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Darwiniana

T >> Thomas Henry Huxley >> Darwiniana




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 Mongol 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°,--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
first two, 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.

It is extremely difficult to estimate, in a proper way, the importance and
the working of the Conditions of Existence. I do not think there were any
of us who had the remotest notion of properly estimating them until the
publication of Mr. Darwin's work, which has placed them before us with
remarkable clearness; and I must endeavour, as far as I can in my own
fashion, to give you some notion of how they work. We shall find it easiest
to take a simple case, and one as free as possible from every kind of
complication.

I will suppose, therefore, that all the habitable part of this globe--the
dry land, amounting to about 51,000,000 square miles--I will suppose that
the whole of that dry land has the same climate, and that it is composed of
the same kind of rock or soil, so that there will be the same station
everywhere; we thus get rid of the peculiar influence of different climates
and stations. I will then imagine that there shall be but one organic being
in the world, and that shall be a plant. In this we start fair. Its food is
to be carbonic acid, water and ammonia, and the saline matters in the soil,
which are, by the supposition, everywhere alike. We take one single plant,
with no opponents, no helpers, and no rivals; it is to be a "fair field,
and no favour." Now, I will ask you to imagine further that it shall be a
plant which shall produce every year fifty seeds, which is a very moderate
number for a plant to produce; and that, by the action of the winds and
currents, these seeds shall be equally and gradually distributed over the
whole surface of the land. I want you now to trace out what will occur, and
you will observe that I am not talking fallaciously any more than a
mathematician does when he expounds his problem. If you show that the
conditions of your problem are such as may actually occur in Nature and do
not transgress any of the known laws of Nature in working out your
proposition, then you are as safe in the conclusion you arrive at as is the
mathematician in arriving at the solution of his problem. In science, the
only way of getting rid of the complications with which a subject of this
kind is environed, is to work in this deductive method. What will be the
result, then? I will suppose that every plant requires one square foot of
ground to live upon; and the result will be that, in the course of nine
years, the plant will have occupied every single available spot in the
whole globe! I have chalked upon the blackboard the figures by which I
arrive at the result:--

Plants. Plants.


1 x 50 in 1st year = 50
50 x 50 " 2nd " = 2,500
2,500 x 50 " 3rd " = 125,000
125,000 x 50 " 4th " = 6,250,000
6,250,000 x 50 " 5th " = 312,500,000
312,500,000 x 50 " 6th " = 15,625,000,000
15,625,000,000 x 50 " 7th " = 781,250,000,000
781,250,000,000 x 50 " 8th " = 39,062,500,000,000
39,062,500,000,000 x 50 " 9th " = 1,953,125,000,000,000

51,000,000 square miles--the )
dry surface of the earth x )
27,878,400--the number of ) = sq. ft. 1,421,798,400,000,000
sq. ft. in 1 sq. mile ) ---------------------
being 531,326,600,000,000
square feet less than would be required at the end of the ninth
year.

You will see from this that, at the end of the first year the single plant
will have produced fifty more of its kind; by the end of the second year
these will have increased to 2,500; and so on, in succeeding years, you get
beyond even trillions; and I am not at all sure that I could tell you what
the proper arithmetical denomination of the total number really is; but, at
any rate, you will understand the meaning of all those noughts. Then you
see that, at the bottom, I have taken the 51,000,000 of square miles,
constituting the surface of the dry land; and as the number of square feet
are placed under and subtracted from the number of seeds that would be
produced in the ninth year, you can see at once that there would be an
immense number more of plants than there would be square feet of ground for
their accommodation. This is certainly quite enough to prove my point; that
between the eighth and ninth year after being planted the single plant
would have stocked the whole available surface of the earth.

This is a thing which is hardly conceivable--it seems hardly
imaginable--yet it is so. It is indeed simply the law of Malthus
exemplified. Mr. Malthus was a clergyman, who worked out this subject most
minutely and truthfully some years ago; he showed quite clearly--and
although he was much abused for his conclusions at the time, they have
never yet been disproved and never will be--he showed that in consequence
of the increase in the number of organic beings in a geometrical ratio,
while the means of existence cannot be made to increase in the same ratio,
that there must come a time when the number of organic beings will be in
excess of the power of production of nutriment, and that thus some check
must arise to the further increase of those organic beings. At the end of
the ninth year we have seen that each plant would not be able to get its
full square foot of ground, and at the end of another year it would have to
share that space with fifty others the produce of the seeds which it would
give off.

What, then, takes place? Every plant grows up, flourishes, occupies its
square foot of ground, and gives off its fifty seeds; but notice this, that
out of this number only one can come to anything; there is thus, as it
were, forty-nine chances to one against its growing up; it depends upon the
most fortuitous circumstances whether any one of these fifty seeds shall
grow up and flourish, or whether it shall die and perish. This is what Mr.
Darwin has drawn attention to, and called the "STRUGGLE FOR EXISTENCE"; and
I have taken this simple case of a plant because some people imagine that
the phrase seems to imply a sort of fight.

I have taken this plant and shown you that this is the result of the ratio
of the increase, the necessary result of the arrival of a time coming for
every species when exactly as many members must be destroyed as are born;
that is the inevitable ultimate result of the rate of production. Now, what
is the result of all this? I have said that there are forty-nine struggling
against every one; and it amounts to this, that the smallest possible start
given to any one seed may give it an advantage which will enable it to get
ahead of all the others; anything that will enable any one of these seeds
to germinate six hours before any of the others will, other things being
alike, enable it to choke them out altogether. I have shown you that there
is no particular in which plants will not vary from each other; it is quite
possible that one of our imaginary plants may vary in such a character as
the thickness of the integument of its seeds; it might happen that one of
the plants might produce seeds having a thinner integument, and that would
enable the seeds of that plant to germinate a little quicker than those of
any of the others, and those seeds would most inevitably extinguish the
forty-nine times as many that were struggling with them.

I have put it in this way, but you see the practical result of the process
is the same as if some person had nurtured the one and destroyed the other
seeds. It does not matter how the variation is produced, so long as it is
once allowed to occur. The variation in the plant once fairly started tends
to become hereditary and reproduce itself; the seeds would spread
themselves in the same way and take part in the struggle with the
forty-nine hundred, or forty-nine thousand, with which they might be
exposed. Thus, by degrees, this variety with some slight organic change or
modification, must spread itself over the whole surface of the habitable
globe, and extirpate or replace the other kinds. That is what is meant by
NATURAL SELECTION; that is the kind of argument by which it is perfectly
demonstrable that the conditions of existence may play exactly the same
part for natural varieties as man does for domesticated varieties. No one
doubts at all that particular circumstances may be more favourable for one
plant and less so for another, and the moment you admit that, you admit the
selective power of nature. Now, although I have been putting a hypothetical
case, you must not suppose that I have been reasoning hypothetically. There
are plenty of direct experiments which bear out what we may call the theory
of natural selection; there is extremely good authority for the statement
that if you take the seed of mixed varieties of wheat and sow it,
collecting the seed next year and sowing it again, at length you will find
that out of all your varieties only two or three have lived, or perhaps
even only one. There were one or two varieties which were best fitted to
get on, and they have killed out the other kinds in just the same way and
with just the same certainty as if you had taken the trouble to remove
them. As I have already said, the operation of nature is exactly the same
as the artificial operation of man.

But if this be true of that simple case, which I put before you, where
there is nothing but the rivalry of one member of a species with others,
what must be the operation of selective conditions, when you recollect as a
matter of fact, that for every species of animal or plant there are fifty
or a hundred species which might all, more or less, be comprehended in the
same climate, food, and station;--that every plant has multitudinous
animals which prey upon it, and which are its direct opponents; and that
these have other animals preying upon them,--that every plant has its
indirect helpers in the birds that scatter abroad its seed, and the animals
that manure it with their dung;--I say, when these things are considered,
it seems impossible that any variation which may arise in a species in
nature should not tend in some way or other either to be a little better or
worse than the previous stock; if it is a little better it will have an
advantage over and tend to extirpate the latter in this crush and struggle;
and if it is a little worse it will itself be extirpated.

I know nothing that more appropriately expresses this, than the phrase,
"the struggle for existence "; because it brings before your minds, in a
vivid sort of way, some of the simplest possible circumstances connected
with it. When a struggle is intense there must be some who are sure to be
trodden down, crushed, and overpowered by others; and there will be some
who just manage to get through only by the help of the slightest accident.
I recollect reading an account of the famous retreat of the French troops,
under Napoleon, from Moscow. Worn out, tired, and dejected, they at length
came to a great river over which there was but one bridge for the passage
of the vast army. Disorganised and demoralised as that army was, the
struggle must certainly have been a terrible one--every one heeding only
himself, and crushing through the ranks and treading down his fellows. The
writer of the narrative, who was himself one of those who were fortunate
enough to succeed in getting over, and not among the thousands who were
left behind or forced into the river, ascribed his escape to the fact that
he saw striding onward through the mass a great strong fellow,--one of the
French Cuirassiers, who had on a large blue cloak-and he had enough
presence of mind to catch and retain a hold of this strong man's cloak. He
says, "I caught hold of his cloak, and although he swore at me and cut at
and struck me by turns, and at last, when he found he could not shake me
off, fell to entreating me to leave go or I should prevent him from
escaping, besides not assisting myself, I still kept tight hold of him, and
would not quit my grasp until he had at last dragged me through." Here you
see was a case of selective saving--if we may so term it--depending for its
success on the strength of the cloth of the Cuirassier's cloak. It is the
same in nature; every species has its bridge of Beresina; it has to fight
its way through and struggle with other species; and when well-nigh
overpowered, it may be that the smallest chance, something in its colour,
perhaps--the minutest circumstance--will turn the scale one way or the
other.

Suppose that by a variation of the black race it had produced the white man
at any time--you know that the Negroes are said to believe this to have
been the case, and to imagine that Cain was the first white man, and that
we are his descendants--suppose that this had ever happened, and that the
first residence of this human being was on the West Coast of Africa. There
is no great structural difference between the white man and the Negro, and
yet there is something so singularly different in the constitution of the
two, that the malarias of that country, which do not hurt the black at all,
cut off and destroy the white. Then you see there would have been a
selective operation performed; if the white man had risen in that way, he
would have been selected out and removed by means of the malaria. Now there
really is a very curious case of selection of this sort among pigs, and it
is a case of selection of colour too. In the woods of Florida there are a
great many pigs, and it is a very curious thing that they are all black,
every one of them. Professor Wyman was there some years ago, and on
noticing no pigs but these black ones, he asked some of the people how it
was that they had no white pigs, and the reply was that in the woods of
Florida there was a root which they called the Paint Root, and that if the
white pigs were to eat any of it, it had the effect of making their hoofs
crack, and they died, but if the black pigs ate any of it, it did not hurt
them at all. Here was a very simple case of natural selection. A skilful
breeder could not more carefully develop the black breed of pigs, and weed
out all the white pigs, than the Paint Root does.

To show you how remarkably indirect may be such natural selective agencies
as I have referred to, I will conclude by noticing a case mentioned by Mr.
Darwin, and which is certainly one of the most curious of its kind. It is
that of the Humble Bee. It has been noticed that there are a great many
more humble bees in the neighbourhood of towns, than out in the open
country; and the explanation of the matter is this: the humble bees build
nests, in which they store their honey and deposit the larvæ and eggs. The
field mice are amazingly fond of the honey and larvæ; therefore, wherever
there are plenty of field mice, as in the country, the humble bees are kept
down; but in the neighbourhood of towns, the number of cats which prowl
about the fields eat up the field mice, and of course the more mice they
eat up the less there are to prey upon the larvæ of the bees--the cats are
therefore the INDIRECT HELPERS of the bees. [Footnote: The humble bees, on
the other hand, are direct helpers of some plants, such as the heartsease
and red clover, which are fertilised by the visits of the bees; and they
are indirect helpers of the numerous insects which are more or less
completely supported by the heartsease and red clover.] Coming back a step
farther we may say that the old maids are also indirect friends of the
humble bees, and indirect enemies of the field mice, as they keep the cats
which eat up the latter! This is an illustration somewhat beneath the
dignity of the subject, perhaps, but it occurs to me in passing, and with
it I will conclude this lecture.



VI. A CRITICAL EXAMINATION OF THE POSITION OF MR. DARWIN'S WORK, "ON THE
ORIGIN OF SPECIES," IN RELATION TO THE COMPLETE THEORY OF THE CAUSES OF THE
PHENOMENA OF ORGANIC NATURE


In the preceding five lectures I have endeavoured to give you an account of
those facts, and of those reasonings from facts, which form the data upon
which all theories regarding the causes of the phenomena of organic nature
must be based. And, although I have had frequent occasion to quote Mr.
Darwin--as all persons hereafter, in speaking upon these subjects, will
have occasion to quote his famous book on the "Origin of Species,"--you
must yet remember that, wherever I have quoted him, it has not been upon
theoretical points, or for statements in any way connected with his
particular speculations, but on matters of fact, brought forward by
himself, or collected by himself, and which appear incidentally in his
book. If a man _will_ make a book, professing to discuss a single
question, an encyclopædia, I cannot help it.

Now, having had an opportunity of considering in this sort of way the
different statements bearing upon all theories whatsoever, I have to lay
before you, as fairly as I can, what is Mr. Darwin's view of the matter and
what position his theories hold, when judged by the principles which I have
previously laid down, as deciding our judgments upon all theories and
hypotheses.

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