The Conditions of Existence
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Thomas H. Huxley >> The Conditions of Existence
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THE CONDITIONS OF EXISTENCE AS AFFECTING THE PERPETUATION OF
LIVING BEINGS
#15 in our series by Thomas H. Huxley
IN the last Lecture I endeavoured to prove to you that, while, as a
general rule, organic beings tend to reproduce their kind, there is in
them, also, a constantly recurring tendency to vary--to vary to a
greater or to a less extent. Such a variety, I pointed out to you,
might arise from causes which we do not understand; we therefore called
it spontaneous; and it might come into existence as a definite and
marked thing, without any gradations between itself and the form which
preceded it. I further pointed out, that such a variety having once
arisen, might be perpetuated to some extent, and indeed to a very
marked extent, without any direct interference, or without any exercise
of that process which we called selection. And then I stated further,
that by such selection, when exercised artificially--if you took care to
breed only from those forms which presented the same peculiarities of
any variety which had arisen in this manner--the variation might be
perpetuated, as far as we can see, indefinitely.
The next question, and it is an important one for us, is this: Is there
any limit to the amount of variation from the primitive stock which can
be produced by this process of selective breeding? In considering this
question, it will be useful to class the characteristics, in respect of
which organic beings vary, under two heads: we may consider structural
characteristics, and we may consider physiological characteristics.
In the first place, as regards structural characteristics, I endeavoured
to show you, by the skeletons which I had upon the table, and by
reference to a great many well-ascertained facts, that the different
breeds of Pigeons, the Carriers, Pouters, and Tumblers, might vary in
any of their internal and important structural characters to a very
great degree; not only might there be changes in the proportions of the
skull, and the characters of the feet and beaks, and so on; but that
there might be an absolute difference in the number of the vertebrae of
the back, as in the sacral vertebrae of the Pouter; and so great is the
extent of the variation in these and similar characters that I pointed
out to you, by reference to the skeletons and the diagrams, that these
extreme varieties may absolutely differ more from one another in their
structural characters than do what naturalists call distinct SPECIES of
pigeons; that is to say, that they differ so much in structure that
there is a greater difference between the Pouter and the Tumbler than
there is between such wild and distinct forms as the Rock Pigeon or the
Ring Pigeon, or the Ring Pigeon and the Stock Dove; and indeed the
differences are of greater value than this, for the structural
differences between these domesticated pigeons are such as would be
admitted by a naturalist, supposing he knew nothing at all about their
origin, to entitle them to constitute even distinct genera.
As I have used this term SPECIES, and shall probably use it a good deal,
I had better perhaps devote a word or two to explaining what I mean by
it.
Animals and plants are divided into groups, which become gradually
smaller, beginning with a KINGDOM, which is divided into SUB-KINGDOMS;
then come the smaller divisions called PROVINCES; and so on from a
PROVINCE to a CLASS from a CLASS to an ORDER, from ORDERS to FAMILIES,
and from these to GENERA, until we come at length to the smallest
groups of animals which can be defined one from the other by constant
characters, which are not sexual; and these are what naturalists call
SPECIES in practice, whatever they may do in theory.
If, in a state of nature, you find any two groups of living beings,
which are separated one from the other by some constantly-recurring
characteristic, I don't care how slight and trivial, so long as it is
defined and constant, and does not depend on sexual peculiarities, then
all naturalists agree in calling them two species; that is what is
meant by the use of the word species--that is to say, it is, for the
practical naturalist, a mere question of structural differences.*
[footnote]* I lay stress here on the 'practical'
signification of "Species." Whether a physiological test
between species exist or not, it is hardly ever applicable
by the practical naturalist.
We have seen now--to repeat this point once more, and it is very
essential that we should rightly understand it--we have seen that
breeds, known to have been derived from a common stock by selection,
may be as different in their structure from the original stock as
species may be distinct from each other.
But is the like true of the physiological characteristics of animals?
Do the physiological differences of varieties amount in degree to those
observed between forms which naturalists call distinct species? This
is a most important point for us to consider.
As regards the great majority of physiological characteristics, there is
no doubt that they are capable of being developed, increased, and
modified by selection.
There is no doubt that breeds may be made as different as species in
many physiological characters. I have already pointed out to you very
briefly the different habits of the breeds of Pigeons, all of which
depend upon their physiological peculiarities,--as the peculiar habit
of tumbling, in the Tumbler--the peculiarities of flight, in the
"homing" birds,--the strange habit of spreading out the tail, and
walking in a peculiar fashion, in the Fantail,--and, lastly, the habit
of blowing out the gullet, so characteristic of the Pouter. These are
all due to physiological modifications, and in all these respects these
birds differ as much from each other as any two ordinary species do.
So with Dogs in their habits and instincts. It is a physiological
peculiarity which leads the Greyhound to chase its prey by sight,--that
enables the Beagle to track it by the scent,--that impels the Terrier
to its rat-hunting propensity,--and that leads the Retriever to its
habit of retrieving. These habits and instincts are all the results of
physiological differences and peculiarities, which have been developed
from a common stock, at least there is every reason to believe so. But
it is a most singular circumstance, that while you may run through
almost the whole series of physiological processes, without finding a
check to your argument, you come at last to a point where you do find a
check, and that is in the reproductive processes. For there is a most
singular circumstance in respect to natural species--at least about some
of them--and it would be sufficient for the purposes of this argument
if it were true of only one of them, but there is, in fact, a great
number of such cases--and that is, that, similar as they may appear to
be to mere races or breeds, they present a marked peculiarity in the
reproductive process. If you breed from the male and female of the same
race, you of course have offspring of the like kind, and if you make
the offspring breed together, you obtain the same result, and if you
breed from these again, you will still have the same kind of offspring;
there is no check. But if you take members of two distinct species,
however similar they may be to each other and make them breed together,
you will find a check, with some modifications and exceptions, however,
which I shall speak of presently. If you cross two such species with
each other, then,--although you may get offspring in the case of the
first cross, yet, if you attempt to breed from the products of that
crossing, which are what are called HYBRIDS--that is, if you couple a
male and a female hybrid--then the result is that in ninety-nine cases
out of a hundred you will get no offspring at all; there will be no
result whatsoever.
The reason of this is quite obvious in some cases; the male hybrids,
although possessing all the external appearances and characteristics of
perfect animals, are physiologically imperfect and deficient in the
structural parts of the reproductive elements necessary to generation.
It is said to be invariably the case with the male mule, the cross
between the Ass and the Mare; and hence it is, that, although crossing
the Horse with the Ass is easy enough, and is constantly done, as far
as I am aware, if you take two mules, a male and a female, and endeavour
to breed from them, you get no offspring whatever; no generation will
take place. This is what is called the sterility of the hybrids
between two distinct species.
You see that this is a very extraordinary circumstance; one does not see
why it should be. The common teleological explanation is, that it is
to prevent the impurity of the blood resulting from the crossing of one
species with another, but you see it does not in reality do anything of
the kind. There is nothing in this fact that hybrids cannot breed with
each other, to establish such a theory; there is nothing to prevent the
Horse breeding with the Ass, or the Ass with the Horse. So that this
explanation breaks down, as a great many explanations of this kind do,
that are only founded on mere assumptions.
Thus you see that there is a great difference between "mongrels," which
are crosses between distinct races, and "hybrids," which are crosses
between distinct species. The mongrels are, so far as we know, fertile
with one another. But between species, in many cases, you cannot
succeed in obtaining even the first cross: at any rate it is quite
certain that the hybrids are often absolutely infertile one with
another.
Here is a feature, then, great or small as it may be, which
distinguishes natural species of animals. Can we find any
approximation to this in the different races known to be produced by
selective breeding from a common stock? Up to the present time the
answer to that question is absolutely a negative one. As far as we
know at present, there is nothing approximating to this check. In
crossing the breeds between the Fantail and the Pouter, the Carrier and
the Tumbler, or any other variety or race you may name--so far as we
know at present--there is no difficulty in breeding together the
mongrels. Take the Carrier and the Fantail, for instance, and let them
represent the Horse and the Ass in the case of distinct species; then
you have, as the result of their breeding, the Carrier-Fantail
mongrel,--we will say the male and female mongrel,--and, as far as we
know, these two when crossed would not be less fertile than the
original cross, or than Carrier with Carrier. Here, you see, is a
physiological contrast between the races produced by selective
modification and natural species. I shall inquire into the value of
this fact, and of some modifying circumstances by and by; for the
present I merely put it broadly before you.
But while considering this question of the limitations of species, a
word must be said about what is called RECURRENCE--the tendency of
races which have been developed by selective breeding from varieties to
return to their primitive type. This is supposed by many to put an
absolute limit to the extent of selective and all other variations.
People say, "It is all very well to talk about producing these
different races, but you know very well that if you turned all these
birds wild, these Pouters, and Carriers, and so on, they would all
return to their primitive stock." This is very commonly assumed to be
a fact, and it is an argument that is commonly brought forward as
conclusive; but if you will take the trouble to inquire into it rather
closely, I think you will find that it is not worth very much. The
first question of course is, Do they thus return to the primitive
stock? And commonly as the thing is assumed and accepted, it is
extremely difficult to get anything like good evidence of it. It is
constantly said, for example, that if domesticated Horses are turned
wild, as they have been in some parts of Asia Minor and South America,
that they return at once to the primitive stock from which they were
bred. But the first answer that you make to this assumption is, to ask
who knows what the primitive stock was; and the second answer is, that
in that case the wild Horses of Asia Minor ought to be exactly like the
wild Horses of South America. If they are both like the same thing,
they ought manifestly to be like each other! The best authorities,
however, tell you that it is quite different. The wild Horse of Asia
is said to be of a dun colour, with a largish head, and a great many
other peculiarities; while the best authorities on the wild Horses of
South America tell you that there is no similarity between their wild
Horses and those of Asia Minor; the cut of their heads is very
different, and they are commonly chestnut or bay-coloured. It is quite
clear, therefore, that as by these facts there ought to have been two
primitive stocks, they go for nothing in support of the assumption that
races recur to one primitive stock, and so far as this evidence is
concerned, it falls to the ground.
Suppose for a moment that it were so, and that domesticated races, when
turned wild, did return to some common condition, I cannot see that
this would prove much more than that similar conditions are likely to
produce similar results; and that when you take back domesticated
animals into what we call natural conditions, you do exactly the same
thing as if you carefully undid all the work you had gone through, for
the purpose of bringing the animal from its wild to its domesticated
state. I do not see anything very wonderful in the fact, if it took
all that trouble to get it from a wild state, that it should go back
into its original state as soon as you removed the conditions which
produced the variation to the domesticated form. There is an important
fact, however, forcibly brought forward by Mr. Darwin, which has been
noticed in connection with the breeding of domesticated pigeons; and it
is, that however different these breeds of pigeons may be from each
other, and we have already noticed the great differences in these
breeds, that if, among any of those variations, you chance to have a
blue pigeon turn up, it will be sure to have the black bars across the
wings, which are characteristic of the original wild stock, the Rock
Pigeon.
Now, this is certainly a very remarkable circumstance; but I do not see
myself how it tells very strongly either one way or the other. I
think, in fact, that this argument in favour of recurrence to the
primitive type might prove a great deal too much for those who so
constantly bring it forward. For example, Mr. Darwin has very forcibly
urged, that nothing is commoner than if you examine a dun horse--and I
had an opportunity of verifying this illustration lately, while in the
islands of the West Highlands, where there are a great many dun
horses--to find that horse exhibit a long black stripe down his back,
very often stripes on his shoulder, and very often stripes on his
legs. I, myself, saw a pony of this description a short time ago, in a
baker's cart, near Rothesay, in Bute: it had the long stripe down the
back, and stripes on the shoulders and legs, just like those of the
Ass, the Quagga, and the Zebra. Now, if we interpret the theory of
recurrence as applied to this case, might it not be said that here was
a case of a variation exhibiting the characters and conditions of an
animal occupying something like an intermediate position between the
Horse, the Ass, the Quagga, and the Zebra, and from which these had
been developed? In the same way with regard even to Man. Every
anatomist will tell you that there is nothing commoner, in dissecting
the human body, than to meet with what are called muscular
variations--that is, if you dissect two bodies very carefully, you will
probably find that the modes of attachment and insertion of the muscles
are not exactly the same in both, there being great peculiarities in
the mode in which the muscles are arranged; and it is very singular,
that in some dissections of the human body you will come upon
arrangements of the muscles very similar indeed to the same parts in the
Apes. Is the conclusion in that case to be, that this is like the
black bars in the case of the Pigeon, and that it indicates a
recurrence to the primitive type from which the animals have been
probably developed? Truly, I think that the opponents of modification
and variation had better leave the argument of recurrence alone, or it
may prove altogether too strong for them.
To sum up,--the evidence as far as we have gone is against the argument
as to any limit to divergences, so far as structure is concerned; and
in favour of a physiological limitation. By selective breeding we can
produce structural divergences as great as those of species, but we
cannot produce equal physiological divergences. For the present I leave
the question there.
Now, the next problem that lies before us--and it is an extremely
important one--is this: Does this selective breeding occur in nature?
Because, if there is no proof of it, all that I have been telling you
goes for nothing in accounting for the origin of species. Are natural
causes competent to play the part of selection in perpetuating
varieties? Here we labour under very great difficulties. In the last
lecture I had occasion to point out to you the extreme difficulty of
obtaining evidence even of the first origin of those varieties which we
know to have occurred in domesticated animals. I told you, that almost
always the origin of these varieties is overlooked, so that I could
only produce two of three cases, as that of Gratio Kelleia and of the
Ancon sheep. People forget, or do not take notice of them until they
come to have a prominence; and if that is true of artificial cases,
under our own eyes, and in animals in our own care, how much more
difficult it must be to have at first hand good evidence of the origin
of varieties in nature! Indeed, I do not know that it is possible by
direct evidence to prove the origin of a variety in nature, or to prove
selective breeding; but I will tell you what we can prove--and this
comes to the same thing--that varieties exist in nature within the
limits of species, and, what is more, that when a variety has come into
existence in nature, there are natural causes and conditions, which are
amply competent to play the part of a selective breeder; and although
that is not quite the evidence that one would like to have--though it
is not direct testimony--yet it is exceeding good and exceedingly
powerful evidence in its way.
As to the first point, of varieties existing among natural species, I
might appeal to the universal experience of every naturalist, and of
any person who has ever turned any attention at all to the
characteristics of plants and animals in a state of nature; but I may as
well take a few definite cases, and I will begin with Man himself.
I am one of those who believe that, at present, there is no evidence
whatever for saying, that mankind sprang originally from any more than
a single pair; I must say, that I cannot see any good ground whatever,
or even any tenable sort of evidence, for believing that there is more
than one species of Man. Nevertheless, as you know, just as there are
numbers of varieties in animals, so there are remarkable varieties of
men. I speak not merely of those broad and distinct variations which
you see at a glance. Everybody, of course, knows the difference
between a Negro and a white man, and can tell a Chinaman from an
Englishman. They each have peculiar characteristics of colour and
physiognomy; but you must recollect that the characters of these races
go very far deeper--they extend to the bony structure, and to the
characters of that most important of all organs to us--the brain; so
that, among men belonging to different races, or even within the same
race, one man shall have a brain a third, or half, or even seventy per
cent. bigger than another; and if you take the whole range of human
brains, you will find a variation in some cases of a hundred per cent.
Apart from these variations in the size of the brain, the characters of
the skull vary. Thus if I draw the figures of a Mongul and of a Negro
head on the blackboard, in the case of the last the breadth would be
about seven-tenths, and in the other it would be nine-tenths of the
total length. So that you see there is abundant evidence of variation
among men in their natural condition. And if you turn to other animals
there is just the same thing. The fox, for example, which has a very
large geographical distribution all over Europe, and parts of Asia, and
on the American Continent, varies greatly. There are mostly large
foxes in the North, and smaller ones in the South. In Germany alone,
the foresters reckon some eight different sorts.
Of the tiger, no one supposes that there is more than one species; they
extend from the hottest parts of Bengal, into the dry, cold, bitter
steppes of Siberia, into a latitude of 50 degrees,--so that they may
even prey upon the reindeer. These tigers have exceedingly different
characteristics, but still they all keep their general features, so that
there is no doubt as to their being tigers. The Siberian tiger has a
thick fur, a small mane, and a longitudinal stripe down the back, while
the tigers of Java and Sumatra differ in many important respects from
the tigers of Northern Asia. So lions vary; so birds vary; and so, if
you go further back and lower down in creation, you find that fishes
vary. In different streams, in the same country even, you will find
the trout to be quite different to each other and easily recognisable by
those who fish in the particular streams. There is the same
differences in leeches; leech collectors can easily point out to you
the differences and the peculiarities which you yourself would probably
pass by; so with fresh-water mussels; so, in fact, with every animal
you can mention.
In plants there is the same kind of variation. Take such a case even as
the common bramble. The botanists are all at war about it; some of them
wanting to make out that there are many species of it, and others
maintaining that they are but many varieties of one species; and they
cannot settle to this day which is a species and which is a variety!
So that there can be no doubt whatsoever that any plant and any animal
may vary in nature; that varieties may arise in the way I have
described,--as spontaneous varieties,--and that those varieties may be
perpetuated in the same way that I have shown you spontaneous varieties
are perpetuated; I say, therefore, that there can be no doubt as to the
origin and perpetuation of varieties in nature.
But the question now is:--Does selection take place in nature? is there
anything like the operation of man in exercising selective breeding,
taking place in nature? You will observe that, at present, I say
nothing about species; I wish to confine myself to the consideration of
the production of those natural races which everybody admits to exist.
The question is, whether in nature there are causes competent to
produce races, just in the same way as man is able to produce by
selection, such races of animals as we have already noticed.
When a variety has arisen, the CONDITIONS OF EXISTENCE are such as to
exercise an influence which is exactly comparable to that of artificial
selection. By Conditions of Existence I mean two things,--there are
conditions which are furnished by the physical, the inorganic world,
and there are conditions of existence which are furnished by the
organic world. There is, in the first place, CLIMATE; under that head
I include only temperature and the varied amount of moisture of
particular places. In the next place there is what is technically
called STATION, which means--given the climate, the particular kind of
place in which an animal or a plant lives or grows; for example, the
station of a fish is in the water, of a fresh-water fish in fresh
water; the station of a marine fish is in the sea, and a marine animal
may have a station higher or deeper. So again with land animals: the
differences in their stations are those of different soils and
neighbourhoods; some being best adapted to a calcareous, and others to
an arenaceous soil. The third condition of existence is FOOD, by which
I mean food in the broadest sense, the supply of the materials necessary
to the existence of an organic being; in the case of a plant the
inorganic matters, such as carbonic acid, water, ammonia, and the
earthy salts or salines; in the case of the animal the inorganic and
organic matters, which we have seen they require; then these are all,
at least the two first, what we may call the inorganic or physical
conditions of existence. Food takes a mid-place, and then come the
organic conditions; by which I mean the conditions which depend upon the
state of the rest of the organic creation, upon the number and kind of
living beings, with which an animal is surrounded. You may class these
under two heads: there are organic beings, which operate as
'opponents', and there are organic beings which operate as 'helpers' to
any given organic creature. The opponents may be of two kinds: there
are the 'indirect opponents', which are what we may call 'rivals'; and
there are the 'direct opponents', those which strive to destroy the
creature; and these we call 'enemies'. By rivals I mean, of course, in
the case of plants, those which require for their support the same kind
of soil and station, and, among animals, those which require the same
kind of station, or food, or climate; those are the indirect opponents;
the direct opponents are, of course, those which prey upon an animal or
vegetable. The 'helpers' may also be regarded as direct and indirect:
in the case of a carnivorous animal, for example, a particular
herbaceous plant may in multiplying be an indirect helper, by enabling
the herbivora on which the carnivore preys to get more food, and thus
to nourish the carnivore more abundantly; the direct helper may be best
illustrated by reference to some parasitic creature, such as the
tape-worm. The tape-worm exists in the human intestines, so that the
fewer there are of men the fewer there will be of tape-worms, other
things being alike. It is a humiliating reflection, perhaps, that we
may be classed as direct helpers to the tape-worm, but the fact is so:
we can all see that if there were no men there would be no tape-worms.