On the Origin of Species by Means of Natural Selection
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That climate acts in main part indirectly by favouring other species, we
may clearly see in the prodigious number of plants in our gardens which 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 amongst the
crowded animals, been disproportionably 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 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 occur; and that of some
social plants being social, that is, abounding in individuals, even on the
extreme confines 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 each other from utter
destruction. I should add that the good effects of frequent intercrossing,
and the ill effects of close interbreeding, probably come into play in some
of these cases; but on this intricate subject I will not here enlarge.
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, has 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
that the land had 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 hundreds yards
distant from one of the old clumps, I counted thirty-two little trees; and
one of them, judging from the rings of growth, had during twenty-six 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 birds. Hence, if certain insectivorous
birds (whose numbers are probably regulated by hawks or beasts of prey)
were to increase in Paraguay, the flies would decrease--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 just have seen in
Staffordshire, the insectivorous birds, and so onwards in ever-increasing
circles of complexity. We began this series by insectivorous birds, and we
have ended with them. Not that in nature the relations can ever be as
simple as this. Battle within battle must ever be recurring with varying
success; and yet in the long-run the forces are so nicely balanced, that
the face of nature remains uniform for long periods of time, though
assuredly the merest trifle would often 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, most
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, in this part of England, is never visited by insects, and
consequently, from its peculiar structure, never can set a seed. Many of
our orchidaceous plants absolutely require the visits of moths to remove
their pollen-masses and thus to fertilise them. I have, also, reason to
believe that humble-bees are indispensable to the fertilisation of the
heartsease (Viola tricolor), for other bees do not visit this flower. From
experiments which I have tried, I have found that the visits of bees, if
not indispensable, are at least highly beneficial to the fertilisation of
our clovers; but humble-bees alone visit the common red clover (Trifolium
pratense), as other bees cannot reach the nectar. Hence I have very little
doubt, 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
degree on the number of field-mice, which destroy their combs and nests;
and Mr. H. 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 Mr. 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
concurring 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
the trees now growing on the ancient Indian mounds, in the Southern United
States, display the same beautiful diversity and proportion of kinds as in
the surrounding virgin forests. What a struggle between the several kinds
of trees must here have gone on during long centuries, 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, and all feeding on each other or on the
trees or 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 must fall to the ground according to definite
laws; but how simple is this problem compared to 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
often 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 almost invariably will 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 quite 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 numbers 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 one of our domestic plants or animals have so exactly the
same strength, habits, and constitution, that the original proportions of a
mixed stock could be kept up for half a dozen generations, if they were
allowed to struggle together, like beings in a state of nature, and if the
seed or young were not annually sorted.
As species of the same genus have usually, though by no means invariably,
some similarity in habits and constitution, and always in structure, the
struggle will generally be more severe between species of the same genus,
when they come into competition with each other, than between 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. One species of charlock
will supplant another, 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 plumed seeds no doubt
stands in the closest relation to the land being already thickly clothed by
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, I suspect that the chief use of the
nutriment in the seed is to favour the growth of the young seedling, 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 wished in imagination to give the plant the power of
increasing in number, we should have to give it some advantage over its
competitors, or over the animals which preyed 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 by
the rigour of the climate alone. 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, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly the same
as in its former home, yet the conditions of its life will generally be
changed in an essential manner. If we wished to increase its average
numbers in its new home, we should have to modify it in a different way to
what we should have done 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 our imagination to give any form some advantage
over another. Probably in no single instance should we know what to do, so
as to succeed. It will convince us of our ignorance on the mutual
relations of all organic beings; a conviction as necessary, as it seems to
be difficult to acquire. All that we can do, is to keep steadily in mind
that each organic being is striving to increase at 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.
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