Produced by Amy E. 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; [1]
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 1: 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.