Infomotions, Inc.Conditions of Existence as Affecting the Perpetuation of Living Beings / Huxley, Thomas Henry, 1825-1895



Author: Huxley, Thomas Henry, 1825-1895
Title: Conditions of Existence as Affecting the Perpetuation of Living Beings
Publisher: Project Gutenberg
Tag(s): species; physiological; variation; varieties; plant; seeds; breeding
Contributor(s): Cunningham, Peter, 1816-1869 [Editor]
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Title:  The Conditions of Existence

Author:  Thomas H. Huxley

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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.

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.





End of The Project Gutenberg Etext of The Conditions of Existence
by Thomas H. Huxley


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