CHAPTER III - STRUGGLE FOR EXISTENCE
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
hilltops: 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,
90 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. 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 Col. 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 Col. 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 forest. 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 be
strictly 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.
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