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 ofour knowledge of the causes of the phenomena of organic nature, --of thepast and of the present, --resolved itself into two subsidiary inquiries:the first was, whether we know anything, either historically orexperimentally, of the mode of origin of living beings; the secondsubsidiary inquiry was, whether, granting the origin, we know anythingabout the perpetuation and modifications of the forms of organic beings. The reply which I had to give to the first question was altogethernegative, and the chief result of my last lecture was, that, neitherhistorically nor experimentally, do we at present know anythingwhatsoever about the origin of living forms. We saw that, historically, we are not likely to know anything about it, although we may perhapslearn something experimentally; but that at present we are an enormousdistance from the goal I indicated. 

I now, then, take up the next question, What do we know of thereproduction, the perpetuation, and the modifications of the formsof living beings, supposing that we have put the question as to theirorigination on one side, and have assumed that at present the causes oftheir origination are beyond us, and that we know nothing about them?Upon this question the state of our knowledge is extremely different; itis exceedingly large, and, if not complete, our experience is certainlymost extensive. It would be impossible to lay it all before you, and themost I can do, or need do to-night, is to take up the principal pointsand put them before you with such prominence as may subserve thepurposes of our present argument. 

The method of the perpetuation of organic beings is of two kinds, --theasexual and the sexual. In the first the perpetuation takes place fromand by a particular act of an individual organism, which sometimes maynot be classed as belonging to any sex at all. In the second case, it isin consequence of the mutual action and interaction of certain portionsof the organisms of usually two distinct individuals, --the male and thefemale. The cases of asexual perpetuation are by no means so common asthe cases of sexual perpetuation; and they are by no means so common inthe animal as in the vegetable world. You are all probably familiar withthe fact, as a matter of experience, that you can propagate plantsby means of what are called "cuttings;" for example, that by taking acutting from a geranium plant, and rearing it properly, by supplying itwith light and warmth and nourishment from the earth, it grows upand takes the form of its parent, having all the properties andpeculiarities of the original plant. 

Sometimes this process, which the gardener performs artificially, takesplace naturally; that is to say, a little bulb, or portion of the plant, detaches itself, drops off, and becomes capable of growing as a separatething. That is the case with many bulbous plants, which throw off inthis way secondary bulbs, which are lodged in the ground and becomedeveloped into plants. This is an asexual process, and from it resultsthe repetition or reproduction of the form of the original being fromwhich the bulb proceeds. 

Among animals the same thing takes place. Among the lower forms ofanimal life, the infusorial animalculae we have already spoken of throwoff certain portions, or break themselves up in various directions, sometimes transversely or sometimes longitudinally; or they may give offbuds, which detach themselves and develop into their proper forms. Thereis the common fresh-water Polype, for instance, which multiplies itselfin this way. Just in the same way as the gardener is able to multiplyand reproduce the peculiarities and characters of particular plantsby means of cuttings, so can the physiological experimentalist--as wasshown by the Abbe Trembley many years ago--so can he do the same thingwith many of the lower forms of animal life. M. De Trembley showed thatyou 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 upand reproduce completely the original form of the animal. These areall cases of asexual multiplication, and there are other instances, and still more extraordinary ones, in which this process takes placenaturally, in a more hidden, a more recondite kind of way. You are allof you familiar with those little green insects, the 'Aphis' or blight, as it is called. These little animals, during a very considerable partof their existence, multiply themselves by means of a kind of internalbudding, 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 moresuccessions; and there is no very good reason to say how soon it mightterminate, or how long it might not go on if the proper conditions ofwarmth and nourishment were kept up. 

Sexual reproduction is quite a distinct matter. Here, in all thesecases, what is required is the detachment of two portions of theparental organisms, which portions we know as the egg and thespermatozoon. In plants it is the ovule and the pollen-grain, as inthe flowering plants, or the ovule and the antherozooid, as in theflowerless. Among all forms of animal life, the spermatozoa proceed fromthe male sex, and the egg is the product of the female. Now, what isremarkable about this mode of reproduction is this, that the egg byitself, or the spermatozoa by themselves, are unable to assume theparental form; but if they be brought into contact with one another, theeffect of the mixture of organic substances proceeding from two sourcesappears to confer an altogether new vigour to the mixed product. Thisprocess is brought about, as we all know, by the sexual intercourse ofthe two sexes, and is called the act of impregnation. The result of thisact on the part of the male and female is, that the formation of a newbeing is set up in the ovule or egg; this ovule or egg soon begins tobe divided and subdivided, and to be fashioned into various complexorganisms, and eventually to develop into the form of one of itsparents, as I explained in the first lecture. These are the processes bywhich the perpetuation of organic beings is secured. Why there should bethe two modes--why this re-invigoration should be required on the partof the female element we do not know; but it is most assuredly thefact, and it is presumable, that, however long the process of asexualmultiplication could be continued, I say there is good reason to believethat it would come to an end if a new commencement were not obtained bya conjunction of the two sexual elements. 

That character which is common to these two distinct processes isthis, that, whether we consider the reproduction, or perpetuation, ormodification of organic beings as they take place asexually, or as theymay take place sexually, --in either case, I say, the offspring has aconstant tendency to assume, speaking generally, the character of theparent. As I said just now, if you take a slip of a plant, and tend itwith care, it will eventually grow up and develop into a plant likethat from which it had sprung; and this tendency is so strong that, asgardeners know, this mode of multiplying by means of cuttings is theonly secure mode of propagating very many varieties of plants; thepeculiarity of the primitive stock seems to be better preserved if youpropagate 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, towhich I have referred, it is most extraordinary that, although cut upinto various pieces, each particular piece will grow up into the form ofthe primitive stock; the head, if separated, will reproduce the bodyand the tail; and if you cut off the tail, you will find that that willreproduce the body and all the rest of the members, without in any waydeviating from the plan of the organism from which these portions havebeen detached. And so far does this go, that some experimentalists havecarefully examined the lower orders of animals, --among them theAbbe Spallanzani, who made a number of experiments upon snails andsalamanders, --and have found that they might mutilate them to anincredible extent; that you might cut off the jaw or the greater partof the head, or the leg or the tail, and repeat the experiment severaltimes, perhaps, cutting off the same member again and again; and yeteach 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 primitivetype. 

It is the same in sexual reproduction: it is a matter of perfectlycommon experience, that the tendency on the part of the offspring alwaysis, speaking broadly, to reproduce the form of the parents. Theproverb has it that the thistle does not bring forth grapes; so, amongourselves, there is always a likeness, more or less marked and distinct, between children and their parents. That is a matter of familiar andordinary observation. We notice the same thing occurring in the casesof the domestic animals--dogs, for instance, and their offspring. Inall these cases of propagation and perpetuation, there seems to bea tendency in the offspring to take the characters of the parentalorganisms. 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, oneof the most marked and striking tendencies of organic beings; but, sideby side with this hereditary tendency there is an equally distinct andremarkable tendency to variation. The tendency to reproduce the originalstock has, as it were, its limits, and side by side with it there is atendency to vary in certain directions, as if there were two opposingpowers working upon the organic being, one tending to take it in astraight line, and the other tending to make it diverge from thatstraight line, first to one side and then to the other. 

So that you see these two tendencies need not precisely contradict oneanother, as the ultimate result may not always be very remote from whatwould have been the case if the line had been quite straight. 

This tendency to variation is less marked in that mode of propagationwhich takes place asexually; it is in that mode that the minorcharacters of animal and vegetable structures are most completelypreserved. Still, it will happen sometimes, that the gardener, when hehas planted a cutting of some favourite plant, will find, contrary tohis expectation, that the slip grows up a little different from theprimitive 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, thatat present we cannot be said to know much about them; but if we turn tothat mode of perpetuation which results from the sexual process, thenwe find variation a perfectly constant occurrence, to a certain extent;and, indeed, I think that a certain amount of variation from theprimitive stock is the necessary result of the method of sexualpropagation itself; for, inasmuch as the thing propagated proceeds fromtwo organisms of different sexes and different makes and temperaments, and as the offspring is to be either of one sex or the other, it isquite clear that it cannot be an exact diagonal of the two, or it wouldbe of no sex at all; it cannot be an exact intermediate form betweenthat 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 maleparent, nor does the female always inherit the precise characteristicsof the mother, --there is always a proportion of the female character inthe male offspring, and of the male character in the female offspring. That must be quite plain to all of you who have looked at allattentively on your own children or those of your neighbours; you willhave noticed how very often it may happen that the son shall exhibit thematernal type of character, or the daughter possess the characteristicsof the father's family. There are all sorts of intermixtures andintermediate conditions between the two, where complexion, or beauty, or fifty other different peculiarities belonging to either side of thehouse, are reproduced in other members of the same family. Indeed, itis sometimes to be remarked in this kind of variation, that the varietybelongs, strictly speaking, to neither of the immediate parents; youwill see a child in a family who is not like either its father or itsmother; but some old person who knew its grandfather or grandmother, or, it may be, an uncle, or, perhaps, even a more distant relative, will seea great similarity between the child and one of these. In this way itconstantly happens that the characteristic of some previous memberof the family comes out and is reproduced and recognised in the mostunexpected manner. 

But apart from that matter of general experience, there are some caseswhich put that curious mixture in a very clear light. You are aware thatthe offspring of the Ass and the Horse, or rather of the he-Ass and theMare, is what is called a Mule; and, on the other hand, the offspringof the Stallion and the she-Ass is what is called a 'Hinny'. I never sawone myself; but they have been very carefully studied. Now, thecurious thing is this, that although you have the same elements inthe experiment in each case, the offspring is entirely different incharacter, according as the male influence comes from the Ass or theHorse. Where the Ass is the male, as in the case of the Mule, you findthat the head is like that of the Ass, that the ears are long, thetail is tufted at the end, the feet are small, and the voice is anunmistakable 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 aremuch more like those of the Mare. Then, if you look at the Hinny, --theresult of the union of the Stallion and the she-Ass, then you find it isthe Horse that has the predominance; that the head is more like thatof the Horse, the ears are shorter, the legs coarser, and the type isaltogether altered; while the voice, instead of being a bray, is theordinary neigh of the Horse. Here, you see, is a most curious thing: youtake exactly the same elements, Ass and Horse, but you combine the sexesin a different manner, and the result is modified accordingly. Youhave in this case, however, a result which is not general anduniversal--there is usually an important preponderance, but not alwayson the same side. 

Here, then, is one intelligible, and, perhaps, necessary cause ofvariation: the fact, that there are two sexes sharing in the productionof the offspring, and that the share taken by each is different andvariable, not only for each combination, but also for different membersof the same family. 

Secondly, there is a variation, to a certain extent--though, inall probability, the influence of this cause has been very muchexaggerated--but there is no doubt that variation is produced, to acertain extent, by what are commonly known as external conditions, --suchas temperature, food, warmth, and moisture. In the long run, everyvariation depends, in some sense, upon external conditions, seeing thateverything 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, thatexternal conditions have a definite effect. You may take a plant whichhas single flowers, and by dealing with the soil, and nourishment, andso on, you may by-and-by convert single flowers into double flowers, and make thorns shoot out into branches. You may thicken or make variousmodifications in the shape of the fruit. In animals, too, you mayproduce analogous changes in this way, as in the case of that deepbronze colour which persons rarely lose after having passed any lengthof time in tropical countries. You may also alter the development ofthe muscles very much, by dint of training; all the world knows thatexercise has a great effect in this way; we always expect to find thearm of a blacksmith hard and wiry, and possessing a large developmentof the brachial muscles. No doubt training, which is one of the formsof external conditions, converts what are originally only instructions, teachings, into habits, or, in other words, into organizations, to agreat extent; but this second cause of variation cannot be consideredto 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 bettername, has been called "spontaneous variation;" which means that whenwe do not know anything about the cause of phenomena, we call itspontaneous. 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 theyare spontaneous. Certainly not in these physical matters, --inthese there is nothing of the kind, --everything depends on previousconditions. But when we cannot trace the cause of phenomena, we callthem spontaneous. 

Of these variations, multitudinous as they are, but little is known withperfect accuracy. I will mention to you some two or three cases, becausethey are very remarkable in themselves, and also because I shall want touse them afterwards. Reaumur, a famous French naturalist, a greatmany years ago, in an essay which he wrote upon the art of hatchingchickens, --which was indeed a very curious essay, --had occasion to speakof variations and monstrosities. One very remarkable case had come underhis notice of a variation in the form of a human member, in the personof a Maltese, of the name of Gratio Kelleia, who was born with sixfingers 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 bornwith that number of fingers and toes, and as we don't know, we call it acase of "spontaneous" variation. There is another remarkable case also. I select these, because they happen to have been observed and noted verycarefully at the time. It frequently happens that a variation occurs, but the persons who notice it do not take any care in noting down theparticulars, until at length, when inquiries come to be made, the exactcircumstances are forgotten; and hence, multitudinous as may be such"spontaneous" variations, it is exceedingly difficult to get at theorigin 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 communicatedby Colonel Humphrey to the President of the Royal Society, --"On a newVariety in the Breed of Sheep, " giving an account of a very remarkablebreed of sheep, which at one time was well known in the northern statesof America, and which went by the name of the Ancon or the Otter breedof sheep. In the year 1791, there was a farmer of the name of SethWright in Massachusetts, who had a flock of sheep, consisting of a ramand, 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 ofsheep came to be noted, and to have the prominence that it now has. Forthe present, I mention only these two cases; but the extent of variationin the breed of animals is perfectly obvious to any one who has studiednatural history with ordinary attention, or to any person who comparesanimals with others of the same kind. It is strictly true that there arenever any two specimens which are exactly alike; however similar, theywill always differ in some certain particular. 

Now let us go back to Atavism, --to the hereditary tendency I spokeof. What will come of a variation when you breed from it, when Atavismcomes, if I may say so, to intersect variation? The two cases of whichI have mentioned the history, give a most excellent illustration ofwhat occurs. Gratio Kelleia, the Maltese, married when he was twenty-twoyears of age, and, as I suppose there were no six-fingered ladies inMalta, he married an ordinary five-fingered person. The result of thatmarriage was four children; the first, who was christened Salvator, hadsix fingers and six toes, like his father; the second was George, whohad five fingers and toes, but one of them was deformed, showing atendency to variation; the third was Andre; he had five fingers and fivetoes, quite perfect; the fourth was a girl, Marie; she had five fingersand five toes, but her thumbs were deformed, showing a tendency towardthe sixth. 

These children grew up, and when they came to adult years, they allmarried, and of course it happened that they all married five-fingeredand five-toed persons. Now let us see what were the results. Salvatorhad four children; they were two boys, a girl, and another boy; thefirst two boys and the girl were six-fingered and six-toed like theirgrandfather; the fourth boy had only five fingers and five toes. Georgehad only four children; there were two girls with six fingers and sixtoes; there was one girl with six fingers and five toes on the rightside, and five fingers and five toes on the left side, so that she washalf and half. The last, a boy, had five fingers and five toes. Thethird, Andre, you will recollect, was perfectly well-formed, and he hadmany children whose hands and feet were all regularly developed. Marie, the last, who, of course, married a man who had only five fingers, hadfour children; the first, a boy, was born with six toes, but the otherthree were normal. 

Now observe what very extraordinary phenomena are presented here. You have an accidental variation arising from what you may call amonstrosity; you have that monstrosity tendency or variation diluted inthe 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 thenormal type; that is to say, that the children would be half and half, some taking the peculiarity of the father, and the others being ofthe purely normal type of the mother; but you see we have a greatpreponderance of the abnormal type. Well, this comes to be mixed oncemore with the pure, the normal type, and the abnormal is again producedin large proportion, notwithstanding the second dilution. Now what wouldhave happened if these abnormal types had intermarried with each other;that is to say, suppose the two boys of Salvator had taken it into theirheads to marry their first cousins, the two first girls of George, theiruncle? You will remember that these are all of the abnormal type oftheir grandfather. The result would probably have been, that theiroffspring would have been in every case a further development of thatabnormal type. You see it is only in the fourth, in the person ofMarie, that the tendency, when it appears but slightly in the secondgeneration, is washed out in the third, while the progeny of Andre, whoescaped in the first instance, escape altogether. 

We have in this case a good example of nature's tendency to theperpetuation of a variation. Here it is certainly a variation whichcarried with it no use or benefit; and yet you see the tendency toperpetuation may be so strong, that, notwithstanding a great admixtureof pure blood, the variety continues itself up to the third generation, which is largely marked with it. In this case, as I have said, therewas no means of the second generation intermarrying with any butfive-fingered persons, and the question naturally suggests itself, Whatwould have been the result of such marriage? Reaumur narrates this caseonly as far as the third generation. Certainly it would have beenan exceedingly curious thing if we could have traced this matter anyfurther; had the cousins intermarried, a six-fingered variety of thehuman 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 breedor raise a flock of sheep like that accidental variety that I havedescribed--and I will tell you why. In that part of Massachusetts whereSeth Wright was living, the fields were separated by fences, and thesheep, which were very active and robust, would roam abroad, and withoutmuch difficulty jump over these fences into other people's farms. Asa matter of course, this exuberant activity on the part of thesheep constantly gave rise to all sorts of quarrels, bickerings, andcontentions among the farmers of the neighbourhood; so it occurred toSeth Wright, who was, like his successors, more or less 'cute, that ifhe could get a stock of sheep like those with the bandy legs, they wouldnot be able to jump over the fences so readily, and he acted upon thatidea. He killed his old ram, and as soon as the young one arrived atmaturity, he bred altogether from it. The result was even more strikingthan in the human experiment which I mentioned just now. ColonelHumphreys testifies that it always happened that the offspring wereeither pure Ancons or pure ordinary sheep; that in no case was thereany mixing of the Ancons with the others. In consequence of this, inthe course of a very few years, the farmer was able to get a veryconsiderable flock of this variety, and a large number of them werespread throughout Massachusetts. Most unfortunately, however--I supposeit was because they were so common--nobody took enough notice of them topreserve their skeletons; and although Colonel Humphreys states that hesent a skeleton to the President of the Royal Society at the same timethat he forwarded his paper, I am afraid that the variety has entirelydisappeared; for a short time after these sheep had become prevalent inthat district, the Merino sheep were introduced; and as their wool wasmuch more valuable, and as they were a quiet race of sheep, and showedno tendency to trespass or jump over fences, the Otter breed of sheep, the wool of which was inferior to that of the Merino, was graduallyallowed to die out. 

You see that these facts illustrate perfectly well what may be done ifyou take care to breed from stocks that are similar to each other. Afterhaving got a variation, if, by crossing a variation with the originalstock, you multiply that variation, and then take care to keep thatvariation distinct from the original stock, and make them breedtogether, --then you may almost certainly produce a race whose tendencyto continue the variation is exceedingly strong. 

This is what is called "selection"; and it is by exactly the sameprocess as that by which Seth Wright bred his Ancon sheep, thatour breeds of cattle, dogs, and fowls, are obtained. There are somepossibilities of exception, but still, speaking broadly, I may say thatthis is the way in which all our varied races of domestic animals havearisen; and you must understand that it is not one peculiarity or onecharacteristic alone in which animals may vary. There is not a singlepeculiarity or characteristic of any kind, bodily or mental, in whichoffspring may not vary to a certain extent from the parent and otheranimals. 

Among ourselves this is well known. The simplest physical peculiarity ismostly reproduced. I know a case of a man whose wife has the lobe ofone of her ears a little flattened. An ordinary observer might scarcelynotice it, and yet every one of her children has an approximation to thesame peculiarity to some extent. If you look at the other extreme, too, the gravest diseases, such as gout, scrofula, and consumption, may behanded down with just the same certainty and persistence as we noticedin the perpetuation of the bandy legs of the Ancon sheep. 

However, these facts are best illustrated in animals, and the extentof the variation, as is well known, is very remarkable in dogs. Forexample, there are some dogs very much smaller than others; indeed, thevariation is so enormous that probably the smallest dog would be aboutthe size of the head of the largest; there are very great variations inthe structural forms not only of the skeleton but also in the shape ofthe skull, and in the proportions of the face and the disposition of theteeth. 

The Pointer, the Retriever, Bulldog, and the Terrier, differ verygreatly, and yet there is every reason to believe that every oneof these races has arisen from the same source, --that all the mostimportant races have arisen by this selective breeding from accidentalvariation. 

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 partialinfusion of error to which I alluded, has been studied very carefully byMr. Darwin, --the case of the domestic pigeons. I dare say there maybe some among you who may be pigeon 'fanciers', and I wish you tounderstand that in approaching the subject, I would speak with allhumility and hesitation, as I regret to say that I am not a pigeonfancier. I know it is a great art and mystery, and a thing upon whicha man must not speak lightly; but I shall endeavour, as far asmy understanding goes, to give you a summary of the published andunpublished information which I have gained from Mr. Darwin. 

Among the enormous variety, --I believe there are somewhere about ahundred and fifty kinds of pigeons, --there are four kinds which maybe selected as representing the extremest divergences of one kind fromanother. Their names are the Carrier, the Pouter, the Fantail, andthe Tumbler. In the large diagrams they are each represented in theirrelative sizes to each other. This first one is the Carrier; you willnotice this large excrescence on its beak; it has a comparatively smallhead; there is a bare space round the eyes; it has a long neck, a verylong beak, very strong legs, large feet, long wings, and so on. Thesecond one is the Pouter, a very large bird, with very long legs andbeak. It is called the Pouter because it is in the habit of causing itsgullet to swell up by inflating it with air. I should tell you that allpigeons have a tendency to do this at times, but in the Pouter it iscarried to an enormous extent. The birds appear to be quite proud oftheir power of swelling and puffing themselves out in this way; and Ithink it is about as droll a sight as you can well see to look at acage full of these pigeons puffing and blowing themselves out in thisridiculous manner. 

The third kind I mentioned--the Fantail--is a small bird, withexceedingly small legs and a very small beak. It is most curiouslydistinguished by the size and extent of its tail, which, instead ofcontaining twelve feathers, may have many more, --say thirty, or evenmore--I believe there are some with as many as forty-two. This bird hasa curious habit of spreading out the feathers of its tail in such away that they reach forward, and touch its head; and if this can beaccomplished, I believe it is looked upon as a point of great beauty. 

But here is the last great variety, --the Tumbler; and of that greatvariety, one of the principal kinds, and one most prized, is thespecimen represented here--the short-faced Tumbler. Its beak is reducedto a mere nothing. Just compare the beak of this one and that of thefirst one, the Carrier--I believe the orthodox comparison of the headand beak of a thoroughly well-bred Tumbler is to stick an oat into acherry, and that will give you the proper relative proportions of thehead and beak. The feet and legs are exceedingly small, and the birdappears to be quite a dwarf when placed side by side with this greatCarrier. 

These are differences enough in regard to their external appearance; butthese differences are by no means the whole or even the most importantof the differences which obtain between these birds. There is hardlya single point of their structure which has not become more or lessaltered; and to give you an idea of how extensive these alterationsare, I have here some very good skeletons, for which I am indebted to myfriend, Mr. Tegetmeier, a great authority in these matters; by meansof which, if you examine them by-and-by, you will be able to see theenormous difference in their bony structures. 

I had the privilege, some time ago, of access to some important MSS. OfMr. Darwin, who, I may tell you, has taken very great pains andspent much valuable time and attention on the investigation of thesevariations, and getting together all the facts that bear upon them. I obtained from these MSS. The following summary of the differencesbetween the domestic breeds of pigeons; that is to say, a notificationof the various points in which their organization differs. In the firstplace, the back of the skull may differ a good deal, and the developmentof the bones of the face may vary a great deal; the back varies a gooddeal; 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 seemsalso to have a kind of independent variation of its own. Then the amountof naked skin round the eyes, and at the base of the beak, may varyenormously; 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 blowingout the gullet, so remarkable in the Pouter, and comparatively so in theothers. There are great differences, too, in the size of the female andthe male, the shape of the body, the number and width of the processesof the ribs, the development of the ribs, and the size, shape, anddevelopment of the breastbone. We may notice, too, --and I mentionthe fact because it has been disputed by what is assumed to be highauthority, --the variation in the number of the sacral vertebrae. Thenumber of these varies from eleven to fourteen, and that without anydiminution 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 thewings. Again, the length of the feet and of the beak, --although theyhave no relation to each other, yet appear to go together, --that is, youhave a long beak wherever you have long feet. There are differences alsoin the periods of the acquirement of the perfect plumage, --the sizeand shape of the eggs, --the nature of flight, and the powers offlight, --so-called "homing" birds having enormous flying powers; [1]while, on the other hand, the little Tumbler is so called because of itsextraordinary faculty of turning head over heels in the air, instead ofpursuing a direct course. And, lastly, the dispositions and voices ofthe birds may vary. Thus the case of the pigeons shows you that thereis hardly a single particular, --whether of instinct, or habit, orbony structure, or of plumage, --of either the internal economy or theexternal shape, in which some variation or change may not take place, which, by selective breeding, may become perpetuated, and form thefoundation of, and give rise to, a new race. 

[Footnote 1: The "Carrier, " I learn from Mr. Tegetmeier, does not'carry'; a high-bred bird of this breed being but a poor flier. Thebirds which fly long distances, and come home, --"homing" birds, --and areconsequently used as carriers, are not "carriers" in the fancy sense. ]

If you carry in your mind's eye these four varieties of pigeons, youwill bear with you as good a notion as you can have, perhaps, of theenormous extent to which a deviation from a primitive type may becarried by means of this process of selective breeding.