The Perpetuation of Living Beings
by
Thomas H. Huxley
This etext was prepared 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;* 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.
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