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Darwiniana

T >> Thomas Henry Huxley >> Darwiniana




Among animals the same thing takes place. Among the lower forms of animal
life, the infusorial animalculæ 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 Abbé 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 non-sexual 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 that little green insect, 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 non-sexual 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 or 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 organs, 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 non-sexual
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 non-sexually, 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 Abbé 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--and as I may very often use it, I will write it up here on this
black-board that you may remember it--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 non-sexually; 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 non-sexual 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. It is a very rare
thing in this country to see 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
organisations, 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. Réaumur, 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 Andrè; 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, Andrè, 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 giving rise to what you may call a monstrosity; you
have that monstrosity 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 Andrè, 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 earned
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? Réaumur 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 trespasser 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 woman who 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.

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