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
until 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.

The causes which check the natural tendency of each species to increase
are most obscure. 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. 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. Climate is important,
and periodical seasons of extreme cold or drought seem to be the most
effective of all checks.

The relations of all animals and plants to each other in the struggle
for existence are most complex, and often unexpected. 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!

The struggle for life is most severe between individuals and varieties
of the same species. The competition is most severe between allied forms
which fill nearly the same place in the economy of Nature. But great is
our ignorance on the mutual relations of all organic beings. 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