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 favorable 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. 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 favorable, 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 anyhow 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, but 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 favorable 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
insured 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.
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. 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
grow, the more vigorous plants gradually kill the less vigorous,
though fully grown plants; thus out of twenty species growing 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 numbers of 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 is 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 1854-55 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 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 favored 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 favored 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 favoring 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
naturalized, for they can not compete with our native plants nor
resist destruction by our native animals.
When a species, owing to highly favorable 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
favored: 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, etc., 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 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 favorable
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.
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 inclosed 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 inclosed, so that cattle could not enter. But how
important an element inclosure is I plainly saw near Farnham, in
Surrey. Here there are extensive heaths, with a few clumps of old
Scotch firs on the distant hilltops: within the last ten years large
spaces have been inclosed, and self-sown firs are now springing
up in multitudes, so close together that all can not 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 uninclosed
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 inclosed, 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 onward
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 fertilize them. I
find from experiments that humblebees are almost indispensable to
the fertilization of the heart’s-ease (Violo tricolor), for other
bees do not visit this flower. I have also found that the visits of
bees are necessary for the fertilization of some kinds of clover;
for instance, 20 heads of Dutch clover (Trifolium repens) yielded
2,290 seeds, but 20 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.
Humblebees alone visit red clover, as other bees can not reach the
nectar. It has been suggested that moths may fertilize 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
humblebees became extinct or very rare in England, the heart’s-ease
and red clover would become very rare, or wholly disappear. The
number of humblebees 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 humblebees,
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 humblebees 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!
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
be strictly said to struggle with each other for existence, as in
the case of locusts and grassfeeding 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 colored 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 can not 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.
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 extension over
parts of the United States of one species of swallow having caused
the decrease of another species. The 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.
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 the 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 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.
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.
NATURAL SELECTION
--CHARLES DARWIN