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Zelmer. + + + + + +THE PERPETUATION OF LIVING BEINGS, HEREDITARY TRANSMISSION AND +VARIATION + +by Thomas Henry Huxley + + + + +The inquiry which we undertook, at our last meeting, into the state of +our knowledge of the causes of the phenomena of organic nature,--of the +past and of the present,--resolved itself into two subsidiary +inquiries: the first was, whether we know anything, either historically +or experimentally, of the mode of origin of living beings; the second +subsidiary inquiry was, whether, granting the origin, we know anything +about the perpetuation and modifications of the forms of organic +beings. The reply which I had to give to the first question was +altogether negative, and the chief result of my last lecture was, that, +neither historically nor experimentally, do we at present know anything +whatsoever about the origin of living forms. We saw that, historically, +we are not likely to know anything about it, although we may perhaps +learn something experimentally; but that at present we are an enormous +distance from the goal I indicated. + +I now, then, take up the next question, What do we know of the +reproduction, the perpetuation, and the modifications of the forms of +living beings, supposing that we have put the question as to their +origination on one side, and have assumed that at present the causes of +their origination are beyond us, and that we know nothing about them? +Upon this question the state of our knowledge is extremely different; +it is exceedingly large, and, if not complete, our experience is +certainly most extensive. It would be impossible to lay it all before +you, and the most I can do, or need do to-night, is to take up the +principal points and put them before you with such prominence as may +subserve the purposes of our present argument. + +The method of the perpetuation of organic beings is of two kinds,--the +asexual and the sexual. In the first the perpetuation takes place from +and by a particular act of an individual organism, which sometimes may +not be classed as belonging to any sex at all. In the second case, it +is in consequence of the mutual action and interaction of certain +portions of the organisms of usually two distinct individuals,--the +male and the female. The cases of asexual perpetuation are by no means +so common as the cases of sexual perpetuation; and they are by no means +so common in the animal as in the vegetable world. You are all +probably familiar with the fact, as a matter of experience, that you +can propagate plants by means of what are called "cuttings;" for +example, that by taking a cutting from a geranium plant, and rearing it +properly, by supplying it with light and warmth and nourishment from +the earth, it grows up and takes the form of its parent, having all the +properties and peculiarities of the original plant. + +Sometimes this process, which the gardener performs artificially, takes +place naturally; that is to say, a little bulb, or portion of the +plant, detaches itself, drops off, and becomes capable of growing as a +separate thing. That is the case with many bulbous plants, which throw +off in this way secondary bulbs, which are lodged in the ground and +become developed into plants. This is an asexual process, and from it +results the repetition or reproduction of the form of the original +being from which the bulb proceeds. + +Among animals the same thing takes place. Among the lower forms of +animal life, the infusorial animalculae we have already spoken of throw +off certain portions, or break themselves up in various directions, +sometimes transversely or sometimes longitudinally; or they may give +off buds, which detach themselves and develop into their proper forms. +There is the common fresh-water Polype, for instance, which multiplies +itself in this way. Just in the same way as the gardener is able to +multiply and reproduce the peculiarities and characters of particular +plants by means of cuttings, so can the physiological +experimentalist--as was shown by the Abbe Trembley many years ago--so +can he do the same thing with many of the lower forms of animal life. +M. de Trembley showed that you could take a polype and cut it into two, +or four, or many pieces, mutilating it in all directions, and the pieces +would still grow up and reproduce completely the original form of the +animal. These are all cases of asexual multiplication, and there are +other instances, and still more extraordinary ones, in which this +process takes place naturally, in a more hidden, a more recondite kind +of way. You are all of you familiar with those little green insects, +the 'Aphis' or blight, as it is called. These little animals, during a +very considerable part of their existence, multiply themselves by means +of a kind of internal budding, the buds being developed into +essentially asexual animals, which are neither male nor female; they +become converted into young 'Aphides', which repeat the process, and +their offspring after them, and so on again; you may go on for nine or +ten, or even twenty or more successions; and there is no very good +reason to say how soon it might terminate, or how long it might not go +on if the proper conditions of warmth and nourishment were kept up. + +Sexual reproduction is quite a distinct matter. Here, in all these +cases, what is required is the detachment of two portions of the +parental organisms, which portions we know as the egg and the +spermatozoon. In plants it is the ovule and the pollen-grain, as in the +flowering plants, or the ovule and the antherozooid, as in the +flowerless. Among all forms of animal life, the spermatozoa proceed +from the male sex, and the egg is the product of the female. Now, what +is remarkable about this mode of reproduction is this, that the egg by +itself, or the spermatozoa by themselves, are unable to assume the +parental form; but if they be brought into contact with one another, +the effect of the mixture of organic substances proceeding from two +sources appears to confer an altogether new vigour to the mixed product. +This process is brought about, as we all know, by the sexual +intercourse of the two sexes, and is called the act of impregnation. +The result of this act on the part of the male and female is, that the +formation of a new being is set up in the ovule or egg; this ovule or +egg soon begins to be divided and subdivided, and to be fashioned into +various complex organisms, and eventually to develop into the form of +one of its parents, as I explained in the first lecture. These are the +processes by which the perpetuation of organic beings is secured. Why +there should be the two modes--why this re-invigoration should be +required on the part of the female element we do not know; but it is +most assuredly the fact, and it is presumable, that, however long the +process of asexual multiplication could be continued, I say there is +good reason to believe that it would come to an end if a new +commencement were not obtained by a conjunction of the two sexual +elements. + +That character which is common to these two distinct processes is this, +that, whether we consider the reproduction, or perpetuation, or +modification of organic beings as they take place asexually, or as they +may take place sexually,--in either case, I say, the offspring has a +constant tendency to assume, speaking generally, the character of the +parent. As I said just now, if you take a slip of a plant, and tend it +with care, it will eventually grow up and develop into a plant like +that from which it had sprung; and this tendency is so strong that, as +gardeners know, this mode of multiplying by means of cuttings is the +only secure mode of propagating very many varieties of plants; the +peculiarity of the primitive stock seems to be better preserved if you +propagate it by means of a slip than if you resort to the sexual mode. + +Again, in experiments upon the lower animals, such as the polype, to +which I have referred, it is most extraordinary that, although cut up +into various pieces, each particular piece will grow up into the form +of the primitive stock; the head, if separated, will reproduce the body +and the tail; and if you cut off the tail, you will find that that will +reproduce the body and all the rest of the members, without in any way +deviating from the plan of the organism from which these portions have +been detached. And so far does this go, that some experimentalists +have carefully examined the lower orders of animals,--among them the +Abbe Spallanzani, who made a number of experiments upon snails and +salamanders,--and have found that they might mutilate them to an +incredible extent; that you might cut off the jaw or the greater part of +the head, or the leg or the tail, and repeat the experiment several +times, perhaps, cutting off the same member again and again; and yet +each of those types would be reproduced according to the primitive +type: nature making no mistake, never putting on a fresh kind of leg, or +head, or tail, but always tending to repeat and to return to the +primitive type. + +It is the same in sexual reproduction: it is a matter of perfectly +common experience, that the tendency on the part of the offspring +always is, speaking broadly, to reproduce the form of the parents. The +proverb has it that the thistle does not bring forth grapes; so, among +ourselves, there is always a likeness, more or less marked and +distinct, between children and their parents. That is a matter of +familiar and ordinary observation. We notice the same thing occurring +in the cases of the domestic animals--dogs, for instance, and their +offspring. In all these cases of propagation and perpetuation, there +seems to be a tendency in the offspring to take the characters of the +parental organisms. To that tendency a special name is given-- it is +called 'Atavism', it expresses this tendency to revert to the ancestral +type, and comes from the Latin word 'atavus', ancestor. + +Well, this 'Atavism' which I shall speak of, is, as I said before, one +of the most marked and striking tendencies of organic beings; but, side +by side with this hereditary tendency there is an equally distinct and +remarkable tendency to variation. The tendency to reproduce the +original stock has, as it were, its limits, and side by side with it +there is a tendency to vary in certain directions, as if there were two +opposing powers working upon the organic being, one tending to take it +in a straight line, and the other tending to make it diverge from that +straight line, first to one side and then to the other. + +So that you see these two tendencies need not precisely contradict one +another, as the ultimate result may not always be very remote from what +would have been the case if the line had been quite straight. + +This tendency to variation is less marked in that mode of propagation +which takes place asexually; it is in that mode that the minor +characters of animal and vegetable structures are most completely +preserved. Still, it will happen sometimes, that the gardener, when he +has planted a cutting of some favourite plant, will find, contrary to +his expectation, that the slip grows up a little different from the +primitive stock--that it produces flowers of a different colour or +make, or some deviation in one way or another. This is what is called +the 'sporting' of plants. + +In animals the phenomena of asexual propagation are so obscure, that at +present we cannot be said to know much about them; but if we turn to +that mode of perpetuation which results from the sexual process, then +we find variation a perfectly constant occurrence, to a certain extent; +and, indeed, I think that a certain amount of variation from the +primitive stock is the necessary result of the method of sexual +propagation itself; for, inasmuch as the thing propagated proceeds from +two organisms of different sexes and different makes and temperaments, +and as the offspring is to be either of one sex or the other, it is +quite clear that it cannot be an exact diagonal of the two, or it would +be of no sex at all; it cannot be an exact intermediate form between +that of each of its parents--it must deviate to one side or the other. +You do not find that the male follows the precise type of the male +parent, nor does the female always inherit the precise characteristics +of the mother,--there is always a proportion of the female character in +the male offspring, and of the male character in the female offspring. +That must be quite plain to all of you who have looked at all +attentively on your own children or those of your neighbours; you will +have noticed how very often it may happen that the son shall exhibit +the maternal type of character, or the daughter possess the +characteristics of the father's family. There are all sorts of +intermixtures and intermediate conditions between the two, where +complexion, or beauty, or fifty other different peculiarities belonging +to either side of the house, are reproduced in other members of the +same family. Indeed, it is sometimes to be remarked in this kind of +variation, that the variety belongs, strictly speaking, to neither of +the immediate parents; you will see a child in a family who is not like +either its father or its mother; but some old person who knew its +grandfather or grandmother, or, it may be, an uncle, or, perhaps, even +a more distant relative, will see a great similarity between the child +and one of these. In this way it constantly happens that the +characteristic of some previous member of the family comes out and is +reproduced and recognised in the most unexpected manner. + +But apart from that matter of general experience, there are some cases +which put that curious mixture in a very clear light. You are aware +that the offspring of the Ass and the Horse, or rather of the he-Ass +and the Mare, is what is called a Mule; and, on the other hand, the +offspring of the Stallion and the she-Ass is what is called a 'Hinny'. +I never saw one myself; but they have been very carefully studied. Now, +the curious thing is this, that although you have the same elements in +the experiment in each case, the offspring is entirely different in +character, according as the male influence comes from the Ass or the +Horse. Where the Ass is the male, as in the case of the Mule, you find +that the head is like that of the Ass, that the ears are long, the tail +is tufted at the end, the feet are small, and the voice is an +unmistakable bray; these are all points of similarity to the Ass; but, +on the other hand, the barrel of the body and the cut of the neck are +much more like those of the Mare. Then, if you look at the Hinny,--the +result of the union of the Stallion and the she-Ass, then you find it is +the Horse that has the predominance; that the head is more like that of +the Horse, the ears are shorter, the legs coarser, and the type is +altogether altered; while the voice, instead of being a bray, is the +ordinary neigh of the Horse. Here, you see, is a most curious thing: +you take exactly the same elements, Ass and Horse, but you combine the +sexes in a different manner, and the result is modified accordingly. +You have in this case, however, a result which is not general and +universal--there is usually an important preponderance, but not always +on the same side. + +Here, then, is one intelligible, and, perhaps, necessary cause of +variation: the fact, that there are two sexes sharing in the production +of the offspring, and that the share taken by each is different and +variable, not only for each combination, but also for different members +of the same family. + +Secondly, there is a variation, to a certain extent--though, in all +probability, the influence of this cause has been very much +exaggerated--but there is no doubt that variation is produced, to a +certain extent, by what are commonly known as external conditions,--such +as temperature, food, warmth, and moisture. In the long run, every +variation depends, in some sense, upon external conditions, seeing that +everything has a cause of its own. I use the term "external +conditions" now in the sense in which it is ordinarily employed: certain +it is, that external conditions have a definite effect. You may take a +plant which has single flowers, and by dealing with the soil, and +nourishment, and so on, you may by-and-by convert single flowers into +double flowers, and make thorns shoot out into branches. You may +thicken or make various modifications in the shape of the fruit. In +animals, too, you may produce analogous changes in this way, as in the +case of that deep bronze colour which persons rarely lose after having +passed any length of time in tropical countries. You may also alter +the development of the muscles very much, by dint of training; all the +world knows that exercise has a great effect in this way; we always +expect to find the arm of a blacksmith hard and wiry, and possessing a +large development of the brachial muscles. No doubt training, which is +one of the forms of external conditions, converts what are originally +only instructions, teachings, into habits, or, in other words, into +organizations, to a great extent; but this second cause of variation +cannot be considered to be by any means a large one. The third cause +that I have to mention, however, is a very extensive one. It is one +that, for want of a better name, has been called "spontaneous +variation;" which means that when we do not know anything about the +cause of phenomena, we call it spontaneous. In the orderly chain of +causes and effects in this world, there are very few things of which it +can be said with truth that they are spontaneous. Certainly not in +these physical matters,--in these there is nothing of the +kind,--everything depends on previous conditions. But when we cannot +trace the cause of phenomena, we call them spontaneous. + +Of these variations, multitudinous as they are, but little is known with +perfect accuracy. I will mention to you some two or three cases, +because they are very remarkable in themselves, and also because I +shall want to use them afterwards. Reaumur, a famous French naturalist, +a great many years ago, in an essay which he wrote upon the art of +hatching chickens,--which was indeed a very curious essay,--had +occasion to speak of variations and monstrosities. One very remarkable +case had come under his notice of a variation in the form of a human +member, in the person of a Maltese, of the name of Gratio Kelleia, who +was born with six fingers upon each hand, and the like number of toes +to each of his feet. That was a case of spontaneous variation. Nobody +knows why he was born with that number of fingers and toes, and as we +don't know, we call it a case of "spontaneous" variation. There is +another remarkable case also. I select these, because they happen to +have been observed and noted very carefully at the time. It frequently +happens that a variation occurs, but the persons who notice it do not +take any care in noting down the particulars, until at length, when +inquiries come to be made, the exact circumstances are forgotten; and +hence, multitudinous as may be such "spontaneous" variations, it is +exceedingly difficult to get at the origin of them. + +The second case is one of which you may find the whole details in the +"Philosophical Transactions" for the year 1813, in a paper communicated +by Colonel Humphrey to the President of the Royal Society,--"On a new +Variety in the Breed of Sheep," giving an account of a very remarkable +breed of sheep, which at one time was well known in the northern states +of America, and which went by the name of the Ancon or the Otter breed +of sheep. In the year 1791, there was a farmer of the name of Seth +Wright in Massachusetts, who had a flock of sheep, consisting of a ram +and, I think, of some twelve or thirteen ewes. Of this flock of ewes, +one at the breeding-time bore a lamb which was very singularly formed; +it had a very long body, very short legs, and those legs were bowed! I +will tell you by-and-by how this singular variation in the breed of +sheep came to be noted, and to have the prominence that it now has. For +the present, I mention only these two cases; but the extent of variation +in the breed of animals is perfectly obvious to any one who has studied +natural history with ordinary attention, or to any person who compares +animals with others of the same kind. It is strictly true that there +are never any two specimens which are exactly alike; however similar, +they will always differ in some certain particular. + +Now let us go back to Atavism,--to the hereditary tendency I spoke of. +What will come of a variation when you breed from it, when Atavism +comes, if I may say so, to intersect variation? The two cases of which +I have mentioned the history, give a most excellent illustration of +what occurs. Gratio Kelleia, the Maltese, married when he was +twenty-two years of age, and, as I suppose there were no six-fingered +ladies in Malta, he married an ordinary five-fingered person. The +result of that marriage was four children; the first, who was christened +Salvator, had six fingers and six toes, like his father; the second was +George, who had five fingers and toes, but one of them was deformed, +showing a tendency to variation; the third was Andre; he had five +fingers and five toes, quite perfect; the fourth was a girl, Marie; she +had five fingers and five toes, but her thumbs were deformed, showing a +tendency toward the sixth. + +These children grew up, and when they came to adult years, they all +married, and of course it happened that they all married five-fingered +and five-toed persons. Now let us see what were the results. Salvator +had four children; they were two boys, a girl, and another boy; the +first two boys and the girl were six-fingered and six-toed like their +grandfather; the fourth boy had only five fingers and five toes. George +had only four children; there were two girls with six fingers and six +toes; there was one girl with six fingers and five toes on the right +side, and five fingers and five toes on the left side, so that she was +half and half. The last, a boy, had five fingers and five toes. The +third, Andre, you will recollect, was perfectly well-formed, and he had +many children whose hands and feet were all regularly developed. Marie, +the last, who, of course, married a man who had only five fingers, had +four children; the first, a boy, was born with six toes, but the other +three were normal. + +Now observe what very extraordinary phenomena are presented here. You +have an accidental variation arising from what you may call a +monstrosity; you have that monstrosity tendency or variation diluted in +the first instance by an admixture with a female of normal construction, +and you would naturally expect that, in the results of such an union, +the monstrosity, if repeated, would be in equal proportion with the +normal type; that is to say, that the children would be half and half, +some taking the peculiarity of the father, and the others being of the +purely normal type of the mother; but you see we have a great +preponderance of the abnormal type. Well, this comes to be mixed once +more with the pure, the normal type, and the abnormal is again produced +in large proportion, notwithstanding the second dilution. Now what +would have happened if these abnormal types had intermarried with each +other; that is to say, suppose the two boys of Salvator had taken it +into their heads to marry their first cousins, the two first girls of +George, their uncle? You will remember that these are all of the +abnormal type of their grandfather. The result would probably have +been, that their offspring would have been in every case a further +development of that abnormal type. You see it is only in the fourth, +in the person of Marie, that the tendency, when it appears but slightly +in the second generation, is washed out in the third, while the progeny +of Andre, who escaped in the first instance, escape altogether. + +We have in this case a good example of nature's tendency to the +perpetuation of a variation. Here it is certainly a variation which +carried with it no use or benefit; and yet you see the tendency to +perpetuation may be so strong, that, notwithstanding a great admixture +of pure blood, the variety continues itself up to the third generation, +which is largely marked with it. In this case, as I have said, there +was no means of the second generation intermarrying with any but +five-fingered persons, and the question naturally suggests itself, What +would have been the result of such marriage? Reaumur narrates this +case only as far as the third generation. Certainly it would have been +an exceedingly curious thing if we could have traced this matter any +further; had the cousins intermarried, a six-fingered variety of the +human race might have been set up. + +To show you that this supposition is by no means an unreasonable one, +let me now point out what took place in the case of Seth Wright's +sheep, where it happened to be a matter of moment to him to obtain a +breed or raise a flock of sheep like that accidental variety that I +have described--and I will tell you why. In that part of Massachusetts +where Seth Wright was living, the fields were separated by fences, and +the sheep, which were very active and robust, would roam abroad, and +without much difficulty jump over these fences into other people's +farms. As a matter of course, this exuberant activity on the part of +the sheep constantly gave rise to all sorts of quarrels, bickerings, +and contentions among the farmers of the neighbourhood; so it occurred +to Seth Wright, who was, like his successors, more or less 'cute, that +if he could get a stock of sheep like those with the bandy legs, they +would not be able to jump over the fences so readily, and he acted upon +that idea. He killed his old ram, and as soon as the young one arrived +at maturity, he bred altogether from it. The result was even more +striking than in the human experiment which I mentioned just now. +Colonel Humphreys testifies that it always happened that the offspring +were either pure Ancons or pure ordinary sheep; that in no case was +there any mixing of the Ancons with the others. In consequence of +this, in the course of a very few years, the farmer was able to get a +very considerable flock of this variety, and a large number of them +were spread throughout Massachusetts. Most unfortunately, however--I +suppose it was because they were so common--nobody took enough notice +of them to preserve their skeletons; and although Colonel Humphreys +states that he sent a skeleton to the President of the Royal Society at +the same time that he forwarded his paper, I am afraid that the variety +has entirely disappeared; for a short time after these sheep had become +prevalent in that district, the Merino sheep were introduced; and as +their wool was much more valuable, and as they were a quiet race of +sheep, and showed no tendency to trespass or jump over fences, the +Otter breed of sheep, the wool of which was inferior to that of the +Merino, was gradually allowed to die out. + +You see that these facts illustrate perfectly well what may be done if +you take care to breed from stocks that are similar to each other. +After having got a variation, if, by crossing a variation with the +original stock, you multiply that variation, and then take care to keep +that variation distinct from the original stock, and make them breed +together,--then you may almost certainly produce a race whose tendency +to continue the variation is exceedingly strong. + +This is what is called "selection"; and it is by exactly the same +process as that by which Seth Wright bred his Ancon sheep, that our +breeds of cattle, dogs, and fowls, are obtained. There are some +possibilities of exception, but still, speaking broadly, I may say that +this is the way in which all our varied races of domestic animals have +arisen; and you must understand that it is not one peculiarity or one +characteristic alone in which animals may vary. There is not a single +peculiarity or characteristic of any kind, bodily or mental, in which +offspring may not vary to a certain extent from the parent and other +animals. + +Among ourselves this is well known. The simplest physical peculiarity +is mostly reproduced. I know a case of a man whose wife has the lobe of +one of her ears a little flattened. An ordinary observer might +scarcely notice it, and yet every one of her children has an +approximation to the same peculiarity to some extent. If you look at +the other extreme, too, the gravest diseases, such as gout, scrofula, +and consumption, may be handed down with just the same certainty and +persistence as we noticed in the perpetuation of the bandy legs of the +Ancon sheep. + +However, these facts are best illustrated in animals, and the extent of +the variation, as is well known, is very remarkable in dogs. For +example, there are some dogs very much smaller than others; indeed, the +variation is so enormous that probably the smallest dog would be about +the size of the head of the largest; there are very great variations in +the structural forms not only of the skeleton but also in the shape of +the skull, and in the proportions of the face and the disposition of +the teeth. + +The Pointer, the Retriever, Bulldog, and the Terrier, differ very +greatly, and yet there is every reason to believe that every one of +these races has arisen from the same source,--that all the most +important races have arisen by this selective breeding from accidental +variation. + +A still more striking case of what may be done by selective breeding, +and it is a better case, because there is no chance of that partial +infusion of error to which I alluded, has been studied very carefully +by Mr. Darwin,--the case of the domestic pigeons. I dare say there may +be some among you who may be pigeon 'fanciers', and I wish you to +understand that in approaching the subject, I would speak with all +humility and hesitation, as I regret to say that I am not a pigeon +fancier. I know it is a great art and mystery, and a thing upon which a +man must not speak lightly; but I shall endeavour, as far as my +understanding goes, to give you a summary of the published and +unpublished information which I have gained from Mr. Darwin. + +Among the enormous variety,--I believe there are somewhere about a +hundred and fifty kinds of pigeons,--there are four kinds which may be +selected as representing the extremest divergences of one kind from +another. Their names are the Carrier, the Pouter, the Fantail, and the +Tumbler. In the large diagrams they are each represented in their +relative sizes to each other. This first one is the Carrier; you will +notice this large excrescence on its beak; it has a comparatively small +head; there is a bare space round the eyes; it has a long neck, a very +long beak, very strong legs, large feet, long wings, and so on. The +second one is the Pouter, a very large bird, with very long legs and +beak. It is called the Pouter because it is in the habit of causing +its gullet to swell up by inflating it with air. I should tell you that +all pigeons have a tendency to do this at times, but in the Pouter it +is carried to an enormous extent. The birds appear to be quite proud +of their power of swelling and puffing themselves out in this way; and +I think it is about as droll a sight as you can well see to look at a +cage full of these pigeons puffing and blowing themselves out in this +ridiculous manner. + +The third kind I mentioned--the Fantail--is a small bird, with +exceedingly small legs and a very small beak. It is most curiously +distinguished by the size and extent of its tail, which, instead of +containing twelve feathers, may have many more,--say thirty, or even +more--I believe there are some with as many as forty-two. This bird +has a curious habit of spreading out the feathers of its tail in such a +way that they reach forward, and touch its head; and if this can be +accomplished, I believe it is looked upon as a point of great beauty. + +But here is the last great variety,--the Tumbler; and of that great +variety, one of the principal kinds, and one most prized, is the +specimen represented here--the short-faced Tumbler. Its beak is +reduced to a mere nothing. Just compare the beak of this one and that +of the first one, the Carrier--I believe the orthodox comparison of the +head and beak of a thoroughly well-bred Tumbler is to stick an oat into +a cherry, and that will give you the proper relative proportions of the +head and beak. The feet and legs are exceedingly small, and the bird +appears to be quite a dwarf when placed side by side with this great +Carrier. + +These are differences enough in regard to their external appearance; but +these differences are by no means the whole or even the most important +of the differences which obtain between these birds. There is hardly a +single point of their structure which has not become more or less +altered; and to give you an idea of how extensive these alterations are, +I have here some very good skeletons, for which I am indebted to my +friend, Mr. Tegetmeier, a great authority in these matters; by means of +which, if you examine them by-and-by, you will be able to see the +enormous difference in their bony structures. + +I had the privilege, some time ago, of access to some important MSS. of +Mr. Darwin, who, I may tell you, has taken very great pains and spent +much valuable time and attention on the investigation of these +variations, and getting together all the facts that bear upon them. I +obtained from these MSS. the following summary of the differences +between the domestic breeds of pigeons; that is to say, a notification +of the various points in which their organization differs. In the +first place, the back of the skull may differ a good deal, and the +development of the bones of the face may vary a great deal; the back +varies a good deal; the shape of the lower jaw varies; the tongue +varies very greatly, not only in correlation to the length and size of +the beak, but it seems also to have a kind of independent variation of +its own. Then the amount of naked skin round the eyes, and at the base +of the beak, may vary enormously; so may the length of the eyelids, the +shape of the nostrils, and the length of the neck. I have already +noticed the habit of blowing out the gullet, so remarkable in the +Pouter, and comparatively so in the others. There are great +differences, too, in the size of the female and the male, the shape of +the body, the number and width of the processes of the ribs, the +development of the ribs, and the size, shape, and development of the +breastbone. We may notice, too,--and I mention the fact because it has +been disputed by what is assumed to be high authority,--the variation +in the number of the sacral vertebrae. The number of these varies from +eleven to fourteen, and that without any diminution in the number of +the vertebrae of the back or of the tail. Then the number and position +of the tail-feathers may vary enormously, and so may the number of the +primary and secondary feathers of the wings. Again, the length of the +feet and of the beak,--although they have no relation to each other, +yet appear to go together,--that is, you have a long beak wherever you +have long feet. There are differences also in the periods of the +acquirement of the perfect plumage,--the size and shape of the +eggs,--the nature of flight, and the powers of flight,--so-called +"homing" birds having enormous flying powers;* while, on the other +hand, the little Tumbler is so called because of its extraordinary +faculty of turning head over heels in the air, instead of pursuing a +direct course. And, lastly, the dispositions and voices of the birds +may vary. Thus the case of the pigeons shows you that there is hardly +a single particular,--whether of instinct, or habit, or bony structure, +or of plumage,--of either the internal economy or the external shape, +in which some variation or change may not take place, which, by +selective breeding, may become perpetuated, and form the foundation of, +and give rise to, a new race. + + [footnote: The "Carrier," I learn from Mr. Tegetmeier, does + not 'carry'; a high-bred bird of this breed being but a + poor flier. The birds which fly long distances, and come + home,--"homing" birds,--and are consequently used as + carriers, are not "carriers" in the fancy sense.] + +If you carry in your mind's eye these four varieties of pigeons, you +will bear with you as good a notion as you can have, perhaps, of the +enormous extent to which a deviation from a primitive type may be +carried by means of this process of selective breeding. + + + + + +End of The Project Gutenberg Etext of The Perpetuation of Living Beings +by Thomas H. Huxley + diff --git a/old/thx0410.zip b/old/thx0410.zip Binary files differnew file mode 100644 index 0000000..43ef937 --- /dev/null +++ b/old/thx0410.zip |