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text-align: left; text-indent: 0;} + .figcenter {margin: auto;} + .figright {float: right;} + .figleft {float: left;} + img.middle { border: none; vertical-align: middle } + + </style> + </head> +<body> + + +<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—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—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—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.</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—like the amnion of the chick + (fig. 7) and mammal—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—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)—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—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 <!-- 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—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 <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—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—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—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—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—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—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—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—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.</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—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—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—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.</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—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—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—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—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—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—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—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—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—the Y chromosome—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> </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—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.</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> </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:—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—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.</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—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—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—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—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—the biometricians—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—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—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—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—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—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. <!-- + 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—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> + + + + + +End of the Project Gutenberg EBook of A Critique of the Theory of Evolution, by +Thomas Hunt Morgan + +*** END OF THIS PROJECT GUTENBERG EBOOK CRITIQUE OF THEORY OF EVOLUTION *** + +***** This file should be named 30701-h.htm or 30701-h.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/3/0/7/0/30701/ + +Produced by Bryan Ness, Keith Edkins, and the Online +Distributed Proofreading Team at https://www.pgdp.net. 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