure
+  lines. In fact, had he not done so his work would not have been
+  essentially different from that of any selection experiment of a pure
+  race of animals or plants. Whether Johannsen <!-- Page 161 --><span
+  class="pagenum"><a name="page161"></a>{161}</span>realized the importance
+  of the condition or not is uncertain&mdash;curiously he laid no emphasis
+  on it in the first edition of his "Elemente der exakten
+  Erblichkeitslehre".</p>
+
+  <p>It has since been pointed out by Jennings and by Pearl that a race
+  that reproduces by self-fertilization as does this bean, automatically
+  becomes pure in all of the factors that make up its germ plasm. Since
+  self-fertilization is the normal process in this bean the purity of the
+  germ plasm already existed when Johannsen began to experiment.</p>
+
+<p class="cenhead"><span class="sc">How Has Selection in Domesticated Animals and Plants Brought About Its Results?</span></p>
+
+  <p>If then selection does not bring about transgressive variation in a
+  general population, how can selection produce anything new? If it can not
+  produce anything new, is there any other way in which selection becomes
+  an agent in evolution?</p>
+
+  <p>We can get some light on this question if we turn to what man has done
+  with his domesticated animals and plants. Through selection, <!-- Page
+  162 --><span class="pagenum"><a name="page162"></a>{162}</span>i.e.,
+  artificial selection, man has undoubtedly brought about changes as
+  remarkable as any shown by wild animals and plants. We know, moreover, a
+  good deal about how these changes have been wrought.</p>
+
+  <p>(1) By crossing different wild species or by crossing wild with races
+  already domesticated new combinations have been made. Parts of one
+  individual have been combined with parts of others, creating new
+  combinations. It is possible even that characters that are entirely new
+  may be produced by the interaction of factors brought into
+  recombination.</p>
+
+  <p>(2) New characters appear from time to time in domesticated and in
+  wild species. These, like the mutants in Drosophila, are fully equipped
+  at the start. Since they breed true and follow Mendel's laws it is
+  possible to combine them with characters of the wild type or with those
+  of other mutant races.</p>
+
+  <p>Amongst the new mutant factors there may be some whose chief effect is
+  on the character that the breeder is already selecting. Such a
+  modification will be likely to attract attention. Superficially it may
+  appear that the <!-- Page 163 --><span class="pagenum"><a
+  name="page163"></a>{163}</span>factor for the original character has
+  varied, while the truth may be that another factor has appeared that has
+  modified a character already present. In fact, many or all Mendelian
+  factors that affect the same organ may be said to be modifiers of each
+  other's effects. Thus the factor for vermilion causes the eye to be one
+  color, and the factor for eosin another color, while eosin vermilion is
+  different from both. Eosin may be said to be a modifier of vermilion or
+  vermilion of eosin. In general, however, it is convenient to use the term
+  "modifier" for cases in which the factor causes a detectable change in a
+  character already present or conspicuous.</p>
+
+<p><!-- Page 164 --><span class="pagenum"><a name="page164"></a>{164}</span></p>
+
+  <div class="figcenter" style="width:41%;">
+      <a href="images/fig_082.jpg"><img style="width:100%" src="images/fig_082.jpg"
+      alt="Fig. 82." title="Fig. 82." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 82. Scheme to indicate
+    influence of the modifying factors, cream and whiting. Neither produces
+    any effect alone but they modify other eye colors such as eosin.</p>
+  </div>
+
+  <p>One of the most interesting, and at the same time most treacherous,
+  kinds of modifying factors is that which produces an effect <i>only</i>
+  when some other factor is present. Thus Bridges has shown that there is a
+  factor called "cream" that does not affect the red color of the eye of
+  the wild fly, yet makes "eosin" much paler (fig. 82). Another factor
+  "whiting" which produces no effect on red makes eosin entirely white.
+  Since cream or whiting may be carried by red eyed flies without their
+  presence being seen until eosin is used, the experimenter must be
+  continually on the lookout for such factors which may lead to erroneous
+  conclusions unless detected. As yet breeders have not realized the
+  important rôle that modifiers have played in their results, but there are
+  indications at least that the heaping up of modifying factors has been
+  one of the ways in which <!-- Page 165 --><span class="pagenum"><a
+  name="page165"></a>{165}</span>highly specialized domesticated animals
+  have been produced. Selection has accomplished this result not by
+  changing factors, but by picking up modifying factors. The demonstration
+  of the presence of these factors has already been made in some cases.
+  Their study promises to be one of the most instructive fields for further
+  work bearing on the selection hypothesis.</p>
+
+  <p>In addition to these well recognized methods by which artificial
+  selection has produced new things we come now to a question that is the
+  very crux of the selection theory today. Our whole conception of
+  selection turns on the answer that we give to this matter and if I appear
+  insistent and go into some detail it is because I think that the matter
+  is worth very careful consideration.</p>
+
+<p class="cenhead"><span class="sc">Are Factors Changed Through Selection?</span></p>
+
+  <p>As we have seen, the variation that we find from individual to
+  individual is due in part to the environment; this can generally be
+  demonstrated. Other differences in an <!-- Page 166 --><span
+  class="pagenum"><a name="page166"></a>{166}</span>ordinary population are
+  recognized as due to different genetic (hereditary) combinations. No one
+  will dispute this statement. But is all the variability accounted for in
+  these two ways? May not a factor itself fluctuate? Is it not <i>a
+  priori</i> probable that factors do fluctuate? Why, in a word, should we
+  regard factors as inviolate when we see that everything else in organisms
+  is more or less in amount? I do not know of any <i>a priori</i> reason
+  why a factor may not fluctuate, unless it is, as I like to think, a
+  chemical molecule. We are, however, dealing here not with generalities
+  but with evidence, and there are three known methods by means of which it
+  has been shown that variability, other than environmental or
+  recombinational, is not due to variability in a factor, nor to various
+  "potencies" possessed by the same factors.</p>
+
+  <p>(1) By making the stock uniform for all of its factors&mdash;chief
+  factors and modifiers alike. Any change in such a stock produced by
+  selection would then be due to a change in one or more of the factors
+  themselves. Johannsen's experiment is an example of this sort.</p>
+
+  <div class="figcenter" style="width:13%;">
+      <a href="images/fig_083a.jpg"><img style="width:100%" src="images/fig_083a.jpg"
+      alt="Fig. 83a." title="Fig. 83a." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 83 a. Drosophila
+    ampelophila with truncate wings.</p>
+  </div>
+
+  <p>(2) The second method is one that is <!-- Page 167 --><span
+  class="pagenum"><a name="page167"></a>{167}</span>capable of
+  <i>demonstrating</i> that the effects of selection are actually due to
+  modifiers. It has been worked out in our laboratory, chiefly by Muller,
+  and used in a particular case to demonstrate that selection produced its
+  effect by isolating modifying factors. For example, a mutant type called
+  truncate appeared, characterized by shorter wings, usually square at the
+  end, (fig. 83a). The wings varied from those of normal length to wings
+  much shorter (fig. 83b). For three years the mutant stock was <!-- Page
+  168 --><span class="pagenum"><a name="page168"></a>{168}</span>bred from
+  individuals having the shorter wings until at last a stock was obtained
+  in which some of the individuals had wings much shorter than the body. By
+  means of linkage experiments it was shown that at least three factors
+  were present that modified the wings. These were isolated by means of
+  their linkage relations, and their mutual influence on the production of
+  truncate wings was shown.</p>
+
+  <div class="figcenter" style="width:32%;">
+      <a href="images/fig_083b.jpg"><img style="width:100%" src="images/fig_083b.jpg"
+      alt="Fig. 83b." title="Fig. 83b." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 83 b. Series of wings of
+    different length shown by truncate stock of D. ampelophila.</p>
+  </div>
+
+<p><!-- Page 169 --><span class="pagenum"><a name="page169"></a>{169}</span></p>
+
+  <p>An experiment of this kind can only be carried out in a case where the
+  groups of linked gens are known. At present Drosophila is the only animal
+  (or plant) sufficiently well known to make this test possible, but this
+  does not prove that the method is of no value. On the contrary it shows
+  that any claim that factors can themselves be changed can have no
+  finality until the claim can be tested out by means of the linkage test.
+  For instance, bar eye (fig. 31) arose as a mutation. All our stock has
+  descended from a single original mutant. But Zeleny has shown that
+  selection within our stock will make the bar eye narrower or broader
+  according to the direction of selection. It remains to be shown in this
+  case how selection has produced its effects, and this can be done by
+  utilizing the same process that was used in the case of truncate.</p>
+
+  <p>Another mutant stock called beaded (fig. 84), has been bred for five
+  years and selected for wings showing more beading. In extreme cases the
+  wings have been reduced to mere stumps (see stumpy, fig. 5), but the
+  stock shows great variability. It is probable here <!-- Page 170 --><span
+  class="pagenum"><a name="page170"></a>{170}</span>as Dexter has shown,
+  that a number of mutant factors that act as modifiers have been picked up
+  in the course of the selection, and when it is recalled that during those
+  five years over 125 new characters have appeared elsewhere it does not
+  seem improbable that factors also have appeared that modify the wings of
+  this stock.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/fig_084.jpg"><img style="width:100%" src="images/fig_084.jpg"
+      alt="Fig. 84." title="Fig. 84." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 84. Two flies showing
+    beaded wings.</p>
+  </div>
+
+  <p>(3) The third method is one that has been developed principally by
+  East for plants; also by MacDowell for rabbits and flies. The <!-- Page
+  171 --><span class="pagenum"><a name="page171"></a>{171}</span>method
+  does not claim to prove that modifiers are present, but it shows why
+  certain results are in harmony with that expectation and can not be
+  accounted for on the basis that a factor has changed. Let me give an
+  example. When a Belgian hare with large body was crossed to a common
+  rabbit with a small body the hybrid was intermediate in size. When the
+  hybrid was crossed back to the smaller type it produced rabbits of
+  various sizes in apparently a continuous series. MacDowell made
+  measurements of the range of variability in the first and in the second
+  generations.</p>
+
+<p class="cenhead"><i>Classification in relation to parents based on skull lengths and ulna
+lengths, to show the relative variability of two measurements and
+of the first generation (F<sub>1</sub>) and the back cross (B. C.)</i></p>
+
+<table class="allbctr" summary="Results of MacDowell" title="Results of MacDowell">
+<tr><td class="allb" colspan="2"> CHARACTER </td><td class="allb" style="text-align:right;"> GENERATION </td><td class="allb" style="text-align:right;"> -13</td><td class="allb" style="text-align:right;"> -12</td><td class="allb" style="text-align:right;"> -11</td><td class="allb" style="text-align:right;"> -10</td><td class="allb" style="text-align:right;"> -9</td><td class="allb" style="text-align:right;"> -8</td><td class="allb" style="text-align:right;"> -7</td><td class="allb" style="text-align:right;"> -6</td><td class="allb" style="text-align:right;"> -5</td><td class="allb" style="text-align:right;"> -4</td><td class="allb" style="text-align:right;"> -3</td><td class="allb" style="text-align:right;"> -2</td><td class="allb" style="text-align:right;"> -1</td><td class="allb" style="text-align:right;"> 0</td><td class="allb" style="text-align:right;"> 1</td><td class="allb" style="text-align:right;"> 2</td><td class="allb" style="text-align:right;"> 3</td><td class="allb" style="text-align:right;"> 4</td><td class="allb" style="text-align:right;"> 5</td></tr>
+<tr><td class="nob" style="text-align:center;"> Length of </td><td class="rightb" rowspan="2" style="vertical-align:middle;"> <a href="images/$lbrace.png"><img src="images/$lbrace.png" class="middle" style="height:5ex; width:0.7em" alt="brace" /></a></td><td class="nob" style="text-align:center;"> F<sub>1</sub> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td></tr>
+<tr><td class="nob" style="text-align:center;"> skull </td><td class="nob" style="text-align:center;"> B.C. </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 6</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 13</td><td class="vertb" style="text-align:right;"> 18</td><td class="vertb" style="text-align:right;"> 42</td><td class="vertb" style="text-align:right;"> 32</td></tr>
+<tr><td class="nob" style="text-align:center;"> Length of </td><td class="rightb" rowspan="2" style="vertical-align:middle;"> <a href="images/$lbrace.png"><img src="images/$lbrace.png" class="middle" style="height:5ex; width:0.7em" alt="brace" /></a></td><td class="nob" style="text-align:center;"> F<sub>1</sub> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> </td></tr>
+<tr><td class="nob" style="text-align:center;"> ulna </td><td class="nob" style="text-align:center;"> B.C. </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 12</td><td class="vertb" style="text-align:right;"> 11</td><td class="vertb" style="text-align:right;"> 20</td><td class="vertb" style="text-align:right;"> 26</td><td class="vertb" style="text-align:right;"> 17</td><td class="vertb" style="text-align:right;"> 19</td></tr>
+</table>
+
+<p class="cenhead"><i>same table continued</i></p>
+
+<table class="allbctr" summary="Results of MacDowell" title="Results of MacDowell">
+<tr><td class="allb" colspan="2"> CHARACTER </td><td class="allb" style="text-align:right;"> GENERATION </td><td class="allb" style="text-align:right;"> 6</td><td class="allb" style="text-align:right;"> 7</td><td class="allb" style="text-align:right;"> 8</td><td class="allb" style="text-align:right;"> 9</td><td class="allb" style="text-align:right;"> 10</td><td class="allb" style="text-align:right;"> 11</td><td class="allb" style="text-align:right;"> 12</td><td class="allb" style="text-align:right;"> 13</td><td class="allb" style="text-align:right;"> 14</td><td class="allb" style="text-align:right;"> 15</td><td class="allb" style="text-align:right;"> 16</td><td class="allb" style="text-align:right;"> 17</td><td class="allb" style="text-align:right;"> 18</td><td class="allb" style="text-align:right;"> 19</td><td class="allb" style="text-align:right;"> 20</td><td class="allb" style="text-align:right;"> 21</td><td class="allb" style="text-align:right;"> 22</td><td class="allb" style="text-align:right;"> 23</td><td class="allb" style="text-align:right;"> 24</td><td class="allb" style="text-align:right;"> 25</td></tr>
+<tr><td class="nob" style="text-align:center;"> Length of </td><td class="rightb" rowspan="2" style="vertical-align:middle;"> <a href="images/$lbrace.png"><img src="images/$lbrace.png" class="middle" style="height:5ex; width:0.7em" alt="brace" /></a></td><td class="nob" style="text-align:center;"> F<sub>1</sub> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 8</td><td class="vertb" style="text-align:right;"> 5</td><td class="vertb" style="text-align:right;"> 10</td><td class="vertb" style="text-align:right;"> 7</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td></tr>
+<tr><td class="nob" style="text-align:center;"> skull </td><td class="nob" style="text-align:center;"> B.C. </td><td class="vertb" style="text-align:right;"> 38</td><td class="vertb" style="text-align:right;"> 34</td><td class="vertb" style="text-align:right;"> 16</td><td class="vertb" style="text-align:right;"> 16</td><td class="vertb" style="text-align:right;"> 8</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td></tr>
+<tr><td class="nob" style="text-align:center;"> Length of </td><td class="rightb" rowspan="2" style="vertical-align:middle;"> <a href="images/$lbrace.png"><img src="images/$lbrace.png" class="middle" style="height:5ex; width:0.7em" alt="brace" /></a></td><td class="nob" style="text-align:center;"> F<sub>1</sub> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 5</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 7</td><td class="vertb" style="text-align:right;"> 3</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td></tr>
+<tr><td class="nob" style="text-align:center;"> ulna </td><td class="nob" style="text-align:center;"> B.C. </td><td class="vertb" style="text-align:right;"> 18</td><td class="vertb" style="text-align:right;"> 15</td><td class="vertb" style="text-align:right;"> 12</td><td class="vertb" style="text-align:right;"> 13</td><td class="vertb" style="text-align:right;"> 15</td><td class="vertb" style="text-align:right;"> 11</td><td class="vertb" style="text-align:right;"> 5</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 4</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> 2</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> 1</td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td><td class="vertb" style="text-align:right;"> </td></tr>
+</table>
+
+  <p>He found that the variability was smaller in the first generation than
+  in the second <!-- Page 172 --><span class="pagenum"><a
+  name="page172"></a>{172}</span>generation (back cross). This is what is
+  expected if several factor-differences were involved, because the hybrids
+  of the first generation are expected to be more uniform in factorial
+  composition than are those in the second generation which are produced by
+  recombination of the factors introduced through their grandparents.
+  Excellent illustrations of the same kinds of results have been found in
+  Indian corn. As shown in figure 85 the length of the cob in F<sub>1</sub>
+  is intermediate between the parent types while in F<sub>2</sub> the range
+  is wider and both of the original types are recovered. East states that
+  similar relations have been found for 18 characters in corn. Emerson has
+  recently furnished further illustrations of the same relations in the
+  length of stalks in beans.</p>
+
+<p><!-- Page 173 --><span class="pagenum"><a name="page173"></a>{173}</span></p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_085.jpg"><img style="width:100%" src="images/fig_085.jpg"
+      alt="Fig. 85." title="Fig. 85." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 85. Cross between two
+    races of Indian corn, one with short cobs and one with long cobs. The
+    range of variability in F<sub>1</sub> is less than that in
+    F<sub>2</sub>. (After East.)</p>
+  </div>
+
+<p><!-- Page 174 --><span class="pagenum"><a name="page174"></a>{174}</span></p>
+
+  <p>A similar case is shown by a cross between fantail and common pigeons
+  (fig. 86). The latter have twelve feathers in the tail, while the
+  selected race from which the fantails came had between 28 and 38 feathers
+  in the tail. The F<sub>1</sub> offspring (forty-one individuals) showed
+  (fig. 87) between 12 and 20 tail feathers, while in F<sub>2</sub> the
+  numbers varied between 12 and 25. Here one of the grand-parental types
+  reappears in large numbers, while the extreme of the other grand-parental
+  type did not reappear (in the counts obtained), although the
+  F<sub>2</sub> number would probably overlap the lower limits of the race
+  of fantail grandparents had not a selected (surviving) lot been taken for
+  the figures given in the table.</p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_086.jpg"><img style="width:100%" src="images/fig_086.jpg"
+      alt="Fig. 86." title="Fig. 86." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 86. Cross of pigeon with
+    normal tail P<sub>1</sub> and fantail P<sub>1</sub>; F<sub>1</sub>,
+    bird below.</p>
+  </div>
+
+<p><!-- Page 175 --><span class="pagenum"><a name="page175"></a>{175}</span></p>
+
+  <div class="figcenter" style="width:35%;">
+      <a href="images/fig_087.jpg"><img style="width:100%" src="images/fig_087.jpg"
+      alt="Fig. 87." title="Fig. 87." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 87. Cross of normal and
+    fantail pigeons. (See Fig. 86.) The F<sub>2</sub> range is wider than
+    that of F<sub>1</sub>. The normal grand-parental type of 12 feathers
+    was recovered in F<sub>2</sub> but the higher numbers characteristic of
+    fantails were not recovered.</p>
+  </div>
+
+<p><!-- Page 176 --><span class="pagenum"><a name="page176"></a>{176}</span></p>
+
+  <p>The preceding account attempts to point out how I should prefer to
+  interpret the problem of selection in the light of the most recent work
+  on breeding. But I would give a very incomplete account of the whole
+  situation if I neglected to include some important work which has led
+  some of my fellow-workers to a very different conclusion.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_088.jpg"><img style="width:100%" src="images/fig_088.jpg"
+      alt="Fig. 88." title="Fig. 88." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 88. Scheme to show classes
+    of hooded rats used by Castle. (After Castle.)</p>
+  </div>
+
+  <p>Castle in particular is the champion of a view based on his results
+  with hooded rats. Starting with individuals which have a narrow black
+  stripe down the back he selected for a narrower stripe in one direction
+  and for a <!-- Page 177 --><span class="pagenum"><a
+  name="page177"></a>{177}</span>broader stripe in the other. As the
+  diagram shows (fig. 88) Castle has succeeded in producing in one
+  direction a race in which the dorsal stripe has disappeared and in the
+  other direction a race in which the black has extended over the back and
+  sides, leaving only a white mark on the belly. Neither of these extremes
+  occurs, he believes, in the ordinary hooded race of domesticated rats. In
+  other words no matter how many of them came under observation the extreme
+  types of his experiment would not be found.</p>
+
+  <p>Castle claims that the factor for hoodedness must be a single
+  Mendelian unit, because if hooded rats are crossed to wild gray rats with
+  uniform coat and their offspring are inbred there are produced in
+  F<sub>2</sub> three uniform rats to one hooded rat. Castle advances the
+  hypothesis that factors&mdash;by which he means Mendelian
+  factors&mdash;may themselves vary in much the same way as do the
+  characters that they stand for. He argues, in so many words, that since
+  we judge a factor by the kind of character it produces, when the
+  character varies the factor that stands for it may have changed. <!--
+  Page 178 --><span class="pagenum"><a name="page178"></a>{178}</span></p>
+
+  <p>As early as 1903 Cuénot had carried out experiments with spotted mice
+  similar to those of Castle with rats. Cuénot found that spotted crossed
+  to uniform coat color gave in F<sub>2</sub> a ratio of three uniform to
+  one spotted, yet selection of those spotted mice with more white in their
+  coat produced mice in successive generations that had more and more
+  white. Conversely Cuénot showed that selection of those spotted mice that
+  had more color in their coat produced mice with more and more color and
+  less white. Cuénot does not however bring up in this connection the
+  question as to how selection in these spotted mice brings about its
+  results.</p>
+
+  <p>Without attempting to discuss these results at the length that they
+  deserve let me briefly state why I think Castle's evidence fails to
+  establish his conclusion.</p>
+
+  <p>In the first place one of the premises may be wrong. The three to one
+  ratio in F<sub>2</sub> by no means proves that all conditions of
+  hoodedness are due to one factor. The result shows at most that one
+  factor that gives the hooded types is a simple Mendelian factor. The
+  changes in this type may be caused by modifying factors <!-- Page 179
+  --><span class="pagenum"><a name="page179"></a>{179}</span>that can show
+  an effect only when hoodedness is itself present. That this is not an
+  imaginary objection but a real one is shown by an experiment that Castle
+  himself made which furnishes the ground for the second objection.</p>
+
+  <p>Second. If the factor has really changed its potency, then if a very
+  dark individual from one end of the series is crossed to a wild rat and
+  the second generation raised we should expect that the hooded
+  F<sub>2</sub> rats would all be dark like their dark grandparent. When
+  Castle made this test he found that there were many grades of hooded rats
+  in the F<sub>2</sub> progeny. They were darker, it is true, as a group
+  than were the original hooded group at the beginning of the selection
+  experiment, but they gave many intermediate grades. Castle attempts to
+  explain this by the assumption that the factor made pure by selection
+  became contaminated by its normal allelomorph in the F<sub>1</sub>
+  parent, but not only does this assumption appear to beg the whole
+  question, but it is in flat contradiction with what we have observed in
+  hundreds of Mendelian cases where no evidence for such a contamination
+  exists. <!-- Page 180 --><span class="pagenum"><a
+  name="page180"></a>{180}</span></p>
+
+  <p>Later Castle crossed some of the extracted rats of average grade
+  (3.01) from the plus series to the same wild race and got F<sub>2</sub>
+  hooded rats from this cross. These F<sub>2</sub> hooded rats did not
+  further approach the ordinary range but were nearer the extreme selected
+  plus hooded rats (3.33) than were the F<sub>2</sub>'s extracted from the
+  first cross (2.59). Castle concludes from this that multiple factors can
+  not account for the result. As a matter of fact, Castle's evidence <i>as
+  published</i> does not establish his conclusion because the wild rats
+  used in the second experiment may have carried plus modifiers. This could
+  only be determined by suitable tests which Castle does not furnish. This
+  is the crucial point, without which the evidence carries no
+  conviction.</p>
+
+  <p>Furthermore, from Castle's point of view, these latest results would
+  seem to increase the difficulty of interpretation of his first
+  F<sub>2</sub> extracted cross, and it is now the first result that calls
+  for explanation if one accepts his later conclusion.</p>
+
+  <p>These and other objections that might be taken up show, I think, that
+  Castle's <!-- Page 181 --><span class="pagenum"><a
+  name="page181"></a>{181}</span>experiment with hooded rats fails entirely
+  to establish his contention of change in potency of the germ or of
+  contamination of factors, while on the contrary they are in entire accord
+  with the view that he is dealing with a case of modifying factors.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/fig_089.jpg"><img style="width:100%" src="images/fig_089.jpg"
+      alt="Fig. 89." title="Fig. 89." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 89. Races of Paramecium.
+    (After Jennings.)</p>
+  </div>
+
+  <p>Equally important are the results that Jennings has obtained with
+  certain protozoa. Paramecium multiplies by dividing across in the <!--
+  Page 182 --><span class="pagenum"><a
+  name="page182"></a>{182}</span>middle, each half replacing its lacking
+  part. Both the small nucleus (micronucleus) and the large nucleus
+  (macronucleus) divide at each division of the body. Jennings found that
+  while individuals descended from a single paramecium vary in size (fig.
+  89), yet the population from a large individual is the same as the
+  population derived from a small individual. In other words, selection
+  produces no result and the probable explanation is, of course, that the
+  different sizes of individuals are due to the environment, while the
+  constancy of the type is genetic. Jennings found a number of races of
+  paramecium of different sizes living under natural conditions. The
+  largest individual of a small race might overlap the smallest individual
+  of other larger races (fig. 89); nevertheless each kind reproduced its
+  particular race. The results are like those of Johannsen in a general
+  way, but differ in that reproduction takes place in paramecium by direct
+  division instead of through self-fertilization as in beans, and also in
+  that the paramecia were probably not homozygous. Since, however, so far
+  as known no "reduction" takes place in <!-- Page 183 --><span
+  class="pagenum"><a name="page183"></a>{183}</span>paramecium at each
+  division, the genetic composition of parent and offspring should be the
+  same. Whether pseudo-parthenogenesis that Woodruff and Erdmann have found
+  occurring in paramecium at intervals involves a redistribution of the
+  hereditary factors is not clear. Jennings's evidence seems incompatible
+  with such a view.</p>
+
+  <div class="figcenter" style="width:32%;">
+      <a href="images/fig_090.jpg"><img style="width:100%" src="images/fig_090.jpg"
+      alt="Fig. 90." title="Fig. 90." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 90. Stylonychia showing
+    division into two. (After Stein.)</p>
+  </div>
+
+  <p>More recently one of Jennings's students, Middleton, has made a
+  careful series of selection experiments with Stylonychia (fig. 90) in
+  which he selected for lines showing more rapid <!-- Page 184 --><span
+  class="pagenum"><a name="page184"></a>{184}</span>or slower rates of
+  division. His observations seem to show that his selection separated two
+  such lines that came from the same original stock. The rapidity of the
+  effects of selection seems to preclude the explanation that
+  pseudo-parthenogenesis has complicated the results. Nevertheless, the
+  results are of such a kind as to suggest that they were due to selection
+  of vegetative (somatic) differences and that no genetic change of factors
+  was involved, for his conclusion that the rapidity with which the effects
+  gained by long selection might be suddenly reversed when selection was
+  reversed is hardly consistent with an interpretation of the results based
+  on changes in the "potencies" of the factors present.</p>
+
+  <p>Equally striking are the interesting experiments that Jennings has
+  recently carried out with Difflugia (fig. 91). This protozoon secretes a
+  shell about itself which has a characteristic shape, and often carries
+  spines. The opening at one end of the shell through which the protoplasm
+  protrudes to make the pseudopodia is surrounded by a rim having a
+  characteristic pattern. The protoplasm contains <!-- Page 185 --><span
+  class="pagenum"><a name="page185"></a>{185}</span>several nuclei and in
+  addition there is scattered material or particles called chromidia that
+  are supposed to be chromatic in nature and related to the material of the
+  nuclei, possibly by direct interchange.</p>
+
+  <div class="figcenter" style="width:19%;">
+      <a href="images/fig_091.jpg"><img style="width:100%" src="images/fig_091.jpg"
+      alt="Fig. 91." title="Fig. 91." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 91. Difflugia Corona.
+    (After Cash.)</p>
+  </div>
+
+  <p>When Difflugia divides, part of the protoplasm protrudes from the
+  opening and a new shell is secreted about this mass which becomes a
+  daughter individual. The behavior of the nucleus and of the chromidia at
+  this time is obscure, but there is some evidence that their materials may
+  be irregularly distributed <!-- Page 186 --><span class="pagenum"><a
+  name="page186"></a>{186}</span>between parent and offspring. If this is
+  correct, and if in the protozoa the chromatin has the same influence that
+  it seems to have in higher animals, the mode of reproduction in Difflugia
+  would be expected to give little more than random sampling of the germ
+  plasm.</p>
+
+  <div class="figcenter" style="width:16%;">
+      <a href="images/fig_092.jpg"><img style="width:100%" src="images/fig_092.jpg"
+      alt="Fig. 92." title="Fig. 92." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 92. Races of Difflugia.
+    (After Leidy.)</p>
+  </div>
+
+  <p>Jennings was able by means of selection to get from the descendants of
+  one original individual a number of different types that themselves bred
+  true, except in so far as selection could affect another change in them.
+  In this connection it is interesting to note that Leidy <!-- Page 187
+  --><span class="pagenum"><a name="page187"></a>{187}</span>has published
+  figures of Difflugia (fig. 92) that show that a great many "types" exist.
+  If through sexual union (a process that occurs in Difflugia) the germ
+  plasm (chromatin) of these wild types has in times past been recombined,
+  then selection would be expected to separate certain types again, if, at
+  division, irregular sampling of the germ plasm takes place. Until these
+  points are settled the bearing of these important experiments of Jennings
+  on the general problem of selection is uncertain.</p>
+
+<p class="cenhead"><span class="sc">How Does Natural Selection Influence the Course of Evolution?</span></p>
+
+  <p>The question still remains: Does selection play any rôle in evolution,
+  and, if so, in what sense? Does the elimination of the unfit influence
+  the course of evolution, except in the negative sense of leaving more
+  room for the fit? There is something further to be said in this
+  connection, although opinions may differ as to whether the following
+  interpretation of the term "natural selection" is the only possible
+  one.</p>
+
+<p><!-- Page 188 --><span class="pagenum"><a name="page188"></a>{188}</span></p>
+
+  <div class="figcenter" style="width:25%;">
+      <a href="images/fig_093.jpg"><img style="width:100%" src="images/fig_093.jpg"
+      alt="Fig. 93." title="Fig. 93." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 93. Evolution of
+    elephant's skulls. (After Dendy.)</p>
+  </div>
+
+  <p>If through a mutation a character appears that is neither advantageous
+  nor disadvantageous, but indifferent, the chance that it may become
+  established in the race is extremely small, although by good luck such a
+  thing may occur rarely. It makes no difference whether the character in
+  question is a dominant or a <!-- Page 189 --><span class="pagenum"><a
+  name="page189"></a>{189}</span>recessive one, the chance of its becoming
+  established is exactly the same. If through a mutation a character
+  appears that has an <i>injurious</i> effect, however slight this may be,
+  it has practically no chance of becoming established.</p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_094.jpg"><img style="width:100%" src="images/fig_094.jpg"
+      alt="Fig. 94." title="Fig. 94." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 94. Evolution of
+    elephant's trunk. (After Lull.)</p>
+  </div>
+
+  <p>If through a mutation a character appears that has a <i>beneficial</i>
+  influence on the individual, the chance that the individual will survive
+  is increased, not only for itself, but for all of its <!-- Page 190
+  --><span class="pagenum"><a name="page190"></a>{190}</span>descendants
+  that come to inherit this character. It is this increase in the number of
+  individuals possessing a particular character, that might have an
+  influence on the course of evolution. This gives a better chance for
+  improvement by several successive steps; but not because the species is
+  more likely to mutate again in the same direction. An imaginary example
+  will illustrate how this happens: When elephants had trunks less than a
+  foot long, the chance of getting trunks more than one foot long was in
+  proportion to the length of trunks already present and to the number of
+  individuals; but increment in trunk length is no more likely to occur
+  from an animal having a trunk more than one foot long than from an animal
+  with a shorter trunk.</p>
+
+  <p>The case is analogous to tossing pennies. At any stage in the game the
+  chance of accumulating a hundred heads is in proportion to the number of
+  heads already obtained, and to the number of throws still to be made. But
+  the number of heads obtained has no influence on the number of heads that
+  will appear in the next throw. <!-- Page 191 --><span class="pagenum"><a
+  name="page191"></a>{191}</span></p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/fig_095.jpg"><img style="width:100%" src="images/fig_095.jpg"
+      alt="Fig. 95." title="Fig. 95." /></a>
+    <p class="poem"><span class="sc">Fig.</span> 95. Evolution of
+    elephant's trunk: above Maeritherium, in the middle Tetrabelodon (After
+    Lancaster); below African elephants (After Gambier Bolton).</p>
+  </div>
+
+<p><!-- Page 192 --><span class="pagenum"><a name="page192"></a>{192}</span></p>
+
+  <p>Owing then to this property of the germ plasm to duplicate itself in a
+  large number of samples not only is an opportunity furnished to an
+  advantageous variation to become extensively multiplied, but the presence
+  of a large number of individuals of a given sort prejudices the probable
+  future result.</p>
+
+  <p>The question may be raised as to whether it is desirable to call
+  selection a <i>creative</i> process. There are so many supernatural and
+  mystical implications that hang around the term creative that one can not
+  be too careful in stating in what sense the term is to be used. If by
+  creative is meant that something is made out of nothing, then of course
+  there is no need for the scientist to try to answer such a question. But
+  if by a creative process is meant that something is made out of something
+  else, then there are two alternatives to be reckoned with.</p>
+
+  <p>First, if it were true that selection of an individual of a certain
+  kind determines that new variations in the same direction occur as a
+  consequence of the selection, then selection would certainly be creative.
+  How this could occur might be quite unintelligible, but of course it <!--
+  Page 193 --><span class="pagenum"><a name="page193"></a>{193}</span>might
+  be claimed that the point is not whether we can explain how creation
+  takes place, but whether we can get verifiable evidence that such a kind
+  of thing happens. This possibility is disposed of by the fact that there
+  is no evidence that selection determines the direction in which variation
+  occurs.</p>
+
+  <p>Second, if you mean by a creative process that by picking out a
+  certain kind of individual and multiplying its numbers a better chance is
+  furnished that a certain end result will be obtained, such a process may
+  be said to be creative. This is, I think, the proper use of the term
+  creative in a mechanistic sense.</p>
+
+<p class="cenhead"><span class="sc">Conclusions</span></p>
+
+  <p>In reviewing the evidence relating to selection I have tried to handle
+  the problem as objectively as I could.</p>
+
+  <p>The evidence shows clearly that the characters of wild animals and
+  plants, as well as those of domesticated races, are inherited both in the
+  wild and in the domesticated forms according to Mendel's Law.</p>
+
+  <p>The causes of the mutations that give rise <!-- Page 194 --><span
+  class="pagenum"><a name="page194"></a>{194}</span>to new characters we do
+  not know, although we have no reason for supposing that they are due to
+  other than natural processes.</p>
+
+  <p>Evolution has taken place by the incorporation into the race of those
+  mutations that are beneficial to the life and reproduction of the
+  organism. Natural selection as here defined means both the increase in
+  the number of individuals that results after a beneficial mutation has
+  occurred (owing to the ability of living matter to propagate) and also
+  that this preponderance of certain kinds of individuals in a population
+  makes some further results more probable than others. More than this,
+  natural selection can not mean, if factors are fixed and are not changed
+  by selection.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 195 --><span class="pagenum"><a name="page195"></a>{195}</span></p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p class="i6"><b>INDEX</b></p>
+    </div>
+
+    <div class="stanza">
+      <p>Abnormal abdomen <a href="#page109">109</a></p>
+      <p>Abraxas <a href="#page78">78</a>-<a href="#page81">81</a></p>
+      <p>Allantois <a href="#page17">17</a></p>
+      <p>Allelomorphs <a href="#page83">83</a>-<a href="#page84">84</a></p>
+      <p>Altenburg <a href="#page112">112</a></p>
+      <p>Amnion <a href="#page16">16</a>-<a href="#page17">17</a></p>
+      <p>Andalusian fowl <a href="#page45">45</a>, <a href="#page46">46</a></p>
+      <p>Annelids <a href="#page22">22</a></p>
+      <p>Antlered wing <a href="#page111">111</a></p>
+      <p>Apterous wing <a href="#page11">11</a></p>
+      <p>Arc wing <a href="#page111">111</a></p>
+      <p>Aristae <a href="#page104">104</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Bar eye <a href="#page67">67</a>, <a href="#page108">108</a>, <a href="#page169">169</a></p>
+      <p>Bateson <a href="#page18">18</a>, <a href="#page34">34</a>, <a href="#page36">36</a></p>
+      <p>Beaded wing <a href="#page11">11</a>, <a href="#page115">115</a></p>
+      <p>Beans <a href="#page147">147</a>-<a href="#page149">149</a>, <a href="#page157">157</a></p>
+      <p>Belgian hare <a href="#page171">171</a></p>
+      <p>Bent wing <a href="#page116">116</a></p>
+      <p>Bergson <a href="#page30">30</a>, <a href="#page31">31</a></p>
+      <p>Bildungstrieb <a href="#page34">34</a></p>
+      <p>Biogenetic law <a href="#page15">15</a>, <a href="#page18">18</a>, <a href="#page19">19</a>, <a href="#page21">21</a></p>
+      <p>Biometricians <a href="#page156">156</a></p>
+      <p>Bird <a href="#page21">21</a>, <a href="#page23">23</a></p>
+      <p>Bithorax <a href="#page65">65</a>, <a href="#page112">112</a>, <a href="#page113">113</a></p>
+      <p>Black body color <a href="#page111">111</a>, <a href="#page133">133</a></p>
+      <p>Blakeslee <a href="#page152">152</a></p>
+      <p>Bridges <a href="#page114">114</a>, <a href="#page143">143</a>, <a href="#page163">163</a></p>
+      <p>British Association <a href="#page36">36</a></p>
+      <p>Brünn <a href="#page40">40</a></p>
+      <p>Buff eye color <a href="#page109">109</a></p>
+      <p>Bufon <a href="#page27">27</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Castle <a href="#page176">176</a>-<a href="#page180">180</a></p>
+      <p>Cat <a href="#page33">33</a></p>
+      <p>Cell <a href="#page90">90</a>, <a href="#page91">91</a></p>
+      <p>Chance variations <a href="#page37">37</a></p>
+      <p>Chick <a href="#page16">16</a>, <a href="#page17">17</a>, <a href="#page20">20</a></p>
+      <p>Chromatin <a href="#page184">184</a></p>
+      <p>Chromosome group of Drosophila <a href="#page102">102</a></p>
+      <p>Chromosomes <a href="#page91">91</a>, <a href="#page95">95</a>, <a href="#page96">96</a>, <a href="#page98">98</a>, <a href="#page130">130</a>, <a href="#page131">131</a>, <a href="#page132">132</a></p>
+      <p>Cleavage <a href="#page21">21</a>, <a href="#page22">22</a>, <a href="#page94">94</a></p>
+      <p>Clover butterfly <a href="#page62">62</a></p>
+      <p>Club wing <a href="#page69">69</a>, <a href="#page70">70</a>, <a href="#page108">108</a></p>
+      <p>Colias philodice <a href="#page62">62</a></p>
+      <p>Color blindness <a href="#page77">77</a>, <a href="#page125">125</a></p>
+      <p>Comb of Drosophila <a href="#page103">103</a></p>
+      <p>Combs of fowls <a href="#page33">33</a>, <a href="#page54">54</a></p>
+      <p>Comparative anatomy <a href="#page7">7</a>, <a href="#page8">8</a>, <a href="#page9">9</a>, <a href="#page14">14</a></p>
+      <p>Corn <a href="#page150">150</a>, <a href="#page153">153</a>, <a href="#page172">172</a></p>
+      <p>Correns <a href="#page41">41</a></p>
+      <p>Cosmogonies <a href="#page27">27</a></p>
+      <p>Cream eye color <a href="#page163">163</a>, <a href="#page164">164</a></p>
+      <p>Crepidula <a href="#page22">22</a></p>
+      <p>Criss-cross inheritance <a href="#page78">78</a></p>
+      <p>Crossing over <a href="#page131">131</a>-<a href="#page133">133</a></p>
+      <p>Cuénot <a href="#page178">178</a></p>
+      <p>Curled wing <a href="#page115">115</a></p>
+      <p>Curved wing <a href="#page111">111</a></p>
+      <p>Curve of probability <a href="#page149">149</a></p>
+      <p>Cut wing <a href="#page11">11</a>, <a href="#page104">104</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Dachs legs <a href="#page112">112</a></p>
+      <p>Dahlgren <a href="#page62">62</a></p>
+      <p>Darwin <a href="#page15">15</a>, <a href="#page24">24</a>, <a href="#page28">28</a>, <a href="#page32">32</a>, <a href="#page35">35</a>-<a href="#page37">37</a>, <a href="#page64">64</a>, <a href="#page145">145</a>, <a href="#page146">146</a>, <a href="#page152">152</a>, <a href="#page154">154</a>-<a href="#page156">156</a></p>
+      <p>Dendy <a href="#page188">188</a></p>
+      <p>De Vries <a href="#page18">18</a>, <a href="#page147">147</a>, <a href="#page156">156</a></p>
+      <p>Dexter <a href="#page170">170</a></p>
+      <p>Dichaete <a href="#page114">114</a></p>
+      <p>Difflugia <a href="#page184">184</a>-<a href="#page187">187</a></p>
+      <p>Discontinuous variation <a href="#page13">13</a></p>
+      <p>Disuse <a href="#page31">31</a></p>
+      <p>Drosophila ampelophila <a href="#page10">10</a>, <a href="#page12">12</a>, <a href="#page13">13</a>, <a href="#page48">48</a>-<a href="#page50">50</a>, <a href="#page60">60</a>, <a href="#page75">75</a>, <a href="#page84">84</a>, <a href="#page85">85</a>, <a href="#page93">93</a>, <a href="#page100">100</a>, <a href="#page103">103</a>, <a href="#page119">119</a>, <a href="#page155">155</a>, <a href="#page162">162</a>, <a href="#page169">169</a></p>
+<!-- Page 196 --><span class="pagenum"><a name="page196"></a>{196}</span>
+      <p>Drosophila repleta <a href="#page76">76</a></p>
+      <p>Duplication of legs <a href="#page109">109</a></p>
+      <p>Dwarf <a href="#page114">114</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>East <a href="#page170">170</a>, <a href="#page172">172</a></p>
+      <p>Ebony <a href="#page50">50</a>, <a href="#page55">55</a>, <a href="#page56">56</a>, <a href="#page115">115</a></p>
+      <p>Egg <a href="#page91">91</a>, <a href="#page94">94</a></p>
+      <p>Elephant <a href="#page191">191</a></p>
+      <p>Elephants' skulls <a href="#page188">188</a></p>
+      <p>Elephants' trunks <a href="#page190">190</a></p>
+      <p>Embryology <a href="#page13">13</a>-<a href="#page23">23</a></p>
+      <p>Emerson <a href="#page172">172</a></p>
+      <p>Environment <a href="#page27">27</a></p>
+      <p>Eosin eye color <a href="#page61">61</a>, <a href="#page107">107</a>, <a href="#page163">163</a></p>
+      <p>Erdmann <a href="#page183">183</a></p>
+      <p>Evolution Creatrice <a href="#page30">30</a></p>
+      <p>Evolution&mdash;three kinds of <a href="#page1">1</a>, <a href="#page2">2</a>, <a href="#page4">4</a></p>
+      <p>Eye color <a href="#page13">13</a></p>
+      <p>Eyeless <a href="#page66">66</a>, <a href="#page115">115</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Factorial theory <a href="#page89">89</a></p>
+      <p>Factors of Drosophila <a href="#page143">143</a></p>
+      <p>Fantails <a href="#page172">172</a>, <a href="#page175">175</a></p>
+      <p>Fertilization <a href="#page91">91</a></p>
+      <p>Fish <a href="#page16">16</a>, <a href="#page20">20</a>, <a href="#page21">21</a></p>
+      <p>Flatworms <a href="#page22">22</a></p>
+      <p>Fluctuations <a href="#page12">12</a></p>
+      <p>Forked bristles <a href="#page106">106</a></p>
+      <p>Fowl <a href="#page77">77</a></p>
+      <p>Fused veins <a href="#page107">107</a>, <a href="#page108">108</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Galton <a href="#page154">154</a></p>
+      <p>Geneticist <a href="#page26">26</a></p>
+      <p>Germ-plasm <a href="#page142">142</a></p>
+      <p>Geoffroy St. Hilaire <a href="#page27">27</a></p>
+      <p>Giant <a href="#page114">114</a></p>
+      <p>Gill-slits <a href="#page20">20</a>, <a href="#page21">21</a>, <a href="#page23">23</a></p>
+      <p>Groups I, II, III, IV <a href="#page100">100</a>-<a href="#page118">118</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Haeckel <a href="#page15">15</a></p>
+      <p>Haemophilia <a href="#page77">77</a></p>
+      <p>Heliotropism <a href="#page106">106</a>, <a href="#page107">107</a></p>
+      <p>Himalyan rabbits <a href="#page83">83</a></p>
+      <p>History <a href="#page1">1</a>, <a href="#page6">6</a></p>
+      <p>Hoge <a href="#page66">66</a></p>
+      <p>Horse, evolution of <a href="#page6">6</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Indian corn <a href="#page172">172</a>, <a href="#page173">173</a></p>
+      <p>Interference <a href="#page137">137</a>, <a href="#page138">138</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Janssens <a href="#page132">132</a></p>
+      <p>Jaunty wing <a href="#page111">111</a></p>
+      <p>Jennings <a href="#page161">161</a>, <a href="#page181">181</a>-<a href="#page184">184</a>, <a href="#page186">186</a></p>
+      <p>Johannsen <a href="#page156">156</a>, <a href="#page157">157</a>, <a href="#page159">159</a>-<a href="#page161">161</a>, <a href="#page166">166</a>, <a href="#page182">182</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Lamarck <a href="#page31">31</a>-<a href="#page34">34</a></p>
+      <p>Langshan <a href="#page77">77</a></p>
+      <p>Leaves <a href="#page147">147</a></p>
+      <p>Leidy <a href="#page186">186</a></p>
+      <p>Lethal <a href="#page105">105</a></p>
+      <p>Linkage groups <a href="#page103">103</a></p>
+      <p>Lizard <a href="#page23">23</a></p>
+      <p>Localization of factors <a href="#page118">118</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>MacDowell <a href="#page155">155</a>, <a href="#page170">170</a>, <a href="#page171">171</a></p>
+      <p>Macritherium <a href="#page191">191</a></p>
+      <p>Mammal <a href="#page16">16</a>, <a href="#page21">21</a>, <a href="#page23">23</a></p>
+      <p>Man <a href="#page20">20</a>, <a href="#page77">77</a>, <a href="#page125">125</a>, <a href="#page126">126</a></p>
+      <p>Map of Chromosomes <a href="#page136">136</a></p>
+      <p>Maroon eye color <a href="#page114">114</a></p>
+      <p>Mendel <a href="#page40">40</a>, <a href="#page41">41</a>, <a href="#page52">52</a>, <a href="#page89">89</a></p>
+      <p>Mendelian heredity <a href="#page39">39</a></p>
+      <p>Mendel's law <a href="#page41">41</a>-<a href="#page59">59</a>, <a href="#page64">64</a>, <a href="#page124">124</a></p>
+      <p>Mendel's second law <a href="#page52">52</a></p>
+      <p>Mesenchyme cells <a href="#page22">22</a></p>
+      <p>Mesoderm cells <a href="#page22">22</a></p>
+      <p>Metaphysician <a href="#page30">30</a></p>
+      <p>Mice <a href="#page33">33</a>, <a href="#page178">178</a></p>
+      <p>Middleton <a href="#page183">183</a></p>
+      <p>Miniature wing <a href="#page108">108</a></p>
+      <p>Mirabilis <a href="#page42">42</a></p>
+      <p>Modifiers <a href="#page163">163</a>, <a href="#page164">164</a>, <a href="#page170">170</a>, <a href="#page171">171</a></p>
+      <p>Molluscs <a href="#page22">22</a></p>
+      <p>Mouse <a href="#page83">83</a></p>
+      <p>Muller <a href="#page112">112</a>, <a href="#page167">167</a></p>
+      <p>Mutations <a href="#page35">35</a>, <a href="#page39">39</a>, <a href="#page84">84</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Nägeli <a href="#page34">34</a>, <a href="#page35">35</a></p>
+      <p>Natural Selection <a href="#page36">36</a>, <a href="#page145">145</a>, <a href="#page146">146</a>, <a href="#page187">187</a>-<a href="#page194">194</a></p>
+<!-- Page 197 --><span class="pagenum"><a name="page197"></a>{197}</span>
+      <p>Nisus formativus <a href="#page34">34</a></p>
+      <p>Non-disjunction <a href="#page139">139</a>-<a href="#page142">142</a></p>
+      <p>Notch wing <a href="#page104">104</a>-<a href="#page106">106</a></p>
+      <p>Nucleus <a href="#page91">91</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Origin of Species <a href="#page35">35</a>, <a href="#page145">145</a></p>
+      <p>Orthogenesis <a href="#page34">34</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Paleontology <a href="#page24">24</a>-<a href="#page27">27</a></p>
+      <p>Papilio polytes <a href="#page63">63</a></p>
+      <p>Papilio turnus <a href="#page63">63</a></p>
+      <p>Paramecium <a href="#page181">181</a>, <a href="#page182">182</a></p>
+      <p>Paratettix <a href="#page81">81</a></p>
+      <p>Peach eye color <a href="#page114">114</a></p>
+      <p>Pea comb <a href="#page54">54</a></p>
+      <p>Pearl <a href="#page161">161</a></p>
+      <p>Peas <a href="#page47">47</a></p>
+      <p>Pigeons <a href="#page172">172</a>, <a href="#page174">174</a>, <a href="#page175">175</a></p>
+      <p>Pink eye color <a href="#page114">114</a>, <a href="#page115">115</a></p>
+      <p>Planarian <a href="#page22">22</a></p>
+      <p>Plymouth Rock <a href="#page77">77</a></p>
+      <p>Podarke <a href="#page22">22</a></p>
+      <p>Polar bodies <a href="#page126">126</a></p>
+      <p>Pole arms <a href="#page5">5</a></p>
+      <p>Protozoa <a href="#page181">181</a></p>
+      <p>Pseudo-parthenogenesis <a href="#page183">183</a></p>
+      <p>Purple eye color <a href="#page109">109</a></p>
+      <p>Purpose <a href="#page4">4</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Rabbits <a href="#page83">83</a>, <a href="#page170">170</a></p>
+      <p>Rats <a href="#page176">176</a>-<a href="#page180">180</a></p>
+      <p>Reduction division <a href="#page182">182</a></p>
+      <p>Reproductive cells <a href="#page96">96</a></p>
+      <p>Ruby eye color <a href="#page106">106</a></p>
+      <p>Rudimentary organ <a href="#page116">116</a></p>
+      <p>Rudimentary wing <a href="#page70">70</a>, <a href="#page71">71</a>, <a href="#page107">107</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Sable body color <a href="#page107">107</a></p>
+      <p>Science definition of <a href="#page6">6</a></p>
+      <p>Segregation <a href="#page41">41</a></p>
+      <p>Selenka <a href="#page94">94</a></p>
+      <p>Sepia eye color <a href="#page13">13</a>, <a href="#page114">114</a></p>
+      <p>Sex chromosomes <a href="#page118">118</a></p>
+      <p>Sex linked inheritance <a href="#page75">75</a>, <a href="#page118">118</a>-<a href="#page130">130</a></p>
+      <p>Sexual dimorphism <a href="#page62">62</a></p>
+      <p>Sheep <a href="#page33">33</a></p>
+      <p>Single comb <a href="#page54">54</a></p>
+      <p>Sooty body color <a href="#page50">50</a>, <a href="#page114">114</a>, <a href="#page115">115</a></p>
+      <p>Speck <a href="#page68">68</a>, <a href="#page69">69</a>, <a href="#page111">111</a></p>
+      <p>Spencer <a href="#page145">145</a></p>
+      <p>Spermatozoön <a href="#page91">91</a>, <a href="#page98">98</a></p>
+      <p>Stars, evolution of <a href="#page6">6</a></p>
+      <p>St. Hilaire <a href="#page27">27</a>-<a href="#page30">30</a></p>
+      <p>Strap wing <a href="#page110">110</a>, <a href="#page111">111</a></p>
+      <p>Stumpy wing <a href="#page11">11</a></p>
+      <p>Sturtevant <a href="#page76">76</a>, <a href="#page143">143</a></p>
+      <p>Stylonychia <a href="#page183">183</a></p>
+      <p>Survival of the fittest <a href="#page146">146</a></p>
+      <p>Systematist <a href="#page85">85</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Tails <a href="#page33">33</a></p>
+      <p>Tan flies <a href="#page106">106</a>, <a href="#page107">107</a></p>
+      <p>Tetrabelodon <a href="#page191">191</a></p>
+      <p>Trefoil <a href="#page111">111</a></p>
+      <p>Truncate wing <a href="#page111">111</a>, <a href="#page112">112</a>, <a href="#page167">167</a>, <a href="#page168">168</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Unfolding principle <a href="#page34">34</a></p>
+      <p>Unio <a href="#page22">22</a></p>
+      <p>Unit character <a href="#page74">74</a>, <a href="#page75">75</a></p>
+      <p>Use <a href="#page31">31</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Variation discontinuous <a href="#page13">13</a></p>
+      <p>Vermilion eye color <a href="#page108">108</a>, <a href="#page163">163</a></p>
+      <p>Vestigial wing <a href="#page11">11</a>, <a href="#page55">55</a>, <a href="#page56">56</a>, <a href="#page109">109</a>, <a href="#page133">133</a></p>
+      <p>Vital force <a href="#page34">34</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Wallace <a href="#page36">36</a></p>
+      <p>Walnut comb <a href="#page54">54</a></p>
+      <p>Weismann <a href="#page17">17</a>, <a href="#page31">31</a>-<a href="#page33">33</a></p>
+      <p>Wilson, E. B. <a href="#page125">125</a></p>
+      <p>Wingless <a href="#page67">67</a></p>
+      <p>Winiwarter <a href="#page126">126</a></p>
+      <p>White eye color <a href="#page13">13</a>, <a href="#page75">75</a>, <a href="#page119">119</a>-<a href="#page130">130</a></p>
+      <p>Whiting eye color <a href="#page163">163</a>, <a href="#page164">164</a></p>
+      <p>Woodruff <a href="#page183">183</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Yellow body color <a href="#page108">108</a>, <a href="#page133">133</a></p>
+      <p>Yolk sac <a href="#page16">16</a>, <a href="#page17">17</a></p>
+    </div>
+
+    <div class="stanza">
+      <p>Zeleny <a href="#page169">169</a></p>
+    </div>
+  </div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
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@@ -0,0 +1,3661 @@
+The Project Gutenberg EBook of A Critique of the Theory of Evolution, by 
+Thomas Hunt Morgan
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Critique of the Theory of Evolution
+
+Author: Thomas Hunt Morgan
+
+Release Date: December 17, 2009 [EBook #30701]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CRITIQUE OF THEORY OF EVOLUTION ***
+
+
+
+
+Produced by Bryan Ness, Keith Edkins, and the Online
+Distributed Proofreading Team at https://www.pgdp.net. Pages
+scanned by Bryan Ness.
+
+
+
+
+
+Transcriber's note: A few typographical errors have been corrected: they
+are listed at the end of the text.
+
+       *       *       *       *       *
+
+
+Princeton University
+
+THE LOUIS CLARK VANUXEM FOUNDATION
+LECTURES FOR 1915-1916
+
+       *       *       *       *       *
+
+The Louis Clark Vanuxem Foundation of Princeton University
+
+was established in 1912 with a bequest of $25,000 under the will of Louis
+Clark Vanuxem, of the Class of 1879. By direction of the executors of Mr.
+Vanuxem's estate, the income of the foundation is to be used for a series
+of public lectures delivered in Princeton annually, at least one half of
+which shall be on subjects of current scientific interest. The lectures are
+to be published and distributed among schools and libraries generally.
+
+The following lectures have already been published or are in press:
+
+    1912-13 The Theory of Permutable Functions, by Vito Volterra
+
+    1913-14 Lectures delivered in connection with the dedication of the
+    Graduate College of Princeton University by Emile Boutroux, Alois
+    Riehl, A. D. Godley, and Arthur Shipley
+
+    1914-15 Romance, by Sir Walter Raleigh
+
+    1915-16 A Critique of the Theory of Evolution, by Thomas Hunt Morgan
+
+       *       *       *       *       *
+
+LOUIS CLARK VANUXEM FOUNDATION
+
+A CRITIQUE
+
+OF THE
+
+THEORY OF EVOLUTION
+
+BY
+
+THOMAS HUNT MORGAN
+
+PROFESSOR OF EXPERIMENTAL ZOOLOGY IN
+COLUMBIA UNIVERSITY
+
+LECTURES DELIVERED AT PRINCETON UNIVERSITY
+FEBRUARY 24, MARCH 1, 8, 15, 1916
+
+PRINCETON UNIVERSITY PRESS
+PRINCETON
+LONDON: HUMPHREY MILFORD
+OXFORD UNIVERSITY PRESS
+1916
+
+       *       *       *       *       *
+
+Copyright, 1916, by
+PRINCETON UNIVERSITY PRESS
+Published October, 1916
+
+[Illustration]
+
+       *       *       *       *       *
+
+
+PREFACE
+
+Occasionally one hears today the statement that we have come to realize
+that we know nothing about evolution. This point of view is a healthy
+reaction to the over-confident belief that we knew everything about
+evolution. There are even those rash enough to think that in the last few
+years we have learned more about evolution than we might have hoped to know
+a few years ago. A _critique_ therefore not only becomes a criticism of the
+older evidence but an appreciation of the new evidence.
+
+In the first lecture an attempt is made to put a new valuation on the
+traditional evidence for evolution. In the second lecture the most recent
+work on heredity is dealt with, for only characters that are inherited can
+become a part of the evolutionary process. In the third lecture the
+physical basis of heredity and the composition of the germ plasm stream are
+examined in the light of new observations; while in the fourth lecture the
+thesis is developed that chance variation combined with a property of
+living things to manifold themselves is the key note of modern evolutionary
+thought.
+
+T. H. MORGAN
+
+_July, 1916_
+
+       *       *       *       *       *
+
+
+                      TABLE OF CONTENTS
+
+                          CHAPTER I
+             A REVALUATION OF THE EVIDENCE ON
+          WHICH THE THEORY OF EVOLUTION WAS BASED
+
+                                                      PAGE
+  PREFACE                                                v
+
+  1. THREE KINDS OF EVOLUTION                          1-7
+
+  2. THE EVIDENCE FOR ORGANIC EVOLUTION               7-27
+     a. The Evidence from Comparative Anatomy         7-14
+     b. The Evidence from Embryology                 14-23
+     c. The Evidence from Paleontology               24-27
+
+  3. THE FOUR GREAT HISTORICAL SPECULATIONS          27-39
+     a. The Environment                              27-31
+        Geoffroy St. Hilaire
+     b. Use and Disuse                               31-34
+        From Lamarck to Weismann
+     c. The Unfolding Principle                      34-36
+        Naegeli and Bateson
+     d. Natural Selection                            36-39
+        Darwin
+
+                         CHAPTER II
+             THE BEARING OF MENDEL'S DISCOVERY ON
+               THE ORIGIN OF HEREDITY CHARACTERS
+
+  1. Mendel's First Discovery--Segregation           41-52
+
+  2. Mendel's Second Discovery--Independent
+     Assortment                                      52-59
+
+  3. The Characters of Wild Animals and Plants
+     Follow the Same Laws of Inheritance as do
+     the Characters of Domesticated Animals and
+     Plants                                          59-84
+     a. Sexual Dimorphism                            61-64
+        Eosin eye color of Drosophila                61-62
+        Color of the Clover Butterfly, Colias
+        philodice                                    62-63
+        Color of Papilio turnus                      63
+        Color pattern of Papilio polytes             63-64
+     b. Duplication of parts                         65-66
+        Thorax of Drosophila                         65
+        Legs of Drosophila                           65-66
+     c. Loss of characters                           66-68
+        "Eyeless" of Drosophila                      66-67
+        Vestigial wings of Drosophila                67
+        Bar eye of Drosophila                        67-68
+     d. Small changes of characters                  68-70
+        "Speck"                                      68
+        Bristles of "club"                           70
+     e. Manifold effects of same factor              71
+     f. Constant but trivial effects may be the
+        product of factors having other vital
+        aspect                                       73
+     g. Sex-linked inheritance                       75-80
+        in Drosophila ampelophila                    75-76
+        in the wild species D. repleta               76
+        in man                                       77
+        in domesticated Fowls                        77-78
+        in the wild moth, Abraxas                    78-80
+    h. Multiple allelomorphs                         81-84
+        in the wild Grouse Locust                    81-83
+        in domesticated mice and rabbits             83
+        in Drosophila ampelophila                    84
+
+  4. MUTATION AND EVOLUTION                          84-88
+
+                         CHAPTER III
+               THE FACTORIAL THEORY OF HEREDITY
+             AND THE COMPOSITION OF THE GERM PLASM
+
+  1. THE CELLULAR BASIS OF ORGANIC EVOLUTION
+     AND HEREDITY                                    89-98
+
+  2. THE MECHANISM OF MENDELIAN HEREDITY
+     DISCOVERED IN THE BEHAVIOR OF THE
+     CHROMOSOMES                                    98-102
+
+  3. THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA
+     AMPELOPHILA                                   103-118
+     a. Group I.                                   104-109
+     b. Group II.                                  109-112
+     c. Group III.                                 112-115
+     d. Group IV.                                  115-118
+
+  4. LOCALIZATION OF FACTORS IN THE CHROMOSOMES    118-142
+     a. The Evidence from Sex Linked Inheritance   118-137
+     b. The Evidence from Interference             137-138
+     c. The Evidence from Non-Disjunction          139-142
+
+  5. HOW MANY GENETIC FACTORS ARE THERE IN
+     THE GERM-PLASM OF A SINGLE INDIVIDUAL?        142-143
+
+  6. CONCLUSIONS                                   144
+
+                         CHAPTER IV
+                   SELECTION AND EVOLUTION
+
+  1. THE THEORY OF NATURAL SELECTION               145-161
+
+  2. HOW HAS SELECTION IN DOMESTICATED ANIMALS
+     AND PLANTS BROUGHT ABOUT ITS RESULTS?         161-165
+
+  3. ARE FACTORS CHANGED THROUGH SELECTION?        165-187
+
+  4. HOW DOES NATURAL SELECTION INFLUENCE
+     THE COURSE OF EVOLUTION?                      187-193
+
+  5. CONCLUSIONS                                   193-194
+
+  INDEX                                            195-197
+
+       *       *       *       *       *
+
+
+CHAPTER I
+
+A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED
+
+We use the word evolution in many ways--to include many different kinds of
+changes. There is hardly any other scientific term that is used so
+carelessly--to imply so much, to mean so little.
+
+THREE KINDS OF EVOLUTION
+
+We speak of the evolution of the stars, of the evolution of the horse, of
+the evolution of the steam engine, as though they were all part of the same
+process. What have they in common? Only this, that each concerns itself
+with the _history_ of something. When the astronomer thinks of the
+_evolution_ of the earth, the moon, the sun and the stars, he has a picture
+of diffuse matter that has slowly condensed. With condensation came heat;
+with heat, action and reaction within the mass until the chemical
+substances that we know today were produced. This is the nebular hypothesis
+of the astronomer. The astronomer explains, or tries to explain, how this
+evolution took place, by an appeal to the physical processes that have been
+worked out in the laboratory, processes which he thinks have existed
+through all the eons during which this evolution was going on and which
+were its immediate causes.
+
+When the biologist thinks of the evolution of animals and plants, a
+different picture presents itself. He thinks of series of animals that have
+lived in the past, whose bones (fig. 1) and shells have been preserved in
+the rocks. He thinks of these animals as having in the past given birth,
+through an unbroken succession of individuals, to the living inhabitants of
+the earth today. He thinks that the old, simpler types of the past have in
+part changed over into the more complex forms of today.
+
+He is thinking as the historian thinks, but he sometimes gets confused and
+thinks that he is explaining evolution when he is only describing it.
+
+[Illustration: FIG. 1. A series of skulls and feet. Eohippus, Mesohippus,
+Meryhippus, Hipparion and Equus. (American Museum of Natural History. After
+Matthews.)]
+
+A third kind of evolution is one for which man himself is responsible, in
+the sense that he has brought it about, often with a definite end in view.
+
+His mind has worked slowly from stage to stage. We can often trace the
+history of the stages through which his psychic processes have passed. The
+evolution of the steam-boat, the steam engine, paintings, clothing,
+instruments of agriculture, of manufacture, or of warfare (fig. 2)
+illustrates the history of human progress. There is an obvious and striking
+similarity between the evolution of man's inventions and the evolution of
+the shells of molluscs and of the bones of mammals, yet in neither case
+does a knowledge of the order in which these things arose explain them. If
+we appeal to the psychologist he will probably tell us that human
+inventions are either the result of happy accidents, that have led to an
+unforeseen, but discovered use; or else the use of the invention was
+foreseen. It is to the latter process more especially that the idea of
+_purpose_ is applied. When we come to review the four great lines of
+evolutionary thought we shall see that this human idea of purpose recurs in
+many forms, suggesting that man has often tried to explain how organic
+evolution has taken place by an appeal to the method which he believes he
+makes use of himself in the inorganic world.
+
+[Illustration: FIG. 2. Evolution of pole arms. (Metropolitan Museum. After
+Dean.)]
+
+What has the evolution of the stars, of the horse and of human inventions
+in common? Only this, that in each case from a simple beginning through a
+series of changes something more complex, or at least different, has come
+into being. To lump all these kinds of changes into one and call them
+evolution is no more than asserting that you believe in consecutive series
+of events (which is history) causally connected (which is science); that
+is, that you believe in history and that you believe in science. But let us
+not forget that we may have complete faith in both without thereby offering
+any explanation of either. It is the business of science to find out
+_specifically_ what kinds of events were involved when the stars evolved in
+the sky, when the horse evolved on the earth, and the steam engine was
+evolved from the mind of man.
+
+Is it not rather an empty generalization to say that any kind of change is
+a process of evolution? At most it means little more than that you want to
+intimate that miraculous intervention is not necessary to account for such
+kinds of histories.
+
+We are concerned here more particularly with the biologists' ideas of
+evolution. My intention is to review the evidence on which the old theory
+rested its case, in the light of some of the newer evidence of recent
+years.
+
+Four great branches of study have furnished the evidence of organic
+evolution. They are:
+
+  Comparative anatomy.
+  Embryology.
+  Paleontology.
+  Experimental Breeding or Genetics.
+
+_The Evidence from Comparative Anatomy_
+
+When we study animals and plants we find that they can be arranged in
+groups according to their resemblances. This is the basis of comparative
+anatomy, which is only an accurate study of facts that are superficially
+obvious to everyone.
+
+The groups are based not on a single difference, but on a very large number
+of resemblances. Let us take for example the group of vertebrates.
+
+[Illustration: FIG. 3. Limb skeletons of extinct and living animals,
+showing the homologous bones: 1, salamander; 2, frog; 3, turtle; 4,
+Aetosaurus; 5, Pleisiosaurus; 6, Ichthyosaurus; 7, Mesosaurus; 8, duck.
+(After Jordan and Kellogg.)]
+
+The hand and the arm of man are similar to the hand and arm of the ape. We
+find the same plan in the forefoot of the rat, the elephant, the horse and
+the opossum. We can identify the same parts in the forefoot of the lizard,
+the frog (fig. 3), and even, though less certainly, in the pectoral fins of
+fishes. Comparison does not end here. We find similarities in the skull and
+back bones of these same animals; in the brain; in the digestive system; in
+the heart and blood vessels; in the muscles.
+
+Each of these systems is very complex, but the same general arrangement is
+found in all. Anyone familiar with the evidence will, I think, probably
+reach the conclusion either that these animals have been created on some
+preconceived plan, or else that they have some other bond that unites them;
+for we find it difficult to believe that such complex, yet similar things
+could have arisen independently. But we try to convince our students of the
+truth of the theory of evolution not so much by calling their attention to
+this relation as by tracing each organ from a simple to a complex
+structure.
+
+I have never known such a course to fail in its intention. In fact, I know
+that the student often becomes so thoroughly convinced that he resents any
+such attempt as that which I am about to make to point out that the
+evidence for his conviction is not above criticism.
+
+[Illustration: FIG. 4. Drosophila ampelophila. a, Female and b, male.]
+
+Because we can often arrange the series of structures in a line extending
+from the very simple to the more complex, we are apt to become unduly
+impressed by this fact and conclude that if we found the complete series we
+should find all the intermediate steps and that they have arisen in the
+order of their complexity. This conclusion is not necessarily correct. Let
+me give some examples that have come under my own observation. We have bred
+for five years the wild fruit fly Drosophila ampelophila (fig. 4) and we
+have found over a hundred and twenty-five new types that breed true. Each
+has arisen independently and suddenly. Every part of the body has been
+affected by one or another of these mutations. For instance many different
+kinds of changes have taken place in the wings and several of these involve
+the size of the wings. If we arrange the latter arbitrarily in the order of
+their size there will be an almost complete series beginning with the
+normal wings and ending with those of apterous flies. Several of these
+types are represented in figure 5. The order in which these mutations
+occurred bears no relation to their size; each originated independently
+from the wild type.
+
+[Illustration: FIG. 5. Mutants of Drosophila ampelophila arranged in order
+of size of wings: (a) cut; (b) beaded; (c) stumpy; (d) another individual
+of stumpy; (f) vestigial (g) apterous.]
+
+The wings of the wild fly are straight (fig. 4). Several types have arisen
+in which the wings are bent upwards and in the most extreme type the wings
+are curled over the back, as seen in figure 54 (g), yet there is no
+historical connection between these stages.
+
+Mutations have occurred involving the pigmentation of the body and wings.
+The head and thorax of the wild Drosophila ampelophila are grayish yellow,
+the abdomen is banded with yellow and black, and the wings are gray. There
+have appeared in our cultures several kinds of darker types ranging to
+almost black flies (fig. 20) and to lighter types that are quite yellow. If
+put in line a series may be made from the darkest flies at one end to the
+light yellow flies at the other. These types, with the fluctuations that
+occur within each type, furnish a complete series of gradations; yet
+historically they have arisen independently of each other.
+
+Many changes in eye color have appeared. As many as thirty or more races
+differing in eye color are now maintained in our cultures. Some of them are
+so similar that they can scarcely be separated from each other. It is
+easily possible beginning with the darkest eye color, sepia, which is deep
+brown, to pick out a perfectly graded series ending with pure white eyes.
+But such a serial arrangement would give a totally false idea of the way
+the different types have arisen; and any conclusion based on the existence
+of such a series might very well be entirely erroneous, for the fact that
+such a series exists bears no relation to the order in which its members
+have appeared.
+
+Suppose that evolution "in the open" had taken place in the same way, by
+means of _discontinuous_ variation. What value then would the evidence from
+comparative anatomy have in so far as it is based on a continuous series of
+variants of any organ?
+
+No one familiar with the entire evidence will doubt for a moment that these
+125 races of Drosophila ampelophila belong to the same species and have had
+a common origin, for while they may differ mainly in one thing they are
+extremely alike in a hundred other things, and in the general relation of
+the parts to each other.
+
+It is in this sense that the evidence from comparative anatomy can be used
+I think as an argument for evolution. It is the resemblances that the
+animals or plants in any group have in common that is the basis for such a
+conclusion; it is not because we can arrange in a continuous series any
+particular variations. In other words, our inference concerning the common
+descent of two or more species is based on the totality of such
+resemblances that still remain in large part after each change has taken
+place. In this sense the argument from comparative anatomy, while not a
+demonstration, carries with it, I think, a high degree of probability.
+
+_The Evidence from Embryology_
+
+In passing from the egg to the adult the individual goes through a series
+of changes. In the course of this development we see not only the
+beginnings of the organs that gradually enlarge and change into those of
+the adult animal, but also see that organs appear and later disappear
+before the adult stage is reached. We find, moreover, that the young
+sometimes resemble in a most striking way the adult stage of groups that we
+place lower in the scale of evolution.
+
+Many years before Darwin advanced his theory of evolution through natural
+selection, the resemblance of the young of higher animals to the adults of
+lower animals had attracted the attention of zoologists and various views,
+often very naive, had been advanced to account for the resemblance. Among
+these speculations there was one practically identical with that adopted by
+Darwin and the post-Darwinians, namely that the higher animals repeat in
+their development the _adult stages_ of lower animals. Later this view
+became one of the cornerstones of the theory of organic evolution. It
+reached its climax in the writings of Haeckel, and I think I may add
+without exaggeration that for twenty-five years it furnished the chief
+inspiration of the school of descriptive embryology. Today it is taught in
+practically all textbooks of biology. Haeckel called this interpretation
+the Biogenetic Law.
+
+[Illustration: FIG. 6. Young trout (Trutta fario) six days after hatching.
+(After Ziegler.)]
+
+It was recognized, of course, that many embryonic stages could not possibly
+represent ancestral animals. A young fish with a huge yolk sac attached
+(fig. 6) could scarcely ever have led a happy, free life as an adult
+individual. Such stages were interpreted, however, as _embryonic_ additions
+to the original ancestral type. The embryo had done something on its own
+account.
+
+In some animals the young have structures that attach them to the mother,
+as does the placenta of the mammals. In other cases the young develop
+membranes about themselves--like the amnion of the chick (fig. 7) and
+mammal--that would have shut off an adult animal from all intercourse with
+the outside world. Hundreds of such embryonic adaptations are known to
+embryologists. These were explained as adaptations and as falsifications of
+the ancestral records.
+
+[Illustration: FIG. 7. Diagram of chick showing relations of amnion,
+allantois and yolk. (After Lillie.)]
+
+At the end of the last century Weismann injected a new idea into our views
+concerning the origin of variations. He urged that variations are germinal,
+i.e. they first appear in the egg and the sperm as changes that later bring
+about modifications in the individual. The idea has been fruitful and is
+generally accepted by most biologists today. It means that the offspring of
+a pair of animals are not affected by the structure or the activities of
+their parents, but the germ plasm is the unmodified stream from which both
+the parent and the young have arisen. Hence their resemblance. Now, it has
+been found that a variation arising in the germ plasm, no matter what its
+cause, may affect any stage in the development of the next individuals that
+arise from it. There is no reason to suppose that such a change produces a
+new character that always sticks itself, as it were, on to the end of the
+old series. This idea of germinal variation therefore carried with it the
+death of the older conception of evolution by superposition.
+
+In more recent times another idea has become current, mainly due to the
+work of Bateson and of de Vries--the idea that variations are
+discontinuous. Such a conception does not fall easily into line with the
+statement of the biogenetic "law"; for actual experience with discontinuous
+variation has taught us that new characters that arise do not add
+themselves to the end of the line of already existing characters but if
+they affect the adult characters they change them without, as it were,
+passing through and beyond them.
+
+[Illustration: FIG. 8. Diagram of head of chick A and B, showing gill
+slits, and aortic arches; and head of fish C showing aortic arches. (After
+Hesse.)]
+
+[Illustration: FIG. 9. Human embryo showing gill slits and aortic arches.
+(After His; from Marshall.)]
+
+I venture to think that these new ideas and this new evidence have played
+havoc with the biogenetic "law". Nevertheless, there is an interpretation
+of the facts that is entirely compatible with the theory of evolution. Let
+me illustrate this by an example.
+
+[Illustration: FIG. 10. Young fish, dorsal view, and side view, showing
+gill slits. (After Kopsch.)]
+
+The embryos of the chick (fig. 8) and of man (fig. 9) possess at an early
+stage in their development gill-slits on the sides of the neck like those
+of fishes. No one familiar with the relations of the parts will for a
+moment doubt that the gill slits of these embryos and of the fish represent
+the same structures. When we look further into the matter we find that
+young fish also possess gill slits (fig. 10 and 11)--even in young stages
+in their development. Is it not then more probable that the mammal and bird
+possess this stage in their development simply because it has never been
+lost? Is not this a more reasonable view than to suppose that the gill
+slits of the embryos of the higher forms represent the adult gill slits of
+the fish that in some mysterious way have been pushed back into the embryo
+of the bird?
+
+[Illustration: FIG. 11. Side views of head of embryo sharks, showing gill
+slits.]
+
+I could give many similar examples. All can be interpreted as embryonic
+survivals rather than as phyletic contractions. Not one of them calls for
+the latter interpretation.
+
+The study of the cleavage pattern of the segmenting egg furnishes the most
+convincing evidence that a different explanation from the one stated in the
+biogenetic law is the more probable explanation.
+
+[Illustration: FIG. 12. Cleavage stages of four types of eggs, showing the
+origin of the mesenchyme cells (stippled) and mesoderm cells (darker); a,
+Planarian; b, Annelid (Podarke); c, Mollusc (Crepidula), d, Mollusc
+(Unio).]
+
+It has been found that the cleavage pattern has the same general
+arrangement in the early stages of flat worms, annelids and molluscs (fig.
+12). Obviously these stages have never been adult ancestors, and obviously
+if their resemblance has any meaning at all, it is that each group has
+retained the same general plan of cleavage, possessed by their common
+ancestor.
+
+Accepting this view, let us ask, does the evidence from embryology favor
+the theory of evolution? I think that it does very strongly. The embryos of
+the mammal, bird, and lizard have gill slits today because gill slits were
+present in the embryos of their ancestors. There is no other view that
+explains so well their presence in the higher forms.
+
+Perhaps someone will say, Well! is not this all that we have contended for!
+Have you not reached the old conclusion in a roundabout way? I think not.
+To my mind there is a wide difference between the old statement that the
+higher animals living today have the original adult stages telescoped into
+their embryos, and the statement that the resemblance between certain
+characters in the embryos of higher animals and corresponding stages in the
+embryos of lower animals is most plausibly explained by the assumption that
+they have descended from the same ancestors, and that their common
+structures are embryonic survivals.
+
+_The Evidence from Paleontology_
+
+The direct evidence furnished by fossil remains is by all odds the
+strongest evidence that we have in favor of organic evolution. Paleontology
+holds the incomparable position of being able to point directly to the
+evidence showing that the animals and plants living in past times are
+connected with those living at the present time, often through an unbroken
+series of stages. Paleontology has triumphed over the weakness of the
+evidence, which Darwin admitted was serious, by filling in many of the
+missing links.
+
+Paleontology has been criticised on the ground that she cannot pretend to
+show the actual ancestors of living forms because, if in the past genera
+and species were as abundant and as diverse as we find them at present, it
+is very improbable that the bones of any individual that happened to be
+preserved are the bones of just that species that took part in the
+evolution. Paleontologists will freely admit that in many cases this is
+probably true, but even then the evidence is, I think, still just as
+valuable and in exactly the same sense as is the evidence from comparative
+anatomy. It suffices to know that there lived in the past a particular
+"group" of animals that had many points in common with those that preceded
+them and with those that came later. Whether these are the actual ancestors
+or not does not so much matter, for the view that from such a group of
+species the later species have been derived is far more probable than any
+other view that has been proposed.
+
+With this unrivalled material and splendid series of gradations,
+paleontology has constructed many stages in the past history of the globe.
+But paleontologists have sometimes gone beyond this descriptive phase of
+the subject and have attempted to formulate the "causes", "laws" and
+"principles" that have led to the development of their series. It has even
+been claimed that paleontologists are in an incomparably better position
+than zoologists to discover such principles, because they know both the
+beginning and the end of the evolutionary series. The retort is obvious. In
+his sweeping and poetic vision the paleontologist may fail completely to
+find out the nature of the pigments that have gone into the painting of his
+picture, and he may confuse a familiarity with the different views he has
+enjoyed of the canvas with a knowledge of how the painting is being done.
+
+My good friend the paleontologist is in greater danger than he realizes,
+when he leaves descriptions and attempts explanation. He has no way to
+check up his speculations and it is notorious that the human mind without
+control has a bad habit of wandering.
+
+When the modern student of variation and heredity--the geneticist--looks
+over the different "continuous" series, from which certain "laws" and
+"principles" have been deduced, he is struck by two facts: that the gaps,
+in some cases, are enormous as compared with the single changes with which
+he is familiar, and (what is more important) that they involve numerous
+parts in many ways. The geneticist says to the paleontologist, since you do
+not know, and from the nature of your case can never know, whether your
+differences are due to one change or to a thousand, you can not with
+certainty tell us anything about the hereditary units which have made the
+process of evolution possible. And without this knowledge there can be no
+understanding of the causes of evolution.
+
+THE FOUR GREAT HISTORICAL SPECULATIONS
+
+Looking backward over the history of the evolution theory we recognize that
+during the hundred and odd years that have elapsed since Buffon, there have
+been four main lines of _speculation_ concerning evolution. We might call
+them the four great cosmogonies or the four modern epics of evolution.
+
+THE ENVIRONMENT
+
+_Geoffroy St. Hilaire_
+
+About the beginning of the last century Geoffroy St. Hilaire, protege, and
+in some respects a disciple of Buffon, was interested as to how living
+species are related to the animals and plants that had preceded them. He
+was familiar with the kind of change that takes place in the embryo if it
+is put into new or changed surroundings, and from this knowledge he
+concluded that as the surface of the earth slowly changed--as the carbon
+dioxide contents in the air altered--as land appeared--and as marine
+animals left the water to inhabit it, they or their embryos responded to
+the new conditions and those that responded favorably gave rise to new
+creations. As the environment changed the fauna and flora changed--change
+for change. Here we have a picture of progressive evolution that carries
+with it an idea of mechanical necessity. If there is anything mystical or
+even improbable in St. Hilaire's argument it does not appear on the
+surface; for he did not assume that the response to the new environment was
+always a favorable one or, as we say, an adaptation. He expressly stated
+that _if_ the response was unfavorable the individual or the race died out.
+He assumed that _sometimes_ the change might be favorable, i.e., that
+certain species, entire groups, would respond in a direction favorable to
+their existence in a new environment and these would come to inherit the
+earth. In this sense he anticipated certain phases of the natural selection
+theory of Darwin, but only in part; for his picture is not one of strife
+within and without the species, but rather the escape of the species from
+the old into a new world.
+
+If then we recognize the intimate bond in chemical constitution of living
+things and of the world in which they develop, what is there improbable in
+St. Hilaire's hypothesis? Why, in a word is not more credit given to St.
+Hilaire in modern evolutionary thought? The reasons are to be found, I
+think, first, in that the evidence to which he appealed was meagre and
+inconclusive; and, second, in that much of his special evidence does not
+seem to us to be applicable. For example the monstrous forms that
+development often assumes in a strange environment, and with which every
+embryologist is only too familiar, rarely if ever furnish combinations, as
+he supposed, that are capable of living. On the contrary, they lead rather
+to the final catastrophe of the organism. And lastly, St. Hilaire's appeal
+to sudden and great transformations, such as a crocodile's egg hatching
+into a bird, has exposed his view to too easy ridicule.
+
+But when all is said, St. Hilaire's conception of evolution contains
+elements that form the background of our thinking to-day, for taken
+broadly, the interaction between the organism and its environment was a
+mechanistic conception of evolution even though the details of the theory
+were inadequate to establish his contention.
+
+In our own time the French metaphysician Bergson in his _Evolution
+Creatrice_ has proposed in mystical form a thought that has at least a
+superficial resemblance to St. Hilaire's conception. The response of living
+things is no longer hit in one species and miss in another; it is precise,
+exact; yet not mechanical in the sense at least in which we usually employ
+the word mechanical. For Bergson claims that the one chief feature of
+living material is that it responds favorably to the situation in which it
+finds itself; at least so far as lies within the possible physical
+limitations of its organization. Evolution has followed no preordained
+plan; it has had no creator; it has brought about its own creation by
+responding adaptively to each situation as it arose.
+
+But note: the man of science believes that the organism responds today as
+it does, because at present it has a chemical and physical constitution
+that gives this response. We find a specific chemical composition and
+generally a specific physical structure already existing. We have no reason
+to suppose that such particular reactions would take place until a specific
+chemical configuration had been acquired. Where did this constitution come
+from? This is the question that the scientist asks himself. I suppose
+Bergson would have to reply that it came into existence at the moment that
+the first specific stimulus was applied. But if this is the answer we have
+passed at once from the realm of observation to the realm of fancy--to a
+realm that is foreign to our experience; for such a view assumes that
+chemical and physical reactions are guided by the needs of the organism
+when the reactions take place inside living beings.
+
+USE AND DISUSE
+
+_From Lamarck to Weismann_
+
+The second of the four great historical explanations appeals to a change
+not immediately connected with the outer world, but to one within the
+organism itself.
+
+Practice makes perfect is a familiar adage. Not only in human affairs do we
+find that a part through use becomes a better tool for performing its task,
+and through disuse degenerates; but in the field of animal behavior we find
+that many of the most essential types of behavior have been learned through
+repeated associations formed by contact with the outside.
+
+It was not so long ago that we were taught that the instincts of animals
+are the inherited experience of their ancestors--lapsed intelligence was
+the current phrase.
+
+Lamarck's name is always associated with the application of the theory of
+the inheritance of acquired characters. Darwin fully endorsed this view and
+made use of it as an explanation in all of his writings about animals.
+Today the theory has few followers amongst trained investigators, but it
+still has a popular vogue that is widespread and vociferous.
+
+To Weismann more than to any other single individual should be ascribed the
+disfavor into which this view has fallen. In a series of brilliant essays
+he laid bare the inadequacy of the supposed evidence on which the
+inheritance of acquired characters rested. Your neighbor's cat, for
+instance, has a short tail, and it is said that it had its tail pinched off
+by a closing door. In its litter of kittens one or more is found without a
+tail. Your neighbor believes that here is a case of cause and effect. He
+may even have known that the mother and grandmother of the cat had natural
+tails. But it has been found that short tail is a dominant character;
+therefore, until we know who was the father of the short-tailed kittens the
+accident to its mother and the normal condition of her maternal ancestry is
+not to the point.
+
+Weismann appealed to common sense. He made few experiments to disprove
+Lamarck's hypothesis. True, he cut off the tails of some mice for a few
+generations but got no tailless offspring and while he gives no exact
+measurements with coefficients of error he did not observe that the tails
+of the descendants had shortened one whit. The combs of fighting cocks and
+the tails of certain breeds of sheep have been cropped for many generations
+and the practice continues today, because their tails are still long. While
+in Lamarck's time there was no evidence opposed to his ingenious theory,
+based as it was on an appeal to the acknowledged facts of improvement that
+take place in the organs of an individual through their own functioning (a
+fact that is as obvious and remarkable today as in the time of Lamarck),
+yet now there is evidence as to whether the effects of use and disuse are
+inherited, and this evidence is not in accord with Lamarck's doctrine.
+
+THE UNFOLDING PRINCIPLE
+
+_Naegeli and Bateson_
+
+I have ventured to put down as one of the four great historical
+explanations, under the heading of the unfolding principle, a conception
+that has taken protean forms. At one extreme it is little more than a
+mystic sentiment to the effect that evolution is the result of an inner
+driving force or principle which goes under many names such as
+Bildungstrieb, nisus formativus, vital force, and orthogenesis.
+Evolutionary thought is replete with variants of this idea, often naively
+expressed, sometimes unconsciously implied. Evolution once meant, in fact,
+an unfolding of what pre-existed in the egg, and the term still carries
+with it something of its original significance.
+
+Naegeli's speculation written several years after Darwin's "Origin of
+Species" may be taken as a typical case. Naegeli thought that there exists
+in living material an innate power to grow and expand. He vehemently
+protested that he meant only a mechanical principle but as he failed to
+refer such a principle to any properties of matter known to physicists and
+chemists his view seems still a mysterious affirmation, as difficult to
+understand as the facts themselves which it purports to explain.
+
+Naegeli compared the process of evolution to the growth of a tree, whose
+ultimate twigs represent the living world of species. Natural selection
+plays only the role of the gardener who prunes the tree into this or that
+shape but who has himself _produced_ nothing. As an imaginative figure of
+speech Naegeli's comparison of the tree might even today seem to hold if we
+substituted "mutations" for "growth", but although we know so little about
+what causes mutations there is no reason for supposing them to be due to an
+inner impulse, and hence they furnish no justification for such a
+hypothesis.
+
+In his recent presidential address before the British Association Bateson
+has inverted this idea. I suspect that his effort was intended as little
+more than a _tour de force_. He claims for it no more than that it is a
+possible line of speculation. Perhaps he thought the time had come to give
+a shock to our too confident views concerning evolution. Be this as it may,
+he has invented a striking paradox. Evolution has taken place through the
+steady loss of inhibiting factors. Living matter was stopped down, so to
+speak, at the beginning of the world. As the stops are lost, new things
+emerge. Living matter has changed only in that it has become simpler.
+
+NATURAL SELECTION
+
+_Darwin_
+
+Of the four great historical speculations about evolution, the doctrine of
+Natural Selection of Darwin and Wallace has met with the most widespread
+acceptance. In the last lecture I intend to examine this theory critically.
+Here we are concerned only with its broadest aspects.
+
+Darwin appealed to _chance variations_ as supplying evolution with the
+material on which natural selection works. If we accept, for the moment,
+this statement as the cardinal doctrine of natural selection it may appear
+that evolution is due, (1) _not_ to an _orderly_ response of the organism
+to its environment, (2) _not_ in the main to the activities of the animal
+through the use or disuse of its parts, (3) _not_ to any innate principle
+of living material itself, and (4) above all _not_ to purpose either from
+within or from without. Darwin made quite clear what he meant by chance. By
+chance he did not mean that the variations were not causal. On the contrary
+he taught that in Science we mean by chance only that the particular
+combination of causes that bring about a variation are not known. They are
+accidents, it is true, but they are causal accidents.
+
+In his famous book on "Animals and Plants under Domestication", Darwin
+dwells at great length on the nature of the conditions that bring about
+variations. If his views seem to us today at times vague, at times
+problematical, and often without a secure basis, nevertheless we find in
+every instance, that Darwin was searching for the _physical causes of
+variation_. He brought, in consequence, conviction to many minds that there
+are abundant indications, even if certain proof is lacking, that the causes
+of variation are to be found in natural processes.
+
+Today the belief that evolution takes place by means of natural processes
+is generally accepted. It does not seem probable that we shall ever again
+have to renew the old contest between evolution and special creation.
+
+But this is not enough. We can never remain satisfied with a negative
+conclusion of this kind. We must find out what natural causes bring about
+variations in animals and plants; and we must also find out what kinds of
+variations are inherited, and how they are inherited. If the circumstantial
+evidence for organic evolution, furnished by comparative anatomy,
+embryology and paleontology is cogent, we should be able to observe
+evolution going on at the present time, i.e. we should be able to observe
+the occurrence of variations and their transmission. This has actually been
+done by the geneticist in the study of mutations and Mendelian heredity, as
+the succeeding lectures will show.
+
+       *       *       *       *       *
+
+
+CHAPTER II
+
+THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITARY CHARACTERS
+
+Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied
+the heredity of certain characters of the common edible pea, in the garden
+of the monastery at Bruenn.
+
+In his account of his work written in 1868, he said:
+
+    "It requires indeed some courage to undertake a labor of such a
+    far-reaching extent; it appears, however, to be the only right way by
+    which we can finally reach the solution of a question the importance of
+    which cannot be over-estimated in connection with the history of the
+    evolution of organic forms."
+
+He tells us also why he selected peas for his work:
+
+    "The selection of the plant group which shall serve for experiments of
+    this kind must be made with all possible care if it be desired to avoid
+    from the outset every risk of questionable results."
+
+    "The experimental plants must necessarily
+
+    1. Possess constant differentiating characters.
+
+    2. The hybrids of such plants must, during the flowering period, be
+    protected from the influence of all foreign pollen, or be easily
+    capable of such protection."
+
+Why do biologists throughout the world to-day agree that Mendel's discovery
+is one of first rank?
+
+A great deal might be said in this connection. What is essential may be
+said in a few words. Biology had been, and is still, largely a descriptive
+and speculative science. _Mendel showed by experimental proof that heredity
+could be explained by a simple mechanism. His discovery has been
+exceedingly fruitful._
+
+Science begins with naive, often mystic conceptions of its problems. It
+reaches its goal whenever it can replace its early guessing by verifiable
+hypotheses and predictable results. This is what Mendel's law did for
+heredity.
+
+MENDEL'S FIRST DISCOVERY--SEGREGATION
+
+[Illustration: FIG. 13. Diagram illustrating a cross between a red (dark)
+and a white variety of four o'clock (Mirabilis jalapa).]
+
+Let us turn to the demonstration of his first law--the law of segregation.
+The first case I choose is not the one given by Mendel but one worked out
+later by Correns. If the common garden plant called four o'clock (Mirabilis
+jalapa) with red flowers is crossed to one having white flowers, the
+offspring are pink (fig. 13). The hybrid, then, is intermediate in the
+color of its flowers between the two parents. If these hybrids are inbred
+the offspring are white, pink and red, in the proportion of 1:2:1. All of
+these had the same ancestry, yet they are of three different kinds. If we
+did not know their history it would be quite impossible to state what the
+ancestry of the white or of the red had been, for they might just as well
+have come from pure white and pure red ancestors respectively as to have
+emerged from the pink hybrids. Moreover, when we test them we find that
+they are as pure as are white or red flowering plants that have had all
+white or all red flowering ancestors.
+
+Mendel's Law explains the results of this cross as shown in figure 14.
+
+The egg cell from the white parent carries the factor for white, the pollen
+cell from the red parent carries the factor for red. The hybrid formed by
+their union carries both factors. The result of their combined action is to
+produce flowers intermediate in color.
+
+When the hybrids mature and their germ cells (eggs or pollen) ripen, each
+carries only one of these factors, either the red or the white, but not
+both. In other words, the two factors that have been brought together in
+the hybrid separate in its germ cells. Half of the egg cells are white
+bearing, half red bearing. Half of the pollen cells are white bearing, half
+red bearing. Chance combinations at fertilization give the three classes of
+individuals of the second generation.
+
+[Illustration: FIG. 14. Diagram illustrating the history of the factors in
+the germ cells of the cross shown in Fig. 13.]
+
+The white flowering plants should forever breed true, as in fact they do.
+The red flowering plants also breed true. The pink flowering plants, having
+the same composition as the hybrids of the first generation, should give
+the same kind of result. They do, indeed, give this result i.e. one white
+to two pink to one red flowered offspring.
+
+[Illustration: FIG. 15. Diagram illustrating a cross between special races
+of white and black fowls, producing the blue (here gray) Andalusian.]
+
+Another case of the same kind is known to breeders of poultry. One of the
+most beautiful of the domesticated breeds is known as the Andalusian. It is
+a slate blue bird shading into blue-black on the neck and back. Breeders
+know that these blue birds do not breed true but produce white, black, and
+blue offspring.
+
+[Illustration: FIG. 16. Diagram showing history of germ cells of cross of
+Fig. 15. The larger circles indicate the color of the birds; their enclosed
+small circles the nature of the factors in the germ cells of such birds.]
+
+The explanation of the failure to produce a pure race of Andalusians is
+that they are like the pink flowers of the four o'clock, i.e., they are a
+hybrid type formed by the meeting of the white and the black germ cells. If
+the whites produced by the Andalusians are bred to the blacks (both being
+pure strains), all the offspring will be blue (fig. 15); if these blues are
+inbred they will give 1 white, to 2 blues, to 1 black. In other words, the
+factor for white and the factor for black separate in the germ cells of the
+hybrid Andalusian birds (fig. 16).
+
+[Illustration: FIG. 17. Diagram of Mendel's cross between yellow (dominant)
+and green (recessive) peas.]
+
+The third case is Mendel's classical case of yellow and green peas (fig.
+17). He crossed a plant belonging to a race having yellow peas with one
+having green peas. The hybrid plants had yellow seeds. These hybrids inbred
+gave three yellows to one green. The explanation (fig. 18) is the same in
+principle as in the preceding cases. The only difference between them is
+that the hybrid which contains both the yellow and the green factors is in
+appearance not intermediate, but like the yellow parent stock. Yellow is
+said therefore to be dominant and green to be recessive.
+
+[Illustration: FIG. 18. Diagram illustrating the history of the factors in
+the cross shown in Fig. 17.]
+
+Another example where one of the contrasted characters is dominant is shown
+by the cross of Drosophila with vestigial wings to the wild type with long
+wings (fig. 19). The F_1 flies have long wings not differing from those of
+the wild fly, so far as can be observed. When two such flies are inbred
+there result three long to one vestigial.
+
+[Illustration: FIG. 19. Diagram illustrating a cross between a fly
+(Drosophila ampelophila) with long wings and a mutant fly with vestigial
+wings.]
+
+The question as to whether a given character is dominant or recessive is a
+matter of no theoretical importance for the principle of segregation,
+although from the notoriety given to it one might easily be misled into the
+erroneous supposition that it was the discovery of this relation that is
+Mendel's crowning achievement.
+
+Let me illustrate by an example in which the hybrid standing between two
+types overlaps them both. There are two mutant races in our cultures of the
+fruit fly Drosophila that have dark body color, one called sooty, another
+which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives
+offspring that are intermediate in color. Some of them are so much like
+sooty that they cannot be distinguished from sooty. At the other extreme
+some of the hybrids are as dark as the lightest of the ebony flies. If
+these hybrids are inbred there is a continuous series of individuals,
+sooties, intermediates and ebonies. Which color here shall we call the
+dominant? If the ebony, then in the second generation we count three
+ebonies to one sooty, putting the hybrids with the ebonies. If the dominant
+is the sooty then we count three sooties to one ebony, putting the hybrids
+with the sooties. The important fact to find out is whether there actually
+exist three classes in the second generation. This can be ascertained even
+when, as in this case, there is a perfectly graded series from one end to
+the other, by testing out individually enough of the flies to show that
+one-fourth of them never produce any descendants but ebonies, one-fourth
+never any but sooties, and one-half of them give rise to both ebony and
+sooty.
+
+[Illustration: FIG. 20. Cross between two allelomorphic races of
+Drosophila, sooty and ebony, that give a completely graded series in F_2.]
+
+MENDEL'S SECOND DISCOVERY--INDEPENDENT ASSORTMENT
+
+Besides his discovery that there are pairs of characters that disjoin, as
+it were, in the germ cells of the hybrid (law of segregation) Mendel made a
+second discovery which also has far-reaching consequences. The following
+case illustrates Mendel's second law.
+
+If a pea that is yellow and round is crossed to one that is green and
+wrinkled (fig. 21), all of the offspring are yellow and round. Inbred,
+these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green
+wrinkled. All the yellows taken together are to the green as 3:1. All the
+round taken together are to the wrinkled as three to one; but some of the
+yellows are now wrinkled and some of the green are now round. There has
+been a recombination of characters, while at the same time the results, for
+each pair of characters taken separately, are in accord with Mendel's Law
+of Segregation, (fig. 22). The second law of Mendel may be called the law
+of independent assortment of different character pairs.
+
+[Illustration: FIG. 21. Cross between yellow-round and green-wrinkled peas,
+giving the 9: 3: 3: 1 ratio in F_2.]
+
+We can, as it were, take the characters of one organism and recombine them
+with those of a different organism. We can explain this result as due to
+the assortment of factors for these characters in the germ cells according
+to a definite law.
+
+[Illustration: FIG. 22. Diagram to show the history of the factor pairs
+yellow-green and round-wrinkled of the cross in Fig. 21.]
+
+As a second illustration let me take the classic case of the combs of
+fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23),
+the offspring have a comb different from either. It is called a walnut
+comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1
+single. This proportion shows that the grandparental types differed in
+respect to two pairs of characters.
+
+[Illustration: FIG. 23. Cross between pea and rose combed fowls. (Charts of
+Baur and Goldschmidt.)]
+
+A fourth case is shown in the fruit fly, where an ebony fly with long wings
+is mated to a grey fly with vestigial wings (fig. 24). The offspring are
+gray with long wings. If these are inbred they give 9 gray long, 3 gray
+vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).
+
+[Illustration: FIG. 24. Cross between long ebony and gray vestigial flies.]
+
+The possibility of interchanging characters might be illustrated over and
+over again. It is true not only when two pairs of characters are involved,
+but when three, four, or more enter the cross.
+
+[Illustration: FIG. 25. Diagram to show the history of the factors in the
+cross shown in Fig. 24.]
+
+It is as though we took individuals apart and put together parts of two,
+three or more individuals by substituting one part for another.
+
+Not only has this power to make whatever combinations we choose great
+practical importance, it has even greater theoretical significance; for, it
+follows that the individual is not in itself the unit in heredity, but that
+within the germ-cells there exist smaller units concerned with the
+transmission of characters.
+
+The older mystical statement of the individual as a unit in heredity has no
+longer any interest in the light of these discoveries, except as a past
+phase of biological history. We see, too, more clearly that the sorting out
+of factors in the germ plasm is a very different process from the influence
+of these factors on the development of the organism. There is today no
+excuse for confusing these two problems.
+
+If mechanistic principles apply also to embryonic development then the
+course of development is capable of being stated as a series of
+chemico-physical reactions and the "_individual_" is merely a term to
+express the sum total of such reactions and should not be interpreted as
+something different from or more than these reactions. So long as so little
+is known of the actual processes involved in development the use of the
+term "individuality", while giving the appearance of profundity, in reality
+often serves merely to cover ignorance and to make a mystery out of a
+mechanism.
+
+THE CHARACTERS OF WILD ANIMALS AND PLANTS FOLLOW THE SAME LAWS OF
+INHERITANCE AS DO THE CHARACTERS OF DOMESTICATED ANIMALS AND PLANTS.
+
+Darwin based many of his conclusions concerning variation and heredity on
+the evidence derived from the garden and from the stock farm. Here he was
+handicapped to some extent, for he had at times to rely on information much
+of which was uncritical, and some of which was worthless.
+
+Today we are at least better informed on _two_ important points; one
+concerning the _kinds_ of variations that furnish to the cultivator the
+materials for his selection; the other concerning the modes of inheritance
+of these variations. We know now that new characters are continually
+appearing in domesticated as well as in wild animals and plants, that these
+characters are often sharply marked off from the original characters, and
+whether the differences are great or whether they are small they are
+transmitted alike according to Mendel's law.
+
+Many of the characteristics of our domesticated animals and cultivated
+plants originated long ago, and only here and there have the records of
+their first appearance been preserved. In only a few instances are these
+records clear and definite, while the complete history of any large group
+of our domesticated products is unknown to us.
+
+Within the last five or six years, however, from a common wild species of
+fly, the fruit fly, Drosophila ampelophila, which we have brought into the
+laboratory, have arisen over a hundred and twenty-five new types whose
+origin is completely known. Let me call attention to a few of the more
+interesting of these types and their modes of inheritance, comparing them
+with wild types in order to show that the kinds of inheritance found in
+domesticated races occur also in wild types. The results will show beyond
+dispute that the characters of wild types are inherited in precisely the
+same way as are the characters of the mutant types--a fact that is not
+generally appreciated except by students of genetics, although it is of the
+most far-reaching significance for the theory of evolution.
+
+A mutant appeared in which the eye color of the female was different from
+that of the male. The eye color of the mutant female is a dark eosin color,
+that of the male yellowish eosin. From the beginning this difference was as
+marked as it is to-day. Breeding experiments show that eosin eye color
+differs from the red color of the eye of the wild fly by a single mutant
+factor. Here then at a single step a type appeared that was sexually
+dimorphic.
+
+Zoologists know that sexual dimorphism is not uncommon in wild species of
+animals, and Darwin proposed the theory of sexual selection to account for
+the difference between the sexes. He assumed that the male preferred
+certain kinds of females differing from himself in a particular character,
+and thus in time through sexual selection, the sexes came to differ from
+each other.
+
+[Illustration: FIG. 26. Clover butterfly (Colias philodice) with two types
+of females, above; and one type of male, below.]
+
+In the case of eosin eye color no such process as that postulated by Darwin
+to account for the differences between the sexes was involved; for the
+single mutation that brought about the change also brought in the
+dimorphism with it.
+
+In recent years zoologists have carefully studied several cases in which
+two types of female are found in the same species. In the common clover
+butterfly, there is a yellow and a white type of female, while the male is
+yellow (fig. 26). It has been shown that a single factor difference
+determines whether the female is yellow or white. The inheritance is,
+according to Gerould, strictly Mendelian.
+
+[Illustration: FIG. 27. Papilio turnus with two types of females above and
+one type of male below.]
+
+In Papilio turnus there exist, in the southern states, two kinds of
+females, one yellow like the male, one black (fig. 27). The evidence here
+is not so certain, but it seems probable that a single factor difference
+determines whether the female shall be yellow or black.
+
+Finally in Papilio polytes of Ceylon and India three different types of
+females appear, (fig. 28 to right) only one of which is like the male. Here
+the analysis of the breeding data shows the possibility of explaining this
+case as due to two pairs Mendelian factors which give in combination the
+three types of female.
+
+[Illustration: FIG. 28. Papilio polytes, with three types of female to
+right and one type of male above to left.]
+
+Taking these cases together, they furnish a much simpler explanation than
+the one proposed by Darwin. They show also that characters like these shown
+by wild species may follow Mendel's law.
+
+[Illustration: FIG. 29. Mutant race of fruit fly with intercalated
+duplicate mesothorax on dorsal side.]
+
+There has appeared in our cultures a fly in which the third division of the
+thorax with its appendages has changed into a segment like the second (fig.
+29). It is smaller than the normal mesothorax and its wings are imperfectly
+developed, but the bristles on the upper surface may have the typical
+arrangement of the normal mesothorax. The mutant shows how great a change
+may result from a single factor difference.
+
+A factor that causes duplication in the legs has also been found. Here the
+interesting fact was discovered (Hoge) that duplication takes place only in
+the cold. At ordinary temperatures the legs are normal.
+
+[Illustration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a'
+normal eye.]
+
+In contrast to the last case, where a character is doubled, is the next one
+in which the eyes are lost (fig. 30). This change also took place at a
+single step. All the flies of this stock however, cannot be said to be
+eyeless, since many of them show pieces of the eye--indeed the variation is
+so wide that the eye may even appear like a normal eye unless carefully
+examined. Formerly we were taught that eyeless animals arose in caves. This
+case shows that they may also arise suddenly in glass milk bottles, by a
+change in a single factor.
+
+I may recall in this connection that wingless flies (fig. 5 f) also arose
+in our cultures by a single mutation. We used to be told that wingless
+insects occurred on desert islands because those insects that had the best
+developed wings had been blown out to sea. Whether this is true or not, I
+will not pretend to say, but at any rate wingless insects may also arise,
+not through a slow process of elimination, but at a single step.
+
+The preceding examples have all related to recessive characters. The next
+one is dominant.
+
+[Illustration: FIG. 31. Mutant race of fruit fly called bar to the right
+(normal to the left). The eye is a narrow vertical bar, the outline of the
+original eye is indicated.]
+
+A single male appeared with a narrow vertical red bar (fig. 31) instead of
+the broad red oval eye. Bred to wild females the new character was found to
+dominate, at least to the extent that the eyes of all its offspring were
+narrower than the normal eye, although not so narrow as the eye of the pure
+stock. Around the bar there is a wide border that corresponds to the region
+occupied by the rest of the eye of the wild fly. It lacks however the
+elements of the eye. It is therefore to be looked upon as a rudimentary
+organ, which is, so to speak, a by-product of the dominant mutation.
+
+The preceding cases have all involved rather great changes in some one
+organ of the body. The following three cases involve slight changes, and
+yet follow the same laws of inheritance as do the larger changes.
+
+[Illustration: FIG. 32. Mutant race of fruit fly, called speck. There is a
+minute black speck at base of wing.]
+
+At the base of the wings a minute black speck appeared (fig. 32). It was
+found to be a Mendelian character. In another case the spines on the thorax
+became forked or kinky (fig. 52b). This stock breeds true, and the
+character is inherited in strictly Mendelian fashion.
+
+[Illustration: FIG. 33. Mutant race of fruit fly called club. The wings
+often remain unexpanded and two bristles present in wild fly (b) are absent
+on side of thorax (c).]
+
+In a certain stock a number of flies appeared in which the wing pads did
+not expand (fig. 33). It was found that this peculiarity is shown in only
+about twenty per cent of the individuals supposed to inherit it. Later it
+was found that this stock lacked two bristles on the sides of the thorax.
+By means of this knowledge the heredity of the character was easily
+determined. It appears that while the expansion of the wing pads fails to
+occur once in five times--probably because it is an environmental effect
+peculiar to this stock,--yet the minute difference of the presence or
+absence of the two lateral bristles is a constant feature of the flies that
+carry this particular factor.
+
+In the preceding cases I have spoken as though a factor influenced only one
+part of the body. It would have been more accurate to have stated that the
+_chief_ effect of the factor was observed in a particular part of the body.
+Most students of genetics realize that a factor difference usually affects
+more than a single character. For example, a mutant stock called
+rudimentary wings has as its principle characteristic very short wings
+(fig. 34). But the factor for rudimentary wings also produces other effects
+as well. The females are almost completely sterile, while the males are
+fertile. The viability of the stock is poor. When flies with rudimentary
+wings are put into competition with wild flies relatively few of the
+rudimentary flies come through, especially if the culture is crowded. The
+hind legs are also shortened. All of these effects are the results of a
+single factor-difference.
+
+[Illustration: FIG. 34. Mutant race of fruit fly, called rudimentary.]
+
+One may venture the guess that some of the specific and varietal
+differences that are characteristic of wild types and which at the same
+time appear to have no survival value, are only by-products of factors
+whose most important effect is on another part of the organism where their
+influence is of vital importance.
+
+It is well known that systematists make use of characters that are constant
+for groups of species, but which do not appear in themselves to have an
+adaptive significance. If we may suppose that the constancy of such
+characters may be only an index of the presence of a factor whose _chief_
+influence is in some other direction or directions, some physiological
+influence, for example, we can give at least a reasonable explanation of
+the constancy of such characters.
+
+I am inclined to think that an overstatement to the effect that each factor
+may affect the entire body, is less likely to do harm than to state that
+each factor affects only a particular character. The reckless use of the
+phrase "unit character" has done much to mislead the uninitiated as to the
+effects that a single change in the germ plasm may produce on the organism.
+Fortunately, the expression "unit character" is being less used by those
+students of genetics who are more careful in regard to the implications of
+their terminology.
+
+There is a class of cases of inheritance, due to the XY chromosomes, that
+is called sex linked inheritance. It is shown both by mutant characters and
+characters of wild species.
+
+For instance, white eye color in Drosophila shows sex linked inheritance.
+If a white eyed male is mated to a wild red eyed female (fig. 35) all the
+offspring have red eyes. If these are inbred, there are three red to one
+white eyed offspring, but white eyes occur only in the males. The
+grandfather has transmitted his peculiarity to half of his grandsons, but
+to none of his granddaughters.
+
+[Illustration: FIG. 35. Diagram showing a cross between a white eyed male
+and a red eyed female of the fruit fly. Sex linked inheritance.]
+
+The reciprocal cross (fig. 36) is also interesting. If a white eyed female
+is bred to a red eyed male, all of the daughters have red eyes and all of
+the sons have white eyes. We call this criss-cross inheritance. If these
+offspring are inbred, they produce equal numbers of red eyed and white eyed
+females and equal numbers of red eyed and white eyed males. The ratio is 1:
+1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the usual 3:1
+Mendelian ratio. Yet, as will be shown later, the result is in entire
+accord with Mendel's principle of segregation.
+
+[Illustration: FIG. 36. Diagram illustrating a cross between a red eyed
+male and white eyed female of the fruit fly (reciprocal cross of that shown
+in Fig. 35).]
+
+It has been shown by Sturtevant that in a wild species of Drosophila, viz.,
+D. repleta, two varieties of individuals exist, in one of which the thorax
+has large splotches and in the other type smaller splotches (fig. 37). The
+factors that differentiate these varieties are sex linked.
+
+[Illustration: FIG. 37. Two types of markings on thorax of Drosophila
+repleta, both found "wild". They show sex linked inheritance.]
+
+Certain types of color blindness (fig. 38) and certain other abnormal
+conditions in man such as haemophilia, are transmitted as sex linked
+characters.
+
+[Illustration: FIG. 38, A. Diagram illustrating inheritance of color
+blindness in man; the iris of the color-blind eye is here black.]
+
+[Illustration: FIG. 38, B. Reciprocal of cross in Fig. 38 a.]
+
+In domestic fowls sex linked inheritance has been found as the
+characteristic method of transmission for at least as many as six
+characters, but here the relation of the sexes is in a sense reversed. For
+instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock
+(fig. 39), the offspring are all barred. If these are inbred half of the
+daughters are black and half are barred; all of the sons are barred. The
+grandmother has transmitted her color to half of her granddaughters but to
+none of her grandsons.
+
+[Illustration: FIG. 39. Sex-linked inheritance in domesticated birds shown
+here in a cross between barred Plymouth Rock male and black Langshan
+female.]
+
+[Illustration: FIG. 40. Reciprocal of Fig. 39.]
+
+In the reciprocal cross (fig. 40) black cock by barred hen, the daughters
+are black and the sons barred--criss-cross inheritance. These inbred give
+black hens and black cocks, barred hens and barred cocks.
+
+There is a case comparable to this found in a wild species of moth, Abraxas
+grossulariata. A wild variation of this type is lighter in color and is
+known as A. lacticolor. When these two types are crossed they exhibit
+exactly the same type of heredity as does the black-barred combination in
+the domestic fowl. As shown in figure 41, lacticolor female bred to
+grossulariata male gives grossulariata sons and daughters. These inbred
+give grossulariata males and females and lacticolor females. Reciprocally
+lacticolor male by grossulariata female, (fig. 42) gives lacticolor
+daughters and grossulariata sons and these inbred give grossulariata males
+and females and lacticolor males and females.
+
+[Illustration: FIG. 41. Sex-linked inheritance in the wild moth, Abraxas
+grossulariata (darker) and A. lacticolor.]
+
+[Illustration: FIG. 42. Reciprocal of Fig. 41.]
+
+[Illustration: FIG. 43. Four wild types of Paratettix in upper line with
+three hybrids below.]
+
+It has been found that there may be even more than two factors that show
+Mendelian segregation when brought together in pairs. For example, in the
+southern States there are several races of the grouse locust (Paratettix)
+that differ from each other markedly in color patterns (fig. 43). When any
+two individuals of these races are crossed they give, as Nabours has shown,
+in F_2 a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there
+are here at least nine characters, any two of which behave as a Mendelian
+pair. These races have arisen in nature and differ definitely and
+strikingly from each other, yet any two differ by only one factor
+difference.
+
+[Illustration: FIG. 44. Diagram illustrating four allelomorphs in mice,
+viz. gray bellied gray (wild type) (above, to left); white bellied gray
+(above, to right); yellow (below, to right); and black (below, to left).]
+
+Similar relations have been found in a number of domesticated races. In
+mice there is a quadruple system represented by the gray house mouse, the
+white bellied, the yellow and the black mouse (fig. 44). In rabbits there
+is probably a triple system, that includes the albino, the Himalayan, and
+the black races. In the silkworm moth there have been described four types
+of larvae, distinguished by different color markings, that form a system of
+quadruple allelomorphs. In Drosophila there is a quintuple system of
+factors in the sex chromosome represented by eye colors, a triple system of
+body colors, and a triple system of factors for eye colors in the third
+chromosome.
+
+MUTATION AND EVOLUTION
+
+What bearing has the appearance of these new types of Drosophila on the
+theory of evolution may be asked. The objection has been raised in fact
+that in the breeding work with Drosophila we are dealing with artificial
+and unnatural conditions. It has been more than implied that results
+obtained from the breeding pen, the seed pan, the flower pot and the milk
+bottle do not apply to evolution in the "open", nature "at large" or to
+"wild" types. To be consistent, this same objection should be extended to
+the use of the spectroscope in the study of the evolution of the stars, to
+the use of the test tube and the balance by the chemist, of the
+galvanometer by the physicist. All these are unnatural instruments used to
+torture Nature's secrets from her. I venture to think that the real
+antithesis is not between unnatural and natural treatment of Nature, but
+rather between controlled or verifiable data on the one hand, and
+unrestrained generalization on the other.
+
+If a systematist were asked whether these new races of Drosophila are
+comparable to wild species, he would not hesitate for a moment. He would
+call them all one species. If he were asked why, he would say, I think,
+"These races differ only in one or two striking points, while in a hundred
+other respects they are identical even to the minutest details." He would
+add, that as large a group of wild species of flies would show on the whole
+the reverse relations, _viz._, they would differ in nearly every detail and
+be identical in only a few points. In all this I entirely agree with the
+systematist, for I do not think such a group of types differing by one
+character each, is comparable to most wild groups of species because the
+difference between wild species is due to a large number of such single
+differences. The characters that have been accumulated in wild species are
+of significance in the maintenance of the species, or at least we are led
+to infer that even though the visible character that we attend to may not
+itself be important, one at least of the other effects of the factors that
+represent these characters is significant. It is, of course, hardly to be
+expected that _any_ random change in as complex a mechanism as an insect
+would improve the mechanism, and as a matter of fact it is doubtful whether
+any of the mutant types so far discovered are better adapted to those
+conditions to which a fly of this structure and habits is already adjusted.
+But this is beside the mark, for modern genetics shows very positively that
+adaptive characters are inherited in exactly the same way as are those that
+are not adaptive; and I have already pointed out that we cannot study a
+single mutant factor without at the same time studying one of the factors
+responsible for normal characters, for the two together constitute the
+Mendelian pair.
+
+And, finally, I want to urge on your attention a question that we are to
+consider in more detail in the last lecture. Evolution of wild species
+appears to have taken place by modifying and improving bit by bit the
+structures and habits that the animal or plant already possessed. We have
+seen that there are thirty mutant factors at least that have an influence
+on eye color, and it is probable that there are at least as many normal
+factors that are involved in the production of the red eye of the wild fly.
+
+Evolution from this point of view has consisted largely in introducing new
+factors that influence characters already present in the animal or plant.
+
+Such a view gives us a somewhat different picture of the process of
+evolution from the old idea of a ferocious struggle between the individuals
+of a species with the survival of the fittest and the annihilation of the
+less fit. Evolution assumes a more peaceful aspect. New and advantageous
+characters survive by incorporating themselves into the race, improving it
+and opening to it new opportunities. In other words, the emphasis may be
+placed less on the competition between the individuals of a species
+(because the destruction of the less fit does not _in itself_ lead to
+anything that is new) than on the appearance of new characters and
+modifications of old characters that become incorporated in the species,
+for on these depends the evolution of the race.
+
+       *       *       *       *       *
+
+
+CHAPTER III
+
+THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM
+
+The discovery that Mendel made with edible peas concerning heredity has
+been found to apply everywhere throughout the plant and animal kingdoms--to
+flowering plants, to insects, snails, crustacea, fishes, amphibians, birds,
+and mammals (including man).
+
+There must be something that these widely separated groups of plants and
+animals have in common--some simple mechanism perhaps--to give such
+definite and orderly series of results. There is, in fact, a mechanism,
+possessed alike by animals and plants, that fulfills every requirement of
+Mendel's principles.
+
+THE CELLULAR BASIS OF ORGANIC EVOLUTION AND HEREDITY
+
+In order to appreciate the full force of the evidence, let me first pass
+rapidly in review a few familiar, historical facts, that preceded the
+discovery of the mechanism in question.
+
+[Illustration: FIG. 45. Typical cell showing the cell wall, the protoplasm
+(with its contained materials); the nucleus with its contained chromatin
+and nuclear sap. (After Dahlgren.)]
+
+Throughout the greater part of the last century, while students of
+evolution and of heredity were engaged in what I may call the more general,
+or, shall I say, the _grosser_ aspects of the subject, there existed
+another group of students who were engaged in working out the minute
+structure of the material basis of the living organism. They found that
+organs such as the brain, the heart, the liver, the lungs, the kidneys,
+etc., are not themselves the units of structure, but that all these organs
+can be reduced to a simpler unit that repeats itself a thousand-fold in
+every organ. We call this unit a cell (fig. 45).
+
+The egg is a cell, and the spermatozoon is a cell. The act of fertilization
+is the union of two cells (fig. 47, upper figure). Simple as the process of
+fertilization appears to us today, its discovery swept aside a vast amount
+of mystical speculation concerning the role of the male and of the female
+in the act of procreation.
+
+Within the cell a new microcosm was revealed. Every cell was found to
+contain a spherical body called the nucleus (fig. 46a). Within the nucleus
+is a network of fibres, a sap fills the interstices of the network. The
+network resolves itself into a definite number of threads at each division
+of the cell (fig. 46 b-e). These threads we call chromosomes. Each species
+of animals and plants possesses a characteristic number of these threads
+which have a definite size and sometimes a specific shape and even
+characteristic granules at different levels. Beyond this point our
+strongest microscopes fail to penetrate. Observation has reached, for the
+time being, its limit.
+
+[Illustration: FIG. 46. A series of cells in process of cell division. The
+chromosomes are the black threads and rods. (After Dahlgren.)]
+
+The story is taken up at this point by a new set of students who have
+worked in an entirely different field. Certain observations and experiments
+that we have not time to consider now, led a number of biologists to
+conclude that the chromosomes are the bearers of the hereditary units. If
+so, there should be many such units carried by _each_ chromosome, for the
+number of chromosomes is limited while the number of independently
+inherited characters is large. In Drosophila it has been demonstrated not
+only that there are exactly as many groups of characters that are inherited
+together as there are pairs of chromosomes, but even that it is possible to
+locate one of these groups in a particular chromosome and to state the
+_relative position_ there of the factors for the characters. If the
+validity of this evidence is accepted, the study of the cell leads us
+finally in a mechanical, but not in a chemical sense, to the ultimate units
+about which the whole process of the transmission of the hereditary factors
+centers.
+
+But before plunging into this somewhat technical matter (that is difficult
+only because it is unfamiliar), certain facts which are familiar for the
+most part should be recalled, because on these turns the whole of the
+subsequent story.
+
+[Illustration: FIG. 47. An egg, and the division of the egg--the so-called
+process of cleavage. (After Selenka.)]
+
+The thousands of cells that make up the cell-state that we call an animal
+or plant come from the fertilized egg. An hour or two after fertilization
+the egg divides into two cells (fig. 47). Then each half divides again.
+Each quarter next divides. The process continues until a large number of
+cells is formed and out of these organs mould themselves.
+
+[Illustration: FIG. 48. Section of the egg of the beetle, Calligrapha,
+showing the pigment at one end where the germ cells will later develop as
+shown in the other two figures. (After Hegner.)]
+
+At every division of the cell the chromosomes also divide. Half of these
+have come from the mother, half from the father. Every cell contains,
+therefore, the sum total of all the chromosomes, and if these are the
+bearers of the hereditary qualities, every cell in the body, whatever its
+function, has a common inheritance.
+
+At an early stage in the development of the animal certain cells are set
+apart to form the organs of reproduction. In some animals these cells can
+be identified early in the cleavage (fig. 48).
+
+The reproductive cells are at first like all the other cells in the body in
+that they contain a full complement of chromosomes, half paternal and half
+maternal in origin (fig. 49). They divide as do the other cells of the body
+for a long time (fig. 49, upper row). At each division each chromosome
+splits lengthwise and its halves migrate to opposite poles of the spindle
+(fig. 49 c).
+
+But there comes a time when a new process appears in the germ cells (fig 49
+e-h). It is essentially the same in the egg and in the sperm cells. The
+discovery of this process we owe to the laborious researches of many
+workers in many countries. The list of their names is long, and I shall not
+even attempt to repeat it. The chromosomes come together in pairs (fig. 49
+a). Each maternal chromosome mates with a paternal chromosome of the same
+kind.
+
+[Illustration: FIG. 49. In the upper row of the diagram a typical process
+of nuclear division, such as takes place in the early germ cells or in the
+body cells. In the lower row the separation of the chromosomes that have
+paired. This sort of separation takes place at one of the two reduction
+divisions.]
+
+Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the
+divisions the double chromosomes separate so that each resulting cell comes
+to contain some maternal and some paternal chromosomes, i.e. one or the
+other member of each pair. At the other division each chromosome simply
+splits as in ordinary cell division.
+
+[Illustration: FIG. 50. The two maturation divisions of the sperm cell.
+Four sperms result, each with half (haploid) the full number (diploid) of
+chromosomes.]
+
+The upshot of the process is that the ripe eggs (fig. 51) and the ripe
+spermatozoa (fig. 50) come to contain only half the total number of
+chromosomes.
+
+[Illustration: FIG. 51. The two maturation divisions of the egg. The
+divisions are unequal, so that two small polar bodies are formed one of
+these subsequently divides. The three polar bodies and the egg are
+comparable to the four sperms.]
+
+When the eggs are fertilized the whole number of chromosomes is restored
+again.
+
+THE MECHANISM OF MENDELIAN HEREDITY DISCOVERED IN THE BEHAVIOR OF THE
+CHROMOSOMES
+
+If the factors in heredity are carried in the chromosomes and if the
+chromosomes are definite structures, we should anticipate that there should
+be as many _groups_ of characters as there are kinds of chromosomes. In
+only one case has a sufficient number of characters been studied to show
+whether there is any correspondence between the number of hereditary groups
+of characters and the number of chromosomes. In the fruit fly, Drosophila
+ampelophila, we have found about 125 characters that are inherited in a
+perfectly definite way. On the opposite page is a list of some of them.
+
+It will be observed in this list that the characters are arranged in four
+groups, Groups I, II, III and IV. Three of these groups are equally large
+or nearly so; Group IV contains only two characters. The characters are put
+into these groups because in heredity the members of each group tend to be
+inherited together, i.e., if two or more enter the cross together they tend
+to remain together through subsequent generations. On the other hand, any
+member of one group is inherited entirely independently of any member of
+the other groups; in the same way as Mendel's yellow-green pair of
+characters is inherited independently of the round-wrinkled pair.
+
+  _Group I_       _Group II_        _Group III_        _Group IV_
+   Abnormal        Antlered          Band               Bent
+   Bar             Apterous          Beaded             Eyeless
+   Bifid           Arc               Cream III
+   Bow             Balloon           Deformed
+   Cherry          Black             Dwarf
+   Chrome          Blistered         Ebony
+   Cleft           Comma             Giant
+   Club            Confluent         Kidney
+   Depressed       Cream II          Low crossing over
+   Dot             Curved            Maroon
+   Eosin           Dachs             Peach
+   Facet           Extra vein        Pink
+   Forked          Fringed           Rough
+   Furrowed        Jaunty            Safranin
+   Fused           Limited           Sepia
+   Green           Little crossover  Sooty
+   Jaunty          Morula            Spineless
+   Lemon           Olive             Spread
+   Lethals, 13     Plexus            Trident
+   Miniature       Purple            Truncate intensifier
+   Notch           Speck             Whitehead
+   Reduplicated    Strap             White ocelli
+   Ruby            Streak
+   Rudimentary     Trefoil
+   Sable           Truncate
+   Shifted         Vestigial
+   Short
+   Skee
+   Spoon
+   Spot
+   Tan
+   Truncate intensifier
+   Vermilion
+   White
+   Yellow
+
+If the factors for these characters are carried by the chromosomes, then we
+should expect that those factors that are carried by the same chromosome
+would be inherited together, provided the chromosomes are definite
+structures in the cell.
+
+[Illustration: FIG. 52. Chromosomes (diploid) of D. ampelophila. The sex
+chromosomes are XX in the female and XY in the male. There are three other
+pairs of chromosomes.]
+
+In the chromosome group of Drosophila, (fig. 52) there are _four_ pairs of
+chromosomes, three of nearly the same size and one much smaller. Not only
+is there agreement between the number of hereditary groups and the number
+of the chromosomes, but even the size relations are the same, for there are
+three great groups of characters and three pairs of large chromosomes, and
+one small group of characters and one pair of small chromosomes.
+
+THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA AMPELOPHILA
+
+The following description of the characters of the wild fly may be useful
+in connection with the account of the modifications of these characters
+that appear in the mutants.
+
+The head and thorax of the wild fly are grayish-yellow, the abdomen is
+banded with alternate stripes of yellow and black. In the male, (fig. 4 to
+right), there are three narrow bands and a black tip. In the female there
+are five black bands (fig. 4 to left). The wings are gray with a surface
+texture of such a kind that at certain angles they are iridescent. The eyes
+are a deep, solid, brick-red. The minute hairs that cover the body have a
+very definite arrangement that is most obvious on the head and thorax.
+There is a definite number of larger hairs called bristles or chaetae which
+have a characteristic position and are used for diagnostic purposes in
+classifying the species. On the foreleg of the male there is a comb-like
+organ formed by a row of bristles; it is absent in the female. The comb is
+a secondary sexual character, and it is, so far as known, functionless.
+
+Some of the characters of the mutant types are shown in figures 53, 54, 55,
+56. The drawing of a single fly is often used here to illustrate more than
+one character. This is done to economize space, but of course there would
+be no difficulty in actually bringing together in the same individual any
+two or more characters belonging to the same group (or to different
+groups). Without colored figures it is not possible to show many of the
+most striking differences of these mutant races; at most dark and light
+coloring can be indicated by the shading of the body, wings, or eyes.
+
+_Group I_
+
+In the six flies drawn in figure 53 there are shown five different wing
+characters. The first of these types (a) is called cut, because the ends of
+the wings look as though they had been cut to a point. The antennae are
+displaced downward and appressed and their bristle-like aristae are
+crumpled.
+
+[Illustration: FIG. 53. Group I. (See text)]
+
+The second figure (b) represents a fly with a notch in the ends of the
+wings. This character is dominant, but the same factor that produces the
+notch in the wings is also a recessive lethal factor; because of this
+latter effect of the character no males of this race exist, and the females
+of the race are never pure but hybrid. Every female with notch wings bred
+to a wild male, will produce in equal numbers notch winged daughters and
+daughters with normal wings. There will be half as many sons as daughters.
+The explanation of this peculiar result is quite simple. Every notch winged
+female has one X chromosome that carries the factor for notch and one X
+chromosome that is "normal". Daughters receiving the former chromosomes are
+notched because the factor for notch is dominant, but they are not killed
+since the lethal effect of the notch factor is recessive to the normal
+allelomorph carried by the other chromosome that the daughters get from
+their father. This normal factor is recessive for notch but dominant for
+life. This same figure (b) is used here to show three other sex linked
+characters. The spines on the thorax are twisted or kinky, which is due to
+a factor called "forked". The effect is best seen on the thorax, but all
+spines on the body are similarly modified; even the minute hairs are also
+affected. Ruby eye color might be here represented--if the eyes in the
+figure were colored. The lighter color of the body and antennae is intended
+to indicate that the character tan is also present. The light color of the
+antennae is the most certain way of identifying tan. The tan flies are
+interesting because they have lost the positive heliotropism that is so
+marked a feature in the behavior of D. ampelophila. As this peculiarity of
+the tan flies is inherited like all the other sex linked characters, it
+follows that when a tan female is bred to a wild male all the sons inherit
+the recessive tan color and indifference to light, while the daughters show
+the dominant sex linked character of their father, i.e., they are "gray",
+and go to the light. Hence when such a brood is disturbed the females fly
+to the light, but the males remain behind.
+
+One of the first mutants that appeared in D. ampelophila was called
+rudimentary on account of the condition of the wings (c). The same mutation
+has appeared independently several times. In the drawing (c) the dark body
+color is intended to indicate "sable" and the lighter color of the eyes is
+intended to indicate eosin. This eye color, which is an allelomorph of
+white, is also interesting because in the female the color is deeper than
+in the male. In other cases of sex linked factors the character is the same
+in the two sexes.
+
+In the fourth figure (d) the third and fourth longitudinal veins of the
+wing are _fused_ into one vein from the base of the wing to the level of
+the first cross-vein and in addition converge and meet near their outer
+ends. The shape of the eye is represented in the figure as different from
+the normal, due to another factor called "bar". This is a dominant
+character, the hybrid condition being also narrow, but not so narrow as the
+pure type. Vermilion eye color might also be here represented--due to a
+factor that has appeared independently on several occasions.
+
+In the fifth figure (e) the wings are shorter and more pointed than in the
+wild fly. This character is called miniature. The light color of the
+drawing may be taken to represent yellow body color, and the light color of
+the eye white eye color.
+
+In the last figure (f) the wings are represented as pads, essentially in
+the same condition that they are in when the fly emerges from the pupa
+case. Not all the flies of this stock have the wings in this condition;
+some have fully expanded wings that appear normal in all respects.
+Nevertheless, about the same percentage of offspring show the pads
+irrespective of whether the parents had pads or expanded wings.
+
+The flies of this stock show, however, another character, which is a
+product of the same factor, and which is constant, i.e., repeated in all
+individuals. The two bristles on the sides of the thorax are constantly
+absent in this race. The lighter color of the eye in the figure may be
+taken to indicate buff--a faint yellowish color. The factor for this eye
+color is another allelomorph of white.
+
+There are many other interesting characters that belong to the first group,
+such as abnormal abdomen, short legs, duplication of the legs, etc. In
+fact, any part of the body may be affected by a sex-linked factor.
+
+_Group II_
+
+In the first figure (a) of figure 54 that contains members of Group II the
+wings are almost entirely absent or "vestigial". This condition arose at a
+single step and breeds true, although it appears to be influenced to some
+extent by temperature, also by modifiers that sometimes appear in the
+stock. Purple eye color belongs in Group II; it resembles the color of the
+eye of the wild fly but is darker and more translucent.
+
+[Illustration: FIG. 54. Group II. (See text.)]
+
+In the second figure (b) the wing is again long and narrow and sometimes
+bent back on itself, as shown here. In several respects the wing resembles
+strap (d) but seems to be due to another factor, called antler,
+insufficiently studied as yet.
+
+In the third figure (c) the wings turn up at the end. This is brought about
+by the presence of the factor called jaunty.
+
+In the fourth figure the wings are long and narrow and several of the veins
+are unrepresented. This character, "strap", is very variable and has not
+yet been thoroughly studied. On the thorax there is a deep black mark
+called trefoil. Even in the wild fly there is a three pronged mark on the
+thorax present in many individuals. Trefoil is a further development and
+modification of this mark and is due to a special factor.
+
+In the fifth figure (e) the wings are arched. The factor is called arc. The
+dark color of the body, and especially of the wings, indicates the factor
+for black.
+
+The sixth figure (f) shows the wings "curved" downwards. In addition there
+is present a minute black speck at the base of each wing, due to another
+factor called speck.
+
+In the seventh figure (g) the wing is truncate. Its end is obliquely
+squared instead of rounded; it may be longer than the body, or shorter when
+other modifying factors are present. The mutation that produces this type
+of wing is of not infrequent occurrence. It has been shown by Muller and
+Altenburg that there are at least two factors that modify this
+character--the chief factor is present in the second chromosome; alone it
+produces the truncate wing in only a certain percentage of cases, but when
+the modifiers are also present about ninety percent of the individuals may
+show the truncate condition of the wing. But the presence of these factors
+makes the stock very infertile, so that it is difficult to maintain.
+
+In the eighth figure (h) the legs are shortened owing to the absence of a
+segment of the tarsus. The stock is called dachs--a nickname given to it
+because the short legs suggested the dachshund.
+
+_Group III_
+
+In figure 55, (a), a mutant type called bithorax is shown. The old
+metathorax is replaced by another mesothorax thrust in between the normal
+mesothorax and the abdomen. It carries a pair of wings that do not
+completely unfold. On this new mesothorax the characteristic arrangement of
+the bristles is shown. Thus at a single step a typical region of the body
+has doubled. The character is recessive.
+
+[Illustration: FIG. 55. Group III. (See text.)]
+
+The size of the adult fly of D. ampelophila varies greatly according to the
+amount of nourishment obtained by the larva. After the fly emerges its size
+remains nearly constant, as in many insects. Two races have, however, been
+separated by Bridges that are different in size as a result of a genetic
+factor. The first of these, called dwarf, is represented by figure 55, (b).
+
+The race is minute, although of course its size is variable, depending on
+food and other conditions. The same figure shows the presence of another
+factor, "sooty", that makes the fly very dark. Maroon eye color might be
+here represented, due to still another factor.
+
+In the third figure (c) the other mutation in size is shown. It is called
+"giant". The flies are twice the size of wild flies. An eye color, called
+peach, might here be represented. It is an allelomorph of pink.
+
+In the fourth figure (d) the mutant called dichaete is shown. It is
+characterized by the absence of two of the bristles on the thorax. Other
+bristles may also be absent, but not so constantly as the two just
+mentioned. Another effect of the same factor is the spread-out condition of
+the wings. The very dark eye color in this figure may be taken to indicate
+the presence of another factor, "sepia", which causes the eyes to assume a
+brown color that becomes black with age. Most of the other mutations in eye
+color that have occurred tend to give a lighter color: this one, which is
+also recessive, makes the eye darker.
+
+In the fifth figure (e) the color of the darkest fly is due to a factor
+called ebony, which is an allelomorph of sooty.
+
+In the sixth figure (f) the wings are beaded, i.e., the margin is defective
+at intervals, giving a beaded-like outline to the wings. This condition is
+very variable and much affected by other factors that influence the shape
+of the wings. The lighter eye color of the drawing may be taken to
+represent pink.
+
+In the seventh figure (g) the wings are curled up over the back. This is a
+recessive character.
+
+_Group IV_
+
+Only two mutants have been obtained that do not belong to any of the
+preceding groups; these are put together in Group IV. It has been shown
+that they are linked to each other and the linkage is so close that it has
+thus far been impossible to obtain the dominant recessive. One of these
+mutants, called "eyeless" (fig. 56, a, a^1), is variable--the eyes are
+often entirely absent or represented by one or more groups of ommatidia.
+The outline of the original eye, so to speak, is strongly marked out and
+its area might be called a rudimentary organ, if such a statement has any
+meaning here.
+
+[Illustration: FIG. 56. Group IV. (See text.)]
+
+The other figure (b) represents "bent", so called from the shape of the
+wings. This mutant is likewise very variable, often indistinguishable from
+the wild type, yet when well developed strikingly different from any other
+mutant.
+
+This brief account of a few of the mutant races that can be most easily
+represented by uncolored figures will serve to show how all parts of the
+body may change, some of the changes being so slight that they would be
+overlooked except by an expert, others so great that in the character
+affected the flies depart far from the original species.
+
+_It is important to note that mutations in the first chromosome are not
+limited to any part of the body nor do they affect more frequently a
+particular part. The same statement holds equally for all of the other
+chromosomes. In fact, since each factor may affect visibly several parts of
+the body at the same time there are no grounds for expecting any special
+relation between a given chromosome and special regions of the body. It can
+not too insistently be urged that when we say a character is the product of
+a particular factor we mean no more than that it is the most conspicuous
+effect of the factor._
+
+If, then, as these and other results to be described point to the
+chromosomes as the bearers of the Mendelian factors, and if, as will be
+shown presently, these factors have a definite location in the chromosomes
+it is clear that the location of the factors in the chromosomes bears no
+spatial relation to the location of the parts of the body to each other.
+
+LOCALIZATION OF FACTORS IN THE CHROMOSOMES
+
+_The Evidence from Sex Linked Inheritance_
+
+When we follow the history of pairs of chromosomes we find that their
+distribution in successive generations is paralleled by the inheritance of
+Mendelian characters. This is best shown in the sex chromosomes (fig. 57).
+In the female there are two of these chromosomes that we call the X
+chromosomes; in the male there are also two but one differs from those of
+the female in its shape, and in the fact that it carries none of the normal
+allelomorphs of the mutant factors. It is called the Y chromosome.
+
+The course followed by the sex chromosomes and that by the characters in
+the case of sex linked inheritance are shown in the next diagram of
+Drosophila illustrating a cross between a white eyed male and a red eyed
+female.
+
+[Illustration: FIG. 57. Scheme of sex determination in Drosophila type.
+Each _mature_ egg contains one X, each mature sperm contains one X, or a Y
+chromosome. Chance union of any egg with any sperm will give either XX
+(female) or XY (male).]
+
+[Illustration: FIG. 58. Cross between white eyed male of D. ampelophila and
+red eyed female. The sex chromosomes are indicated by the rods. A black rod
+indicates that the chromosome carries the factor for red; the open
+chromosome the factor for white eye color.]
+
+The first of these represents a cross between a white eyed male and a red
+eyed female (fig. 58, top row). The X chromosome in the male is represented
+by an open bar, the Y chromosome is bent. In the female the two X
+chromosomes are black. Each egg of such a female will contain one "black" X
+after the polar bodies have been thrown off. In the male there will be two
+classes of sperm--the female-producing, carrying the (open) X, and the
+male-producing, carrying the Y chromosome. Any egg fertilized by an X
+bearing sperm will produce a female that will have red eyes because the X
+(black) chromosome it gets from the mother carries the dominant factor for
+red. Any egg fertilized by a Y-bearing sperm will produce a male that will
+also have red eyes because he gets his (black) X chromosome from his
+mother.
+
+When, then, these two F_1 flies (second row) are inbred the following
+combinations are expected. Each egg will contain a black X (red eye
+producing) or a white X (white eye producing) after the polar bodies have
+been extruded. The male will produce two kinds of sperms, of which the
+female producing will contain a black X (red eye producing). Since any egg
+may by chance be fertilized by any sperm there will result the four classes
+of individuals shown on the bottom row of the diagram. All the females will
+have red eyes, because irrespective of the two kinds of eggs involved all
+the female-producing sperm carry a black X. Half of the males have red eyes
+because half of the eggs have had each a red-producing X chromosome. The
+other half of the males have white eyes, because the other half of the eggs
+had each a white-producing X chromosome. Other evidence has shown that the
+Y chromosome of the male is indifferent, so far as these Mendelian factors
+are concerned.
+
+[Illustration: FIG. 59. Cross between red eyed male and white eyed female;
+reciprocal cross of Fig. 58.]
+
+The reciprocal experiment is illustrated in figure 59. A white eyed female
+is mated to a red eyed male (top row). All the mature eggs of such a female
+contain one white-producing X chromosome represented by the open bar in the
+diagram. The red eyed male contains female-producing X-bearing sperm that
+carry the factor for red eye color, and male-producing Y chromosomes. Any
+egg fertilized by an X-bearing sperm will become a red eyed female because
+the X chromosome that comes from the father carries the dominant factor for
+red eye color. Any egg fertilized by a Y-bearing sperm will become a male
+with white eyes because the only X chromosome that the male contains comes
+from his mother and is white producing.
+
+When these two F_1 flies are inbred (middle row) the following combinations
+are expected. Half the eggs will contain each a white producing X
+chromosome and half red producing. The female-producing sperms will each
+contain a white X and the male-producing sperms will each contain an
+indifferent Y chromosome. Chance meetings of egg and sperm will give the
+four F_2 classes (bottom row). These consist of white eyed and red eyed
+females and white eyed and red eyed males. The ratio here is 1:1 and not
+three to one (3:1) as in other Mendelian cases. But Mendel's law of
+segregation is not transgressed, as the preceding analysis has shown; for,
+the chromosomes have followed strictly the course laid down on Mendel's
+principle for the distribution of factors. The peculiar result in this case
+is due to the fact that the F_1 male gets his single factor for eye color
+from his mother only and it is linked to or contained in a body (the X
+chromosome) that is involved in producing the females, while the mate of
+this body--the Y chromosome--is indifferent with regard to these factors,
+yet active as a mate to X in synapsis.
+
+[Illustration: FIG. 60. Diagram of sex determination in type with XX female
+and XO male (after Wilson).]
+
+In man there are several characters that show exactly this same kind of
+inheritance. Color blindness, or at least certain kinds of color blindness,
+appear to follow the same scheme. A color blind father transmits through
+his daughters his peculiarity to half of his grandsons, but to none of his
+grand-daughters (fig. 38A). The result is the same as in the case of the
+white eyed male of Drosophila. Color blind women are rather unusual, which
+is expected from the method of inheritance of this character, but in the
+few known cases where such color blind women have married normal husbands
+the sons have inherited the peculiarity from the mother (fig. 38B). Here
+again the result is the same as for the similar combination in Drosophila.
+
+[Illustration: FIG. 61. Spermatogenesis in man. There are 47 chromosomes
+(diploid) in the male. After reduction half of the sperm carry 24
+chromosomes (one of which is X) and half carry 23 chromosomes (no X).]
+
+In man the sex formula appears to be XX for the female and XO for the male
+(fig. 60), and since the relation is essentially the same as that in
+Drosophila the chromosome explanation is the same. According to von
+Winiwarter there are 48 chromosomes in the female and 47 in the male (fig.
+61). After the extrusion of the polar bodies there are 24 chromosomes in
+the egg. In the male at one of the two maturation divisions the X
+chromosome passes to one pole undivided (fig. 61, C). In consequence there
+are two classes of sperms in man; female producing containing 24
+chromosomes, and male producing containing 23 chromosomes. If the factor
+for color blindness is carried by the X chromosome its inheritance in man
+works out on the same chromosome scheme and in the same way as does white
+eye color (or any other sex linked character) in the fly, for the O sperm
+in man is equivalent to the Y sperm in the fly.
+
+In these cases we have been dealing with a single pair of characters. Let
+us now take a case where two pairs of sex linked characters enter the cross
+at the same time, and preferably a case where the two recessives enter the
+cross from the same parent.
+
+If a female with white eyes and yellow wings is crossed to a wild male with
+red eyes and gray wings (fig. 62), the sons are yellow and have white eyes
+and the daughters are gray and have red eyes. If two F_1 flies are mated
+they will produce the following classes.
+
+[Illustration: FIG. 62. Cross between a white eyed, yellow winged female of
+D. ampelophila and a red eyed, gray winged male. Two pairs of sex linked
+characters, viz., white-red and yellow-gray are involved. (See text.)]
+
+  Yellow  Gray  Yellow  Gray
+  White   Red   Red     White
+  ------------  -------------
+        |             |
+       99.%          1.%
+
+Not only have the two grandparental combinations reappeared, but in
+addition two new combinations, viz., grey white and yellow red. The two
+original combinations far exceed in numbers the new or exchange
+combinations. If we follow the history of the X chromosomes we discover
+that the _larger classes_ of grandchildren appear in accord with the way in
+which the X chromosomes are transmitted from one generation to the next.
+
+The _smaller classes_ of grandchildren, the exchange combinations or
+cross-overs, as we call them, can be explained by the assumption that at
+some stage in their history an interchange of parts has taken place between
+the chromosomes. This is indicated in the diagrams.
+
+The most important fact brought out by the experiment is that the factors
+that went in together tend to stick together. It makes no difference in
+what combination the members of the two pairs of characters enter, they
+tend to remain in that combination.
+
+If one admits that the sex chromosomes carry these factors for the
+sex-linked characters--and the evidence is certainly very strong in favor
+of this view--it follows necessarily from these facts that at some time in
+their history there has been an interchange between the two sex chromosomes
+in the female.
+
+There are several stages in the conjugation of the chromosomes at which
+such an interchange between the members of a pair might occur. There is
+further a small amount of direct evidence, unfortunately very meagre at
+present, showing that an interchange does actually occur.
+
+At the ripening period of the germ cell the members of each pair of
+chromosomes come together (fig. 49, e). In several forms they have been
+described as meeting at one end and then progressively coming to lie side
+by side as shown in fig. 63, e, f, g, h, i. At the end of the process they
+appear to have completely united along their length (fig. 63, j, k, l). It
+is always a maternal and a paternal chromosome that meet in this way and
+always two of the same kind. It has been observed that as the members of a
+pair come together they occasionally twist around each other (fig. 63, g,
+l, and 64, and 65). In consequence a part of one chromosome comes to be now
+on one side and now on the other side of its mate.
+
+[Illustration: FIG. 63. Conjugation of chromosomes (side to side union) in
+the spermatogenesis of Batracoseps. (After Janssens.)]
+
+When the chromosomes separate at the next division of the germ cell the
+part on one side passes to one pole, the part on the other to the opposite
+pole, (figs. 64 and 65). Whenever the chromosomes do not untwist at this
+time there must result an interchange of pieces where they were crossed
+over each other.
+
+[Illustration: FIG. 64. Scheme to illustrate a method of crossing over of
+the chromosomes.]
+
+Janssens has found at the time of separation evidence in favor of the view
+that some such interchange probably takes place.
+
+We find this same process of interchange of characters taking place in each
+of the other three groups of Drosophila. An example will show this for the
+Group II.
+
+[Illustration: FIG. 65. Scheme to illustrate double crossing over.]
+
+If a black vestigial male is crossed to a gray long-winged female (fig. 66)
+the offspring are gray long. If an F_1 female is back-crossed to a black
+vestigial male the following kinds of flies are produced:
+
+  Black        Gray       Black   Gray
+  vestigial    long       long    vestigial
+  -----------------       -----------------
+          |                      |
+         83%                    17%
+
+The combinations that entered are more common in the F_2 generations than
+the cross-over classes, showing that there is linkage of the factors that
+entered together.
+
+Another curious fact is brought out if instead of back-crossing the F_1
+female we back-cross the F_1 male to a black vestigial female. Their
+offspring are now of only two kinds, black vestigial and gray long. This
+means that in the male there is no crossing-over or interchange of pieces.
+This relation holds not only for the Group II but for all the other groups
+as well.
+
+Why interchange takes place in the female of Drosophila and not in the male
+we do not know at present. We might surmise that when in the male the
+members of a pair come together they do not twist around each other, hence
+no crossing-over results.
+
+[Illustration: FIG. 66. Cross between black vestigial and gray long flies.
+Two pairs of factors involved in the second group. The F_1 female is back
+crossed (to right) to black vestigial male; and the F_1 male is back
+crossed to black vestigial female (to left). Crossing over takes place in
+the F_1 female but not in the F_1 male.]
+
+Crossing-over took place between white and yellow only once in a hundred
+times. Other characters show different values, but the same value under the
+same conditions is obtained from the same pair of characters.
+
+[Illustration: FIG. 67. Map of four chromosomes of D. ampelophila locating
+those factors in each group that have been most fully studied.]
+
+If we assume that the nearer together the factors lie in the chromosome the
+less likely is a twist to occur between them, and conversely the farther
+apart they lie the more likely is a twist to occur between them, we can
+understand how the linkage is different for different pairs of factors.
+
+On this basis we have made out chromosomal maps for each chromosome (fig.
+67). The diagram indicates those loci that have been most accurately
+placed.
+
+_The Evidence from Interference_
+
+There is a considerable body of information that we have obtained that
+corroborates the location of the factors in the chromosome. This evidence
+is too technical to take up in any detail, but there is one result that is
+so important that I must attempt to explain it. If, as I assume, crossing
+over is brought about by twisting of the chromosomes, and if owing to the
+material of the chromosomes there is a most frequent distance of internode,
+then, when crossing over between nodes takes place at same level at a-b in
+figure 68, the region on each side of that point, a to A and b to B, should
+be protected, so to speak, from further crossing over. This in fact we have
+found to be the case. No other explanation so far proposed will account for
+this extraordinary relation.
+
+[Illustration: FIG. 68. Scheme to indicate that when the members of a pair
+of chromosomes cross (at a-b) the region on each side is protected
+inversely to the distance from a-b.]
+
+What advantage, may be asked, is there in obtaining numerical data of this
+kind? It is this:--whenever a new character appears we need only determine
+in which of the four groups it lies and its distance from two members
+within that group. With this information we can predict with a high degree
+of probability what results it will give with any other member of any
+group. Thus we can do on paper what would require many months of labor by
+making the actual experiment. In a word we can predict what will happen in
+a situation where prediction is impossible without this numerical
+information.
+
+_The Evidence from Non-Disjunction_
+
+In the course of the work on Drosophila exceptions appeared in one strain
+where certain individuals did not conform to the scheme of sex linked
+inheritance. For a moment the hypothesis seemed to fail, but a careful
+examination led to the suspicion that in this strain something had happened
+to the sex chromosomes. It was seen that if in some way the X chromosomes
+failed to disjoin in certain eggs, the exceptions could be explained. The
+analysis led to the suggestion that if the Y chromosome had got into the
+female line the results would be accounted for, since its presence there
+would be expected to cause this peculiar non-disjunction of the X
+chromosomes.
+
+That this was the explanation was shown when the material was examined. The
+females that gave these results were found by Bridges to have two X's and a
+Y chromosome.
+
+The normal chromosome group of the female is shown in figure 52 and the
+chromosome group of one of the exceptional females is shown in figure 69.
+In a female of this kind there are three sex chromosomes X X Y which are
+homologous in the sense that in normal individuals the two present are
+mates and separate at the reduction division. If in the X X Y individual X
+and X conjugate and separate at reduction and the unmated Y is free to move
+to either pole of the spindle, two kinds of mature eggs will result, viz.,
+X and XY. If, on the other hand, X and Y conjugate and separate at
+reduction and the remaining X is free to go to either pole, four kinds of
+eggs will result--XY--X--XX--Y. As a total result four kinds of eggs are
+expected: viz. many XY and X eggs and a few XX and Y eggs.
+
+[Illustration: FIG. 69. Figure of the chromosome group of an XXY female,
+that gives non-disjunction.]
+
+These four kinds of eggs may be fertilized either by female-producing
+sperms or male-producing sperms, as indicated in the diagram (fig. 70).
+
+[Illustration: FIG. 70. Scheme showing the results of fertilizing white
+bearing eggs (4 kinds) resulting from non-disjunction. The upper half of
+the diagram gives the results when these eggs are fertilized by normal red
+bearing, female producing sperm, the lower half by normal, male producing
+sperm.]
+
+If such an XXY female carried white bearing Xs (open X in the figures), and
+the male carried a red bearing X (black X in the figures) it will be seen
+that there should result an exceptional class of sons that are red, and an
+exceptional class of daughters that are white. Tests of these exceptions
+show that they behave subsequently in heredity as their composition
+requires. Other tests may also be made of the other classes of offspring.
+Bridges has shown that they fulfill all the requirements predicted. Thus a
+result that seemed in contradiction with the chromosome hypothesis has
+turned out to give a brilliant confirmation of that theory both genetically
+and cytologically.
+
+HOW MANY GENETIC FACTORS ARE THERE IN THE GERM-PLASM OF A SINGLE INDIVIDUAL
+
+In passing I invite your attention to a speculation based on our maps of
+the chromosomes--a speculation which I must insist does not pretend to be
+more than a guess but has at least the interest of being the first guess
+that we have ever been in position to make as to how many factors go
+towards the makeup of the germ plasm.
+
+We have found practically no factors less than .04 of a unit apart. If our
+map includes the entire length of the chromosomes and if we assume factors
+are uniformly distributed along the chromosome at distances equal to the
+shortest distance yet observed, viz. .04, then we can calculate roughly how
+many hereditary factors there are in Drosophila. The calculation gives
+about 7500 factors. The reader should be cautioned against accepting the
+above assumptions as strictly true, for crossing-over values are known to
+differ according to different environmental conditions (as shown by Bridges
+for age), and to differ even in different parts of the chromosome as a
+result of the presence of specific genetic factors (as shown by
+Sturtevant). Since all the chromosomes except the X chromosomes are double
+we must double our estimate to give the _total_ number of factors, but the
+half number is the number of the different kinds of factors of Drosophila.
+
+CONCLUSIONS
+
+I have passed in review a long series of researches as to the nature of the
+hereditary material. We have in consequence of this work arrived within
+sight of a result that seemed a few years ago far beyond our reach. The
+mechanism of heredity has, I think, been discovered--discovered not by a
+flash of intuition but as the result of patient and careful study of the
+evidence itself.
+
+With the discovery of this mechanism I venture the opinion that the problem
+of heredity has been solved. We know how the factors carried by the parents
+are sorted out to the germ cells. The explanation does not pretend to state
+how factors arise or how they influence the development of the embryo. But
+these have never been an integral part of the doctrine of heredity. The
+problems which they present must be worked out in their own field. So, I
+repeat, the mechanism of the chromosomes offers a satisfactory solution of
+the traditional problem of heredity.
+
+       *       *       *       *       *
+
+
+CHAPTER IV
+
+SELECTION AND EVOLUTION
+
+Darwin's Theory of Natural Selection still holds today first place in every
+discussion of evolution, and for this very reason the theory calls for
+careful scrutiny; for it is not difficult to show that the expression
+"natural selection" is to many men a metaphor that carries many meanings,
+and sometimes different meanings to different men. While I heartily agree
+with my fellow biologists in ascribing to Darwin himself, and to his work,
+the first place in biological philosophy, yet recognition of this claim
+should not deter us from a careful analysis of the situation in the light
+of work that has been done since Darwin's time.
+
+THE THEORY OF NATURAL SELECTION
+
+In his great book on the _Origin of Species_, Darwin tried to do two
+things: first, to show that the evidence bearing on evolution makes that
+explanation probable. No such great body of evidence had ever been brought
+together before, and it wrought, as we all know, a revolution in our modes
+of thinking.
+
+Darwin also set himself the task of showing _how_ evolution might have
+taken place. He pointed to the influence of the environment, to the effects
+of use and disuse, and to natural selection. It is to the last theory that
+his name is especially attached. He appealed to a fact familiar to
+everyone, that no two individuals are identical and that some of the
+differences that they show are inherited. He argued that those individuals
+that are best suited to their environment are the most probable ones to
+survive and to leave most offspring. In consequence their descendants
+should in time replace through competition the less well-adapted
+individuals of the species. This is the process Darwin called natural
+selection, and Spencer the survival of the fittest.
+
+Stated in these general terms there is nothing in the theory to which
+anyone is likely to take exception. But let us examine the argument more
+critically.
+
+[Illustration: FIG. 71. Series of leaves of a tree arranged according to
+size. (After de Vries.)]
+
+If we measure, or weigh, or classify any character shown by the individuals
+of a population, we find differences. We recognize that some of the
+differences are due to the varied experiences that the individuals have
+encountered in the course of their lives, i.e. to their environment, but we
+also recognize that some of the differences may be due to individuals
+having different inheritances--different germ plasms. Some familiar
+examples will help to bring home this relation.
+
+If the leaves of a tree are arranged according to size (fig. 71), we find a
+continuous series, but there are more leaves of medium size than extremes.
+If a lot of beans be sorted out according to their weights, and those
+between certain weights put into cylinders, the cylinders, when arranged
+according to the size of the beans, will appear as shown in figure 72. An
+imaginary line running over the tops of the piles will give a curve (fig.
+73) that corresponds to the curve of probability (fig. 74).
+
+[Illustration: FIG. 72. Beans put into cylindrical jars according to the
+sizes of the beans. The jars arranged according to size of contained beans.
+(After de Vries.)]
+
+[Illustration: FIG. 73. A curve resulting from arrangement of beans
+according to size. (After de Vries.)]
+
+If we stand men in lines according to their height (fig. 75) we get a
+similar arrangement.
+
+[Illustration: FIG. 74. Curve of probability.]
+
+[Illustration: FIG. 75. Students arranged according to size. (After
+Blakeslee.)]
+
+The differences in size shown by the individual beans or by the individual
+men are due in part to heredity, in part to the environment in which they
+have developed. This is a familiar fact of almost every-day observation. It
+is well shown in the following example. In figure 76 the two boys and the
+two varieties of corn, which they are holding, differ in height. The
+pedigrees of the boys (fig. 77) make it probable that their height is
+largely inherited and the two races of corn are known to belong to a tall
+and a short race respectively. Here, then, the chief effect or difference
+is due to heredity. On the other hand, if individuals of the same race
+develop in a favorable environment the result is different from the
+development in an unfavorable environment, as shown in figure 78. Here to
+the right the corn is crowded and in consequence dwarfed, while to the left
+the same kind of corn has had more room to develop and is taller.
+
+[Illustration: FIG. 76. A short and a tall boy each holding a stalk of
+corn--one stalk of a race of short corn, the other of tall corn. (After
+Blakeslee.)]
+
+[Illustration: FIG. 77. Pedigree of boys shown in Fig. 76. (After
+Blakeslee.)]
+
+Darwin knew that if selection of particular kinds of individuals of a
+population takes place the next generation is affected. If the taller men
+of a community are selected _the average_ of their offspring will be taller
+than the average of the former population. If selection for tallness again
+takes place, still taller men will _on the average_ arise. If, amongst
+these, selection again makes a choice the process would, he thought,
+continue (fig. 79).
+
+[Illustration: FIG. 78. A race of corn reared under different conditions.]
+
+We now recognize that this statement contains an important truth, but we
+have found that it contains only a part of the truth. Any one who repeats
+for himself this kind of selection experiment will find that while his
+average class will often change in the direction of his selection, the
+process slows down as a rule rather suddenly (fig. 80). He finds, moreover,
+that the limits of variability are not necessarily transcended as the
+process continues even although the average may for a while be increased.
+More tall men may be produced by selection of this kind, but the tallest
+men are not necessarily any taller than the tallest in the original
+population.
+
+[Illustration: FIG. 79. Curves showing how (hypothetically) selection might
+be supposed to bring about progress in direction of selection. (After
+Goldschmidt.)]
+
+Selection, then, has not produced anything new, but only more of certain
+kinds of individuals. Evolution, however, means producing more new things,
+not more of what already exists.
+
+Darwin seems to have thought that the range of variation shown by the
+offspring of a given individual about that type of individual would be as
+wide as the range shown by the original population (fig. 79), but Galton's
+work has made it clear that this is not the case in a general or mixed
+population. If the offspring of individuals continued to show, as Darwin
+seems to have thought, as wide a range on each side of their parents' size,
+so to speak, as did the original population, then it would follow that
+selection could slide successive generations along in the direction of
+selection.
+
+[Illustration: FIG. 80. Diagram illustrating the results of selection for
+extra bristles in D. ampelophila. Selection at first produces decided
+effects which soon slow down and then cease. (MacDowell.)]
+
+Darwin himself was extraordinarily careful, however, in the statements he
+made in this connection and it is rather by implication than by actual
+reference that one can ascribe this meaning to his views. His
+contemporaries and many of his followers, however, appear to have accepted
+this _sliding scale_ interpretation as the cardinal doctrine of evolution.
+If this is doubted or my statement is challenged then one must explain why
+de Vries' mutation theory met with so little enthusiasm amongst the older
+group of zoologists and botanists; and one must explain why Johannsen's
+splendid work met with such bitter opposition from the English school--the
+biometricians--who amongst the post-Darwinian school are assumed to be the
+lineal descendants of Darwin.
+
+And in this connection we should not forget that just this sort of process
+was supposed to take place in the inheritance of use and disuse. What is
+gained in one generation forms the basis for further gains in the next
+generation. Now, Darwin not only believed that acquired characters are
+inherited but turned more and more to this explanation in his later
+writings. Let us, however, not make too much of the matter; for it is much
+less important to find out whether Darwin's ideas were vague, than it is to
+make sure that our own ideas are clear.
+
+If I have made several statements here that appear dogmatic let me now
+attempt to justify them, or at least give the evidence which seems to me to
+make them probable.
+
+The work of the Danish botanist, Johannsen, has given us the most carefully
+analyzed case of selection that has ever been obtained. There are,
+moreover, special reasons why the material that he used is better suited to
+give definite information than any other so far studied. Johannsen worked
+with the common bean, weighing the seeds or else measuring them. These
+beans if taken from many plants at random give the typical curve of
+probability (fig. 74). The plant multiplies by self-fertilization. Taking
+advantage of this fact Johannsen kept the seeds of each plant separate from
+the others, and raised from them a new generation. When curves were made
+from these new groups it was found that some of them had different modes
+from that of the original general population (fig. 81 A-E, bottom group).
+They are shown in the upper groups (A, B, C, D, E). But do not understand
+me to say that the offspring of each bean gave a different mode.
+
+[Illustration: FIG. 81. Pure lines of beans. The lower figure gives the
+general population, the other figures give the pure lines within the
+population. (After Johannsen.)]
+
+On the contrary, some of the lines would be the same.
+
+The result means that the general population is made up of definite kinds
+of individuals that may have been sorted out.
+
+That his conclusion is correct is shown by rearing a new generation from
+any plant or indeed from several plants of any one of these lines. Each
+line repeats the same modal class. There is no further breaking up into
+groups. Within the line it does not matter at all whether one chooses a big
+bean or a little one--they will give the same result. In a word, the germ
+plasm in each of these lines is pure, or homozygous, as we say. The
+differences that we find between the weights (or sizes) of the individual
+beans are due to external conditions to which they have been subjected.
+
+In a word, Johannsen's work shows that the frequency distribution of a pure
+line is due to factors that are extrinsic to the germ plasm. It does not
+matter then which individuals in a pure line are used to breed from, for
+they all carry the same germ plasm.
+
+We can now understand more clearly how selection acting on a general
+population brings about results in the direction of selection.
+
+An individual is picked out from the population in order to get a
+particular kind of germ plasm. Although the different classes of
+individuals may overlap, so that one can not always judge an individual
+from its appearance, nevertheless on the whole chance favors the picking
+out of the kind of germ plasm sought.
+
+In species with separate sexes there is the further difficulty that two
+individuals must be chosen for each mating, and superficial examination of
+them does not insure that they belong to the same group--their germ plasm
+cannot be inspected. Hence selection of biparental forms is a precarious
+process, now going forward, now backwards, now standing still. In time,
+however, the process forward is almost certain to take place if the
+selection is from a heterogeneous population. Johannsen's work was
+simplified because he started with pure lines. In fact, had he not done so
+his work would not have been essentially different from that of any
+selection experiment of a pure race of animals or plants. Whether Johannsen
+realized the importance of the condition or not is uncertain--curiously he
+laid no emphasis on it in the first edition of his "Elemente der exakten
+Erblichkeitslehre".
+
+It has since been pointed out by Jennings and by Pearl that a race that
+reproduces by self-fertilization as does this bean, automatically becomes
+pure in all of the factors that make up its germ plasm. Since
+self-fertilization is the normal process in this bean the purity of the
+germ plasm already existed when Johannsen began to experiment.
+
+HOW HAS SELECTION IN DOMESTICATED ANIMALS AND PLANTS BROUGHT ABOUT ITS
+RESULTS?
+
+If then selection does not bring about transgressive variation in a general
+population, how can selection produce anything new? If it can not produce
+anything new, is there any other way in which selection becomes an agent in
+evolution?
+
+We can get some light on this question if we turn to what man has done with
+his domesticated animals and plants. Through selection, i.e., artificial
+selection, man has undoubtedly brought about changes as remarkable as any
+shown by wild animals and plants. We know, moreover, a good deal about how
+these changes have been wrought.
+
+(1) By crossing different wild species or by crossing wild with races
+already domesticated new combinations have been made. Parts of one
+individual have been combined with parts of others, creating new
+combinations. It is possible even that characters that are entirely new may
+be produced by the interaction of factors brought into recombination.
+
+(2) New characters appear from time to time in domesticated and in wild
+species. These, like the mutants in Drosophila, are fully equipped at the
+start. Since they breed true and follow Mendel's laws it is possible to
+combine them with characters of the wild type or with those of other mutant
+races.
+
+Amongst the new mutant factors there may be some whose chief effect is on
+the character that the breeder is already selecting. Such a modification
+will be likely to attract attention. Superficially it may appear that the
+factor for the original character has varied, while the truth may be that
+another factor has appeared that has modified a character already present.
+In fact, many or all Mendelian factors that affect the same organ may be
+said to be modifiers of each other's effects. Thus the factor for vermilion
+causes the eye to be one color, and the factor for eosin another color,
+while eosin vermilion is different from both. Eosin may be said to be a
+modifier of vermilion or vermilion of eosin. In general, however, it is
+convenient to use the term "modifier" for cases in which the factor causes
+a detectable change in a character already present or conspicuous.
+
+[Illustration: FIG. 82. Scheme to indicate influence of the modifying
+factors, cream and whiting. Neither produces any effect alone but they
+modify other eye colors such as eosin.]
+
+One of the most interesting, and at the same time most treacherous, kinds
+of modifying factors is that which produces an effect _only_ when some
+other factor is present. Thus Bridges has shown that there is a factor
+called "cream" that does not affect the red color of the eye of the wild
+fly, yet makes "eosin" much paler (fig. 82). Another factor "whiting" which
+produces no effect on red makes eosin entirely white. Since cream or
+whiting may be carried by red eyed flies without their presence being seen
+until eosin is used, the experimenter must be continually on the lookout
+for such factors which may lead to erroneous conclusions unless detected.
+As yet breeders have not realized the important role that modifiers have
+played in their results, but there are indications at least that the
+heaping up of modifying factors has been one of the ways in which highly
+specialized domesticated animals have been produced. Selection has
+accomplished this result not by changing factors, but by picking up
+modifying factors. The demonstration of the presence of these factors has
+already been made in some cases. Their study promises to be one of the most
+instructive fields for further work bearing on the selection hypothesis.
+
+In addition to these well recognized methods by which artificial selection
+has produced new things we come now to a question that is the very crux of
+the selection theory today. Our whole conception of selection turns on the
+answer that we give to this matter and if I appear insistent and go into
+some detail it is because I think that the matter is worth very careful
+consideration.
+
+ARE FACTORS CHANGED THROUGH SELECTION?
+
+As we have seen, the variation that we find from individual to individual
+is due in part to the environment; this can generally be demonstrated.
+Other differences in an ordinary population are recognized as due to
+different genetic (hereditary) combinations. No one will dispute this
+statement. But is all the variability accounted for in these two ways? May
+not a factor itself fluctuate? Is it not _a priori_ probable that factors
+do fluctuate? Why, in a word, should we regard factors as inviolate when we
+see that everything else in organisms is more or less in amount? I do not
+know of any _a priori_ reason why a factor may not fluctuate, unless it is,
+as I like to think, a chemical molecule. We are, however, dealing here not
+with generalities but with evidence, and there are three known methods by
+means of which it has been shown that variability, other than environmental
+or recombinational, is not due to variability in a factor, nor to various
+"potencies" possessed by the same factors.
+
+(1) By making the stock uniform for all of its factors--chief factors and
+modifiers alike. Any change in such a stock produced by selection would
+then be due to a change in one or more of the factors themselves.
+Johannsen's experiment is an example of this sort.
+
+[Illustration: FIG. 83 a. Drosophila ampelophila with truncate wings.]
+
+(2) The second method is one that is capable of _demonstrating_ that the
+effects of selection are actually due to modifiers. It has been worked out
+in our laboratory, chiefly by Muller, and used in a particular case to
+demonstrate that selection produced its effect by isolating modifying
+factors. For example, a mutant type called truncate appeared, characterized
+by shorter wings, usually square at the end, (fig. 83a). The wings varied
+from those of normal length to wings much shorter (fig. 83b). For three
+years the mutant stock was bred from individuals having the shorter wings
+until at last a stock was obtained in which some of the individuals had
+wings much shorter than the body. By means of linkage experiments it was
+shown that at least three factors were present that modified the wings.
+These were isolated by means of their linkage relations, and their mutual
+influence on the production of truncate wings was shown.
+
+[Illustration: FIG. 83 b. Series of wings of different length shown by
+truncate stock of D. ampelophila.]
+
+An experiment of this kind can only be carried out in a case where the
+groups of linked gens are known. At present Drosophila is the only animal
+(or plant) sufficiently well known to make this test possible, but this
+does not prove that the method is of no value. On the contrary it shows
+that any claim that factors can themselves be changed can have no finality
+until the claim can be tested out by means of the linkage test. For
+instance, bar eye (fig. 31) arose as a mutation. All our stock has
+descended from a single original mutant. But Zeleny has shown that
+selection within our stock will make the bar eye narrower or broader
+according to the direction of selection. It remains to be shown in this
+case how selection has produced its effects, and this can be done by
+utilizing the same process that was used in the case of truncate.
+
+Another mutant stock called beaded (fig. 84), has been bred for five years
+and selected for wings showing more beading. In extreme cases the wings
+have been reduced to mere stumps (see stumpy, fig. 5), but the stock shows
+great variability. It is probable here as Dexter has shown, that a number
+of mutant factors that act as modifiers have been picked up in the course
+of the selection, and when it is recalled that during those five years over
+125 new characters have appeared elsewhere it does not seem improbable that
+factors also have appeared that modify the wings of this stock.
+
+[Illustration: FIG. 84. Two flies showing beaded wings.]
+
+(3) The third method is one that has been developed principally by East for
+plants; also by MacDowell for rabbits and flies. The method does not claim
+to prove that modifiers are present, but it shows why certain results are
+in harmony with that expectation and can not be accounted for on the basis
+that a factor has changed. Let me give an example. When a Belgian hare with
+large body was crossed to a common rabbit with a small body the hybrid was
+intermediate in size. When the hybrid was crossed back to the smaller type
+it produced rabbits of various sizes in apparently a continuous series.
+MacDowell made measurements of the range of variability in the first and in
+the second generations.
+
+    _Classification in relation to parents based on skull lengths and ulna
+    lengths, to show the relative variability of two measurements and of
+    the first generation (F_1) and the back cross (B. C.)_
+
+  CHARACTER|GENERATION|-13|-12|-11|-10| -9| -8| -7| -6| -5| -4| -3| -2| -1|
+  ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
+  Length of{   F_1    |   |   |   |   |   |   |   |   |   |   |   |   |   |
+    skull  {   B.C.   |   |   |   |   |   |   |   |   |   |   |   |   |  3|
+  Length of{   F_1    |   |   |   |   |   |   |   |   |   |   |   |   |   |
+    ulna   {   B.C.   |  1|   |   |   |   |  1|   |  2|  3|  1|  2|  4|  4|
+
+  _same table continued_
+
+  CHARACTER|GENERATION|  0|  1|  2|  3|  4|  5|  6|  7|  8|  9| 10| 11| 12|
+  ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
+  Length of{   F_1    |   |   |   |   |   |   |   |  2|  2|  8|  5| 10|  7|
+    skull  {   B.C.   |  6|  4| 13| 18| 42| 32| 38| 34| 16| 16|  8|  4|  3|
+  Length of{   F_1    |   |  1|   |   |  2|   |  1|  1|  1|  2|  2|  5|  3|
+    ulna   {   B.C.   | 12| 11| 20| 26| 17| 19| 18| 15| 12| 13| 15| 11|  5|
+
+  _same table continued_
+
+  CHARACTER|GENERATION| 13| 14| 15| 16| 17| 18| 19| 20| 21| 22| 23| 24| 25|
+  ---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
+  Length of{   F_1    |  3|  2|  2|   |   |   |   |   |   |   |   |   |   |
+    skull  {   B.C.   |  1|   |   |   |   |   |   |   |   |   |   |   |   |
+  Length of{   F_1    |  1|  7|  3|  2|  1|   |   |   |  2|   |  1|   |  1|
+    ulna   {   B.C.   |  2|  4|  2|  2|   |   |  1|  1|   |   |   |   |   |
+
+He found that the variability was smaller in the first generation than in
+the second generation (back cross). This is what is expected if several
+factor-differences were involved, because the hybrids of the first
+generation are expected to be more uniform in factorial composition than
+are those in the second generation which are produced by recombination of
+the factors introduced through their grandparents. Excellent illustrations
+of the same kinds of results have been found in Indian corn. As shown in
+figure 85 the length of the cob in F_1 is intermediate between the parent
+types while in F_2 the range is wider and both of the original types are
+recovered. East states that similar relations have been found for 18
+characters in corn. Emerson has recently furnished further illustrations of
+the same relations in the length of stalks in beans.
+
+[Illustration: FIG. 85. Cross between two races of Indian corn, one with
+short cobs and one with long cobs. The range of variability in F_1 is less
+than that in F_2. (After East.)]
+
+A similar case is shown by a cross between fantail and common pigeons (fig.
+86). The latter have twelve feathers in the tail, while the selected race
+from which the fantails came had between 28 and 38 feathers in the tail.
+The F_1 offspring (forty-one individuals) showed (fig. 87) between 12 and
+20 tail feathers, while in F_2 the numbers varied between 12 and 25. Here
+one of the grand-parental types reappears in large numbers, while the
+extreme of the other grand-parental type did not reappear (in the counts
+obtained), although the F_2 number would probably overlap the lower limits
+of the race of fantail grandparents had not a selected (surviving) lot been
+taken for the figures given in the table.
+
+[Illustration: FIG. 86. Cross of pigeon with normal tail P_1 and fantail
+P_1; F_1, bird below.]
+
+[Illustration: FIG. 87. Cross of normal and fantail pigeons. (See Fig. 86.)
+The F_2 range is wider than that of F_1. The normal grand-parental type of
+12 feathers was recovered in F_2 but the higher numbers characteristic of
+fantails were not recovered.]
+
+The preceding account attempts to point out how I should prefer to
+interpret the problem of selection in the light of the most recent work on
+breeding. But I would give a very incomplete account of the whole situation
+if I neglected to include some important work which has led some of my
+fellow-workers to a very different conclusion.
+
+[Illustration: FIG. 88. Scheme to show classes of hooded rats used by
+Castle. (After Castle.)]
+
+Castle in particular is the champion of a view based on his results with
+hooded rats. Starting with individuals which have a narrow black stripe
+down the back he selected for a narrower stripe in one direction and for a
+broader stripe in the other. As the diagram shows (fig. 88) Castle has
+succeeded in producing in one direction a race in which the dorsal stripe
+has disappeared and in the other direction a race in which the black has
+extended over the back and sides, leaving only a white mark on the belly.
+Neither of these extremes occurs, he believes, in the ordinary hooded race
+of domesticated rats. In other words no matter how many of them came under
+observation the extreme types of his experiment would not be found.
+
+Castle claims that the factor for hoodedness must be a single Mendelian
+unit, because if hooded rats are crossed to wild gray rats with uniform
+coat and their offspring are inbred there are produced in F_2 three uniform
+rats to one hooded rat. Castle advances the hypothesis that factors--by
+which he means Mendelian factors--may themselves vary in much the same way
+as do the characters that they stand for. He argues, in so many words, that
+since we judge a factor by the kind of character it produces, when the
+character varies the factor that stands for it may have changed.
+
+As early as 1903 Cuenot had carried out experiments with spotted mice
+similar to those of Castle with rats. Cuenot found that spotted crossed to
+uniform coat color gave in F_2 a ratio of three uniform to one spotted, yet
+selection of those spotted mice with more white in their coat produced mice
+in successive generations that had more and more white. Conversely Cuenot
+showed that selection of those spotted mice that had more color in their
+coat produced mice with more and more color and less white. Cuenot does not
+however bring up in this connection the question as to how selection in
+these spotted mice brings about its results.
+
+Without attempting to discuss these results at the length that they deserve
+let me briefly state why I think Castle's evidence fails to establish his
+conclusion.
+
+In the first place one of the premises may be wrong. The three to one ratio
+in F_2 by no means proves that all conditions of hoodedness are due to one
+factor. The result shows at most that one factor that gives the hooded
+types is a simple Mendelian factor. The changes in this type may be caused
+by modifying factors that can show an effect only when hoodedness is itself
+present. That this is not an imaginary objection but a real one is shown by
+an experiment that Castle himself made which furnishes the ground for the
+second objection.
+
+Second. If the factor has really changed its potency, then if a very dark
+individual from one end of the series is crossed to a wild rat and the
+second generation raised we should expect that the hooded F_2 rats would
+all be dark like their dark grandparent. When Castle made this test he
+found that there were many grades of hooded rats in the F_2 progeny. They
+were darker, it is true, as a group than were the original hooded group at
+the beginning of the selection experiment, but they gave many intermediate
+grades. Castle attempts to explain this by the assumption that the factor
+made pure by selection became contaminated by its normal allelomorph in the
+F_1 parent, but not only does this assumption appear to beg the whole
+question, but it is in flat contradiction with what we have observed in
+hundreds of Mendelian cases where no evidence for such a contamination
+exists.
+
+Later Castle crossed some of the extracted rats of average grade (3.01)
+from the plus series to the same wild race and got F_2 hooded rats from
+this cross. These F_2 hooded rats did not further approach the ordinary
+range but were nearer the extreme selected plus hooded rats (3.33) than
+were the F_2's extracted from the first cross (2.59). Castle concludes from
+this that multiple factors can not account for the result. As a matter of
+fact, Castle's evidence _as published_ does not establish his conclusion
+because the wild rats used in the second experiment may have carried plus
+modifiers. This could only be determined by suitable tests which Castle
+does not furnish. This is the crucial point, without which the evidence
+carries no conviction.
+
+Furthermore, from Castle's point of view, these latest results would seem
+to increase the difficulty of interpretation of his first F_2 extracted
+cross, and it is now the first result that calls for explanation if one
+accepts his later conclusion.
+
+These and other objections that might be taken up show, I think, that
+Castle's experiment with hooded rats fails entirely to establish his
+contention of change in potency of the germ or of contamination of factors,
+while on the contrary they are in entire accord with the view that he is
+dealing with a case of modifying factors.
+
+[Illustration: FIG. 89. Races of Paramecium. (After Jennings.)]
+
+Equally important are the results that Jennings has obtained with certain
+protozoa. Paramecium multiplies by dividing across in the middle, each half
+replacing its lacking part. Both the small nucleus (micronucleus) and the
+large nucleus (macronucleus) divide at each division of the body. Jennings
+found that while individuals descended from a single paramecium vary in
+size (fig. 89), yet the population from a large individual is the same as
+the population derived from a small individual. In other words, selection
+produces no result and the probable explanation is, of course, that the
+different sizes of individuals are due to the environment, while the
+constancy of the type is genetic. Jennings found a number of races of
+paramecium of different sizes living under natural conditions. The largest
+individual of a small race might overlap the smallest individual of other
+larger races (fig. 89); nevertheless each kind reproduced its particular
+race. The results are like those of Johannsen in a general way, but differ
+in that reproduction takes place in paramecium by direct division instead
+of through self-fertilization as in beans, and also in that the paramecia
+were probably not homozygous. Since, however, so far as known no
+"reduction" takes place in paramecium at each division, the genetic
+composition of parent and offspring should be the same. Whether
+pseudo-parthenogenesis that Woodruff and Erdmann have found occurring in
+paramecium at intervals involves a redistribution of the hereditary factors
+is not clear. Jennings's evidence seems incompatible with such a view.
+
+[Illustration: FIG. 90. Stylonychia showing division into two. (After
+Stein.)]
+
+More recently one of Jennings's students, Middleton, has made a careful
+series of selection experiments with Stylonychia (fig. 90) in which he
+selected for lines showing more rapid or slower rates of division. His
+observations seem to show that his selection separated two such lines that
+came from the same original stock. The rapidity of the effects of selection
+seems to preclude the explanation that pseudo-parthenogenesis has
+complicated the results. Nevertheless, the results are of such a kind as to
+suggest that they were due to selection of vegetative (somatic) differences
+and that no genetic change of factors was involved, for his conclusion that
+the rapidity with which the effects gained by long selection might be
+suddenly reversed when selection was reversed is hardly consistent with an
+interpretation of the results based on changes in the "potencies" of the
+factors present.
+
+Equally striking are the interesting experiments that Jennings has recently
+carried out with Difflugia (fig. 91). This protozoon secretes a shell about
+itself which has a characteristic shape, and often carries spines. The
+opening at one end of the shell through which the protoplasm protrudes to
+make the pseudopodia is surrounded by a rim having a characteristic
+pattern. The protoplasm contains several nuclei and in addition there is
+scattered material or particles called chromidia that are supposed to be
+chromatic in nature and related to the material of the nuclei, possibly by
+direct interchange.
+
+[Illustration: FIG. 91. Difflugia Corona. (After Cash.)]
+
+When Difflugia divides, part of the protoplasm protrudes from the opening
+and a new shell is secreted about this mass which becomes a daughter
+individual. The behavior of the nucleus and of the chromidia at this time
+is obscure, but there is some evidence that their materials may be
+irregularly distributed between parent and offspring. If this is correct,
+and if in the protozoa the chromatin has the same influence that it seems
+to have in higher animals, the mode of reproduction in Difflugia would be
+expected to give little more than random sampling of the germ plasm.
+
+[Illustration: FIG. 92. Races of Difflugia. (After Leidy.)]
+
+Jennings was able by means of selection to get from the descendants of one
+original individual a number of different types that themselves bred true,
+except in so far as selection could affect another change in them. In this
+connection it is interesting to note that Leidy has published figures of
+Difflugia (fig. 92) that show that a great many "types" exist. If through
+sexual union (a process that occurs in Difflugia) the germ plasm
+(chromatin) of these wild types has in times past been recombined, then
+selection would be expected to separate certain types again, if, at
+division, irregular sampling of the germ plasm takes place. Until these
+points are settled the bearing of these important experiments of Jennings
+on the general problem of selection is uncertain.
+
+HOW DOES NATURAL SELECTION INFLUENCE THE COURSE OF EVOLUTION?
+
+The question still remains: Does selection play any role in evolution, and,
+if so, in what sense? Does the elimination of the unfit influence the
+course of evolution, except in the negative sense of leaving more room for
+the fit? There is something further to be said in this connection, although
+opinions may differ as to whether the following interpretation of the term
+"natural selection" is the only possible one.
+
+[Illustration: FIG. 93. Evolution of elephant's skulls. (After Dendy.)]
+
+If through a mutation a character appears that is neither advantageous nor
+disadvantageous, but indifferent, the chance that it may become established
+in the race is extremely small, although by good luck such a thing may
+occur rarely. It makes no difference whether the character in question is a
+dominant or a recessive one, the chance of its becoming established is
+exactly the same. If through a mutation a character appears that has an
+_injurious_ effect, however slight this may be, it has practically no
+chance of becoming established.
+
+[Illustration: FIG. 94. Evolution of elephant's trunk. (After Lull.)]
+
+If through a mutation a character appears that has a _beneficial_ influence
+on the individual, the chance that the individual will survive is
+increased, not only for itself, but for all of its descendants that come to
+inherit this character. It is this increase in the number of individuals
+possessing a particular character, that might have an influence on the
+course of evolution. This gives a better chance for improvement by several
+successive steps; but not because the species is more likely to mutate
+again in the same direction. An imaginary example will illustrate how this
+happens: When elephants had trunks less than a foot long, the chance of
+getting trunks more than one foot long was in proportion to the length of
+trunks already present and to the number of individuals; but increment in
+trunk length is no more likely to occur from an animal having a trunk more
+than one foot long than from an animal with a shorter trunk.
+
+The case is analogous to tossing pennies. At any stage in the game the
+chance of accumulating a hundred heads is in proportion to the number of
+heads already obtained, and to the number of throws still to be made. But
+the number of heads obtained has no influence on the number of heads that
+will appear in the next throw.
+
+[Illustration: FIG. 95. Evolution of elephant's trunk: above Maeritherium,
+in the middle Tetrabelodon (After Lancaster); below African elephants
+(After Gambier Bolton).]
+
+Owing then to this property of the germ plasm to duplicate itself in a
+large number of samples not only is an opportunity furnished to an
+advantageous variation to become extensively multiplied, but the presence
+of a large number of individuals of a given sort prejudices the probable
+future result.
+
+The question may be raised as to whether it is desirable to call selection
+a _creative_ process. There are so many supernatural and mystical
+implications that hang around the term creative that one can not be too
+careful in stating in what sense the term is to be used. If by creative is
+meant that something is made out of nothing, then of course there is no
+need for the scientist to try to answer such a question. But if by a
+creative process is meant that something is made out of something else,
+then there are two alternatives to be reckoned with.
+
+First, if it were true that selection of an individual of a certain kind
+determines that new variations in the same direction occur as a consequence
+of the selection, then selection would certainly be creative. How this
+could occur might be quite unintelligible, but of course it might be
+claimed that the point is not whether we can explain how creation takes
+place, but whether we can get verifiable evidence that such a kind of thing
+happens. This possibility is disposed of by the fact that there is no
+evidence that selection determines the direction in which variation occurs.
+
+Second, if you mean by a creative process that by picking out a certain
+kind of individual and multiplying its numbers a better chance is furnished
+that a certain end result will be obtained, such a process may be said to
+be creative. This is, I think, the proper use of the term creative in a
+mechanistic sense.
+
+CONCLUSIONS
+
+In reviewing the evidence relating to selection I have tried to handle the
+problem as objectively as I could.
+
+The evidence shows clearly that the characters of wild animals and plants,
+as well as those of domesticated races, are inherited both in the wild and
+in the domesticated forms according to Mendel's Law.
+
+The causes of the mutations that give rise to new characters we do not
+know, although we have no reason for supposing that they are due to other
+than natural processes.
+
+Evolution has taken place by the incorporation into the race of those
+mutations that are beneficial to the life and reproduction of the organism.
+Natural selection as here defined means both the increase in the number of
+individuals that results after a beneficial mutation has occurred (owing to
+the ability of living matter to propagate) and also that this preponderance
+of certain kinds of individuals in a population makes some further results
+more probable than others. More than this, natural selection can not mean,
+if factors are fixed and are not changed by selection.
+
+       *       *       *       *       *
+
+
+        INDEX
+
+  Abnormal abdomen 109
+  Abraxas 78-81
+  Allantois 17
+  Allelomorphs 83-84
+  Altenburg 112
+  Amnion 16-17
+  Andalusian fowl 45, 46
+  Annelids 22
+  Antlered wing 111
+  Apterous wing 11
+  Arc wing 111
+  Aristae 104
+
+  Bar eye 67, 108, 169
+  Bateson 18, 34, 36
+  Beaded wing 11, 115
+  Beans 147-149, 157
+  Belgian hare 171
+  Bent wing 116
+  Bergson 30, 31
+  Bildungstrieb 34
+  Biogenetic law 15, 18, 19, 21
+  Biometricians 156
+  Bird 21, 23
+  Bithorax 65, 112, 113
+  Black body color 111, 133
+  Blakeslee 152
+  Bridges 114, 143, 163
+  British Association 36
+  Bruenn 40
+  Buff eye color 109
+  Bufon 27
+
+  Castle 176-180
+  Cat 33
+  Cell 90, 91
+  Chance variations 37
+  Chick 16, 17, 20
+  Chromatin 184
+  Chromosome group of Drosophila 102
+  Chromosomes 91, 95, 96, 98, 130, 131, 132
+  Cleavage 21, 22, 94
+  Clover butterfly 62
+  Club wing 69, 70, 108
+  Colias philodice 62
+  Color blindness 77, 125
+  Comb of Drosophila 103
+  Combs of fowls 33, 54
+  Comparative anatomy 7, 8, 9, 14
+  Corn 150, 153, 172
+  Correns 41
+  Cosmogonies 27
+  Cream eye color 163, 164
+  Crepidula 22
+  Criss-cross inheritance 78
+  Crossing over 131-133
+  Cuenot 178
+  Curled wing 115
+  Curved wing 111
+  Curve of probability 149
+  Cut wing 11, 104
+
+  Dachs legs 112
+  Dahlgren 62
+  Darwin 15, 24, 28, 32, 35-37, 64, 145, 146, 152, 154-156
+  Dendy 188
+  De Vries 18, 147, 156
+  Dexter 170
+  Dichaete 114
+  Difflugia 184-187
+  Discontinuous variation 13
+  Disuse 31
+  Drosophila ampelophila 10, 12, 13, 48-50, 60, 75, 84, 85, 93, 100, 103,
+      119, 155, 162, 169
+  Drosophila repleta 76
+  Duplication of legs 109
+  Dwarf 114
+
+  East 170, 172
+  Ebony 50, 55, 56, 115
+  Egg 91, 94
+  Elephant 191
+  Elephants' skulls 188
+  Elephants' trunks 190
+  Embryology 13-23
+  Emerson 172
+  Environment 27
+  Eosin eye color 61, 107, 163
+  Erdmann 183
+  Evolution Creatrice 30
+  Evolution--three kinds of 1, 2, 4
+  Eye color 13
+  Eyeless 66, 115
+
+  Factorial theory 89
+  Factors of Drosophila 143
+  Fantails 172, 175
+  Fertilization 91
+  Fish 16, 20, 21
+  Flatworms 22
+  Fluctuations 12
+  Forked bristles 106
+  Fowl 77
+  Fused veins 107, 108
+
+  Galton 154
+  Geneticist 26
+  Germ-plasm 142
+  Geoffroy St. Hilaire 27
+  Giant 114
+  Gill-slits 20, 21, 23
+  Groups I, II, III, IV 100-118
+
+  Haeckel 15
+  Haemophilia 77
+  Heliotropism 106, 107
+  Himalyan rabbits 83
+  History 1, 6
+  Hoge 66
+  Horse, evolution of 6
+
+  Indian corn 172, 173
+  Interference 137, 138
+
+  Janssens 132
+  Jaunty wing 111
+  Jennings 161, 181-184, 186
+  Johannsen 156, 157, 159-161, 166, 182
+
+  Lamarck 31-34
+  Langshan 77
+  Leaves 147
+  Leidy 186
+  Lethal 105
+  Linkage groups 103
+  Lizard 23
+  Localization of factors 118
+
+  MacDowell 155, 170, 171
+  Macritherium 191
+  Mammal 16, 21, 23
+  Man 20, 77, 125, 126
+  Map of Chromosomes 136
+  Maroon eye color 114
+  Mendel 40, 41, 52, 89
+  Mendelian heredity 39
+  Mendel's law 41-59, 64, 124
+  Mendel's second law 52
+  Mesenchyme cells 22
+  Mesoderm cells 22
+  Metaphysician 30
+  Mice 33, 178
+  Middleton 183
+  Miniature wing 108
+  Mirabilis 42
+  Modifiers 163, 164, 170, 171
+  Molluscs 22
+  Mouse 83
+  Muller 112, 167
+  Mutations 35, 39, 84
+
+  Naegeli 34, 35
+  Natural Selection 36, 145, 146, 187-194
+  Nisus formativus 34
+  Non-disjunction 139-142
+  Notch wing 104-106
+  Nucleus 91
+
+  Origin of Species 35, 145
+  Orthogenesis 34
+
+  Paleontology 24-27
+  Papilio polytes 63
+  Papilio turnus 63
+  Paramecium 181, 182
+  Paratettix 81
+  Peach eye color 114
+  Pea comb 54
+  Pearl 161
+  Peas 47
+  Pigeons 172, 174, 175
+  Pink eye color 114, 115
+  Planarian 22
+  Plymouth Rock 77
+  Podarke 22
+  Polar bodies 126
+  Pole arms 5
+  Protozoa 181
+  Pseudo-parthenogenesis 183
+  Purple eye color 109
+  Purpose 4
+
+  Rabbits 83, 170
+  Rats 176-180
+  Reduction division 182
+  Reproductive cells 96
+  Ruby eye color 106
+  Rudimentary organ 116
+  Rudimentary wing 70, 71, 107
+
+  Sable body color 107
+  Science definition of 6
+  Segregation 41
+  Selenka 94
+  Sepia eye color 13, 114
+  Sex chromosomes 118
+  Sex linked inheritance 75, 118-130
+  Sexual dimorphism 62
+  Sheep 33
+  Single comb 54
+  Sooty body color 50, 114, 115
+  Speck 68, 69, 111
+  Spencer 145
+  Spermatozoon 91, 98
+  Stars, evolution of 6
+  St. Hilaire 27-30
+  Strap wing 110, 111
+  Stumpy wing 11
+  Sturtevant 76, 143
+  Stylonychia 183
+  Survival of the fittest 146
+  Systematist 85
+
+  Tails 33
+  Tan flies 106, 107
+  Tetrabelodon 191
+  Trefoil 111
+  Truncate wing 111, 112, 167, 168
+
+  Unfolding principle 34
+  Unio 22
+  Unit character 74, 75
+  Use 31
+
+  Variation discontinuous 13
+  Vermilion eye color 108, 163
+  Vestigial wing 11, 55, 56, 109, 133
+  Vital force 34
+
+  Wallace 36
+  Walnut comb 54
+  Weismann 17, 31-33
+  Wilson, E. B. 125
+  Wingless 67
+  Winiwarter 126
+  White eye color 13, 75, 119-130
+  Whiting eye color 163, 164
+  Woodruff 183
+
+  Yellow body color 108, 133
+  Yolk sac 16, 17
+
+  Zeleny 169
+
+       *       *       *       *       *
+
+
+Corrections made to printed original.
+
+page 104, "shown in figures 53, 54, 55, 56": '52, 53, 54, 55' in original.
+
+
+
+
+
+
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