The Conditions of Existence
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Thomas H. Huxley >> The Conditions of Existence
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 sq. miles--the dry surface of the earth x 27,878,400--the
number of sq. ft. in 1 sq. mile = sq. ft. 1,421,798,400,000,000 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 eat 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 develope 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 larvae and eggs. The field mice are amazingly fond of the
honey and larvae; 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 larvae of the bees--the cats are therefore the INDIRECT
HELPERS of the bees!* 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.
[footnote] *The humble bees, on the other hand, are direct helpers of
some plants, such as the heartsease and red clover, which are
fertilized 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.