THE CONDITIONS OF EXISTENCE AS AFFECTING
THE PERPETUATION OF LIVING
BEINGS.
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 vertebræ of
the back, as in the sacral vertebræ 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.[53]
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 habits
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 or 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°,--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 recognizable 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 × 50 in 1st year = 50
50 × 50 " 2nd " = 2,500
2,500 × 50 " 3rd " = 125,000
125,000 × 50 " 4th " = 6,250,000
6,250,000 × 50 " 5th " = 312,500,000
312,500,000 × 50 " 6th " = 15,625,000,000
15,625,000,000 × 50 " 7th " = 781,250,000,000
781,250,000,000 × 50 " 8th " = 39,062,500,000,000
39,062,500,000,000 × 50 " 9th " = 1,953,125,000,000,000
51,000,000 sq. miles--the dry surface}
of the earth × 27,878,400--the } = sq. ft. 1,421,798,400,000,000
number of 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 2500; 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 clergy-man, 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. Disorganized and
demoralized 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.[54]
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.