STRUGGLE FOR EXISTENCE.
Its bearing on natural selection—The term used in a wide
sense—Geometrical ratio of increase—Rapid increase of naturalised
animals and plants—Nature of the checks to increase—Competition
universal—Effects of climate—Protection from the number of
individuals—Complex relations of all animals and plants throughout
nature—Struggle for life most severe between individuals and varieties
of the same species: often severe between species of the same genus—The
relation of organism to organism the most important of all relations.
Before entering on the subject of this chapter I must make a few
preliminary remarks to show how the struggle for existence bears on
natural selection. It has been seen in the last chapter that among
organic beings in a state of nature there is some individual
variability: indeed I am not aware that this has ever been disputed. It
is immaterial for us whether a multitude of doubtful forms be called
species or sub-species or varieties; what rank, for instance, the two
or three hundred doubtful forms of British plants are entitled to hold,
if the existence of any well-marked varieties be admitted. But the mere
existence of individual variability and of some few well-marked
varieties, though necessary as the foundation for the work, helps us
but little in understanding how species arise in nature. How have all
those exquisite adaptations of one part of the organisation to another
part, and to the conditions of life and of one organic being to another
being, been perfected? We see these beautiful co-adaptations most
plainly in the woodpecker and the mistletoe; and only a little less
plainly in the humblest parasite which clings to the hairs of a
quadruped or feathers of a bird; in the structure of the beetle which
dives through the water; in the plumed seed which is wafted by the
gentlest breeze; in short, we see beautiful adaptations everywhere and
in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow from the struggle for life. Owing to this struggle,
variations, however slight and from whatever cause proceeding, if they
be in any degree profitable to the individuals of a species, in their
infinitely complex relations to other organic beings and to their
physical conditions of life, will tend to the preservation of such
individuals, and will generally be inherited by the offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term natural
selection, in order to mark its relation to man’s power of selection.
But the expression often used by Mr. Herbert Spencer, of the Survival
of the Fittest, is more accurate, and is sometimes equally convenient.
We have seen that man by selection can certainly produce great results,
and can adapt organic beings to his own uses, through the accumulation
of slight but useful variations, given to him by the hand of Nature.
But Natural Selection, we shall hereafter see, is a power incessantly
ready for action, and is as immeasurably superior to man’s feeble
efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for existence.
In my future work this subject will be treated, as it well deserves, at
greater length. The elder De Candolle and Lyell have largely and
philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently
the result of his great horticultural knowledge. Nothing is easier than
to admit in words the truth of the universal struggle for life, or more
difficult—at least I found it so—than constantly to bear this
conclusion in mind. Yet unless it be thoroughly engrained in the mind,
the whole economy of nature, with every fact on distribution, rarity,
abundance, extinction, and variation, will be dimly seen or quite
misunderstood. We behold the face of nature bright with gladness, we
often see superabundance of food; we do not see or we forget that the
birds which are idly singing round us mostly live on insects or seeds,
and are thus constantly destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind, that, though food
may be now superabundant, it is not so at all seasons of each recurring
year.
_The Term, Struggle for Existence, used in a large sense._
I should premise that I use this term in a large and metaphorical
sense, including dependence of one being on another, and including
(which is more important) not only the life of the individual, but
success in leaving progeny. Two canine animals, in a time of dearth,
may be truly said to struggle with each other which shall get food and
live. But a plant on the edge of a desert is said to struggle for life
against the drought, though more properly it should be said to be
dependent on the moisture. A plant which annually produces a thousand
seeds, of which only one of an average comes to maturity, may be more
truly said to struggle with the plants of the same and other kinds
which already clothe the ground. The mistletoe is dependent on the
apple and a few other trees, but can only in a far-fetched sense be
said to struggle with these trees, for, if too many of these parasites
grow on the same tree, it languishes and dies. But several seedling
mistletoes, growing close together on the same branch, may more truly
be said to struggle with each other. As the mistletoe is disseminated
by birds, its existence depends on them; and it may metaphorically be
said to struggle with other fruit-bearing plants, in tempting the birds
to devour and thus disseminate its seeds. In these several senses,
which pass into each other, I use for convenience sake the general term
of Struggle for Existence.
_Geometrical Ratio of Increase._
A struggle for existence inevitably follows from the high rate at which
all organic beings tend to increase. Every being, which during its
natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season or
occasional year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no country
could support the product. Hence, as more individuals are produced than
can possibly survive, there must in every case be a struggle for
existence, either one individual with another of the same species, or
with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with manifold
force to the whole animal and vegetable kingdoms; for in this case
there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be now increasing,
more or less rapidly, in numbers, all cannot do so, for the world would
not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding man
has doubled in twenty-five years, and at this rate, in less than a
thousand years, there would literally not be standing room for his
progeny. Linnæus has calculated that if an annual plant produced only
two seeds—and there is no plant so unproductive as this—and their
seedlings next year produced two, and so on, then in twenty years there
would be a million plants. The elephant is reckoned the slowest breeder
of all known animals, and I have taken some pains to estimate its
probable minimum rate of natural increase; it will be safest to assume
that it begins breeding when thirty years old, and goes on breeding
till ninety years old, bringing forth six young in the interval, and
surviving till one hundred years old; if this be so, after a period of
from 740 to 750 years there would be nearly nineteen million elephants
alive descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several parts of
the world; if the statements of the rate of increase of slow-breeding
cattle and horses in South America, and latterly in Australia, had not
been well authenticated, they would have been incredible. So it is with
plants; cases could be given of introduced plants which have become
common throughout whole islands in a period of less than ten years.
Several of the plants, such as the cardoon and a tall thistle, which
are now the commonest over the wide plains of La Plata, clothing square
leagues of surface almost to the exclusion of every other plant, have
been introduced from Europe; and there are plants which now range in
India, as I hear from Dr. Falconer, from Cape Comorin to the Himalaya,
which have been imported from America since its discovery. In such
cases, and endless others could be given, no one supposes that the
fertility of the animals or plants has been suddenly and temporarily
increased in any sensible degree. The obvious explanation is that the
conditions of life have been highly favourable, and that there has
consequently been less destruction of the old and young and that nearly
all the young have been enabled to breed. Their geometrical ratio of
increase, the result of which never fails to be surprising, simply
explains their extraordinarily rapid increase and wide diffusion in
their new homes.
In a state of nature almost every full-grown plant annually produces
seed, and among animals there are very few which do not annually pair.
Hence we may confidently assert that all plants and animals are tending
to increase at a geometrical ratio—that all would rapidly stock every
station in which they could any how exist, and that this geometrical
tendency to increase must be checked by destruction at some period of
life. Our familiarity with the larger domestic animals tends, I think,
to mislead us; we see no great destruction falling on them, and we do
not keep in mind that thousands are annually slaughtered for food, and
that in a state of nature an equal number would have somehow to be
disposed of.
The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is, that
the slow breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two. The Fulmar
petrel lays but one egg, yet it is believed to be the most numerous
bird in the world. One fly deposits hundreds of eggs, and another, like
the hippobosca, a single one. But this difference does not determine
how many individuals of the two species can be supported in a district.
A large number of eggs is of some importance to those species which
depend on a fluctuating amount of food, for it allows them rapidly to
increase in number. But the real importance of a large number of eggs
or seeds is to make up for much destruction at some period of life; and
this period in the great majority of cases is an early one. If an
animal can in any way protect its own eggs or young, a small number may
be produced, and yet the average stock be fully kept up; but if many
eggs or young are destroyed, many must be produced or the species will
become extinct. It would suffice to keep up the full number of a tree,
which lived on an average for a thousand years, if a single seed were
produced once in a thousand years, supposing that this seed were never
destroyed and could be ensured to germinate in a fitting place; so
that, in all cases, the average number of any animal or plant depends
only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind—never to forget that every single organic
being may be said to be striving to the utmost to increase in numbers;
that each lives by a struggle at some period of its life; that heavy
destruction inevitably falls either on the young or old during each
generation or at recurrent intervals. Lighten any check, mitigate the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount.
_Nature of the Checks to Increase._
The causes which check the natural tendency of each species to increase
are most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will it tend to increase still further.
We know not exactly what the checks are even in a single instance. Nor
will this surprise any one who reflects how ignorant we are on this
head, even in regard to mankind, although so incomparably better known
than any other animal. This subject of the checks to increase has been
ably treated by several authors, and I hope in a future work to discuss
it at considerable length, more especially in regard to the feral
animals of South America. Here I will make only a few remarks, just to
recall to the reader’s mind some of the chief points. Eggs or very
young animals seem generally to suffer most, but this is not invariably
the case. With plants there is a vast destruction of seeds, but from
some observations which I have made it appears that the seedlings
suffer most from germinating in ground already thickly stocked with
other plants. Seedlings, also, are destroyed in vast numbers by various
enemies; for instance, on a piece of ground three feet long and two
wide, dug and cleared, and where there could be no choking from other
plants, I marked all the seedlings of our native weeds as they came up,
and out of 357 no less than 295 were destroyed, chiefly by slugs and
insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more
vigorous plants gradually kill the less vigorous, though fully grown
plants; thus out of twenty species grown on a little plot of mown turf
(three feet by four) nine species perished, from the other species
being allowed to grow up freely.
The amount of food for each species, of course, gives the extreme limit
to which each can increase; but very frequently it is not the obtaining
food, but the serving as prey to other animals, which determines the
average number of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of game were shot
during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game
than at present, although hundreds of thousands of game animals are now
annually shot. On the other hand, in some cases, as with the elephant,
none are destroyed by beasts of prey; for even the tiger in India most
rarely dares to attack a young elephant protected by its dam.
Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought seem to be
the most effective of all checks. I estimated (chiefly from the greatly
reduced numbers of nests in the spring) that the winter of 1854-5
destroyed four-fifths of the birds in my own grounds; and this is a
tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle
for existence; but in so far as climate chiefly acts in reducing food,
it brings on the most severe struggle between the individuals, whether
of the same or of distinct species, which subsist on the same kind of
food. Even when climate, for instance, extreme cold, acts directly, it
will be the least vigorous individuals, or those which have got least
food through the advancing winter, which will suffer the most. When we
travel from south to north, or from a damp region to a dry, we
invariably see some species gradually getting rarer and rarer, and
finally disappearing; and the change of climate being conspicuous, we
are tempted to attribute the whole effect to its direct action. But
this is a false view; we forget that each species, even where it most
abounds, is constantly suffering enormous destruction at some period of
its life, from enemies or from competitors for the same place and food;
and if these enemies or competitors be in the least degree favoured by
any slight change of climate, they will increase in numbers; and as
each area is already fully stocked with inhabitants, the other species
must decrease. When we travel southward and see a species decreasing in
numbers, we may feel sure that the cause lies quite as much in other
species being favoured, as in this one being hurt. So it is when we
travel northward, but in a somewhat lesser degree, for the number of
species of all kinds, and therefore of competitors, decreases
northward; hence in going northward, or in ascending a mountain, we far
oftener meet with stunted forms, due to the _directly_ injurious action
of climate, than we do in proceeding southward or in descending a
mountain. When we reach the Arctic regions, or snow-capped summits, or
absolute deserts, the struggle for life is almost exclusively with the
elements.
That climate acts in main part indirectly by favouring other species we
clearly see in the prodigious number of plants which in our gardens can
perfectly well endure our climate, but which never become naturalised,
for they cannot compete with our native plants nor resist destruction
by our native animals.
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics—at least, this
seems generally to occur with our game animals—often ensue; and here we
have a limiting check independent of the struggle for life. But even
some of these so-called epidemics appear to be due to parasitic worms,
which have from some cause, possibly in part through facility of
diffusion among the crowded animals, been disproportionally favoured:
and here comes in a sort of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of corn
and rape-seed, &c., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can
the birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their
numbers are checked during the winter; but any one who has tried knows
how troublesome it is to get seed from a few wheat or other such plants
in a garden; I have in this case lost every single seed. This view of
the necessity of a large stock of the same species for its
preservation, explains, I believe, some singular facts in nature such
as that of very rare plants being sometimes extremely abundant, in the
few spots where they do exist; and that of some social plants being
social, that is abounding in individuals, even on the extreme verge of
their range. For in such cases, we may believe, that a plant could
exist only where the conditions of its life were so favourable that
many could exist together, and thus save the species from utter
destruction. I should add that the good effects of intercrossing, and
the ill effects of close interbreeding, no doubt come into play in many
of these cases; but I will not here enlarge on this subject.
_Complex Relations of all Animals and Plants to each other in the
Struggle for Existence._
Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance,
which, though a simple one, interested me. In Staffordshire, on the
estate of a relation, where I had ample means of investigation, there
was a large and extremely barren heath, which had never been touched by
the hand of man; but several hundred acres of exactly the same nature
had been enclosed twenty-five years previously and planted with Scotch
fir. The change in the native vegetation of the planted part of the
heath was most remarkable, more than is generally seen in passing from
one quite different soil to another: not only the proportional numbers
of the heath-plants were wholly changed, but twelve species of plants
(not counting grasses and carices) flourished in the plantations, which
could not be found on the heath. The effect on the insects must have
been still greater, for six insectivorous birds were very common in the
plantations, which were not to be seen on the heath; and the heath was
frequented by two or three distinct insectivorous birds. Here we see
how potent has been the effect of the introduction of a single tree,
nothing whatever else having been done, with the exception of the land
having been enclosed, so that cattle could not enter. But how important
an element enclosure is, I plainly saw near Farnham, in Surrey. Here
there are extensive heaths, with a few clumps of old Scotch firs on the
distant hill-tops: within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in multitudes, so
close together that all cannot live. When I ascertained that these
young trees had not been sown or planted I was so much surprised at
their numbers that I went to several points of view, whence I could
examine hundreds of acres of the unenclosed heath, and literally I
could not see a single Scotch fir, except the old planted clumps. But
on looking closely between the stems of the heath, I found a multitude
of seedlings and little trees, which had been perpetually browsed down
by the cattle. In one square yard, at a point some hundred yards
distant from one of the old clumps, I counted thirty-two little trees;
and one of them, with twenty-six rings of growth, had, during many
years tried to raise its head above the stems of the heath, and had
failed. No wonder that, as soon as the land was enclosed, it became
thickly clothed with vigorously growing young firs. Yet the heath was
so extremely barren and so extensive that no one would ever have
imagined that cattle would have so closely and effectually searched it
for food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous as
they are, must be habitually checked by some means, probably by other
parasitic insects. Hence, if certain insectivorous birds were to
decrease in Paraguay, the parasitic insects would probably increase;
and this would lessen the number of the navel-frequenting flies—then
cattle and horses would become feral, and this would certainly greatly
alter (as indeed I have observed in parts of South America) the
vegetation: this again would largely affect the insects; and this, as
we have just seen in Staffordshire, the insectivorous birds, and so
onwards in ever-increasing circles of complexity. Not that under nature
the relations will ever be as simple as this. Battle within battle must
be continually recurring with varying success; and yet in the long-run
the forces are so nicely balanced that the face of nature remains for
long periods of time uniform, though assuredly the merest trifle would
give the victory to one organic being over another. Nevertheless, so
profound is our ignorance, and so high our presumption, that we marvel
when we hear of the extinction of an organic being; and as we do not
see the cause, we invoke cataclysms to desolate the world, or invent
laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals,
remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic
Lobelia fulgens is never visited in my garden by insects, and
consequently, from its peculiar structure, never sets a seed. Nearly
all our orchidaceous plants absolutely require the visits of insects
to remove their pollen-masses and thus to fertilise them. I find from
experiments that humble-bees are almost indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do
not visit this flower. I have also found that the visits of bees are
necessary for the fertilisation of some kinds of clover; for instance
twenty heads of Dutch clover (Trifolium repens) yielded 2,290 seeds,
but twenty other heads, protected from bees, produced not one. Again,
100 heads of red clover (T. pratense) produced 2,700 seeds, but the
same number of protected heads produced not a single seed. Humble bees
alone visit red clover, as other bees cannot reach the nectar. It has
been suggested that moths may fertilise the clovers; but I doubt
whether they could do so in the case of the red clover, from their
weight not being sufficient to depress the wing petals. Hence we may
infer as highly probable that, if the whole genus of humble-bees
became extinct or very rare in England, the heartsease and red clover
would become very rare, or wholly disappear. The number of humble-bees
in any district depends in a great measure upon the number of
field-mice, which destroy their combs and nests; and Colonel Newman,
who has long attended to the habits of humble-bees, believes that
“more than two-thirds of them are thus destroyed all over England.”
Now the number of mice is largely dependent, as every one knows, on
the number of cats; and Colonel Newman says, “Near villages and small
towns I have found the nests of humble-bees more numerous than
elsewhere, which I attribute to the number of cats that destroy the
mice.” Hence it is quite credible that the presence of a feline animal
in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain
flowers in that district!
In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally the
most potent, but all will concur in determining the average number, or
even the existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what
we call chance. But how false a view is this! Every one has heard that
when an American forest is cut down, a very different vegetation
springs up; but it has been observed that ancient Indian ruins in the
Southern United States, which must formerly have been cleared of trees,
now display the same beautiful diversity and proportion of kinds as in
the surrounding virgin forests. What a struggle must have gone on
during long centuries between the several kinds of trees, each annually
scattering its seeds by the thousand; what war between insect and
insect—between insects, snails, and other animals with birds and beasts
of prey—all striving to increase, all feeding on each other, or on the
trees, their seeds and seedlings, or on the other plants which first
clothed the ground and thus checked the growth of the trees. Throw up a
handful of feathers, and all fall to the ground according to definite
laws; but how simple is the problem where each shall fall compared to
that of the action and reaction of the innumerable plants and animals
which have determined, in the course of centuries, the proportional
numbers and kinds of trees now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This
is likewise sometimes the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle will almost invariably be
most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed to
the same dangers. In the case of varieties of the same species, the
struggle will generally be almost equally severe, and we sometimes see
the contest soon decided: for instance, if several varieties of wheat
be sown together, and the mixed seed be resown, some of the varieties
which best suit the soil or climate, or are naturally the most fertile,
will beat the others and so yield more seed, and will consequently in a
few years supplant the other varieties. To keep up a mixed stock of
even such extremely close varieties as the variously coloured
sweet-peas, they must be each year harvested separately, and the seed
then mixed in due proportion, otherwise the weaker kinds will steadily
decrease in number and disappear. So again with the varieties of sheep:
it has been asserted that certain mountain-varieties will starve out
other mountain-varieties, so that they cannot be kept together. The
same result has followed from keeping together different varieties of
the medicinal leech. It may even be doubted whether the varieties of
any of our domestic plants or animals have so exactly the same
strength, habits, and constitution, that the original proportions of a
mixed stock (crossing being prevented) could be kept up for
half-a-dozen generations, if they were allowed to struggle together, in
the same manner as beings in a state of nature, and if the seed or
young were not annually preserved in due proportion.
_Struggle for Life most severe between Individuals and Varieties of the
same Species._
As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them, if
they come into competition with each other, than between the species of
distinct genera. We see this in the recent extension over parts of the
United States of one species of swallow having caused the decrease of
another species. The recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush. How frequently we
hear of one species of rat taking the place of another species under
the most different climates! In Russia the small Asiatic cockroach has
everywhere driven before it its great congener. In Australia the
imported hive-bee is rapidly exterminating the small, stingless native
bee. One species of charlock has been known to supplant another
species; and so in other cases. We can dimly see why the competition
should be most severe between allied forms, which fill nearly the same
place in the economy of nature; but probably in no one case could we
precisely say why one species has been victorious over another in the
great battle of life.
A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related,
in the most essential yet often hidden manner, to that of all other
organic beings, with which it comes into competition for food or
residence, or from which it has to escape, or on which it preys. This
is obvious in the structure of the teeth and talons of the tiger; and
in that of the legs and claws of the parasite which clings to the hair
on the tiger’s body. But in the beautifully plumed seed of the
dandelion, and in the flattened and fringed legs of the water-beetle,
the relation seems at first confined to the elements of air and water.
Yet the advantage of the plumed seeds no doubt stands in the closest
relation to the land being already thickly clothed with other plants;
so that the seeds may be widely distributed and fall on unoccupied
ground. In the water-beetle, the structure of its legs, so well adapted
for diving, allows it to compete with other aquatic insects, to hunt
for its own prey, and to escape serving as prey to other animals.
The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds, as peas and
beans, when sown in the midst of long grass, it may be suspected that
the chief use of the nutriment in the seed is to favour the growth of
the seedlings, whilst struggling with other plants growing vigorously
all around.
Look at a plant in the midst of its range! Why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case
we can clearly see that if we wish in imagination to give the plant the
power of increasing in numbers, we should have to give it some
advantage over its competitors, or over the animals which prey on it.
On the confines of its geographical range, a change of constitution
with respect to climate would clearly be an advantage to our plant; but
we have reason to believe that only a few plants or animals range so
far, that they are destroyed exclusively by the rigour of the climate.
Not until we reach the extreme confines of life, in the Arctic regions
or on the borders of an utter desert, will competition cease. The land
may be extremely cold or dry, yet there will be competition between
some few species, or between the individuals of the same species, for
the warmest or dampest spots.
Hence we can see that when a plant or animal is placed in a new
country, among new competitors, the conditions of its life will
generally be changed in an essential manner, although the climate may
be exactly the same as in its former home. If its average numbers are
to increase in its new home, we should have to modify it in a different
way to what we should have had to do in its native country; for we
should have to give it some advantage over a different set of
competitors or enemies.
It is good thus to try in imagination to give any one species an
advantage over another. Probably in no single instance should we know
what to do. This ought to convince us of our ignorance on the mutual
relations of all organic beings; a conviction as necessary, as it is
difficult to acquire. All that we can do is to keep steadily in mind
that each organic being is striving to increase in a geometrical ratio;
that each, at some period of its life, during some season of the year,
during each generation, or at intervals, has to struggle for life and
to suffer great destruction. When we reflect on this struggle we may
console ourselves with the full belief that the war of nature is not
incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply.