t be standing-room for his progeny.
Linnaeus 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.

The struggle for life is 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 similarity 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. 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 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