d.

It will be observed that this argument rests entirely on the assumption,
that _varieties_ occurring in a state of nature are in all respects
analogous to or even identical with those of domestic animals, and are
governed by the same laws as regards their permanence or further
variation. But it is the object of the present paper to show that this
assumption is altogether false, that there is a general principle in
nature which will cause many _varieties_ to survive the parent species,
and to give rise to successive variations departing further and further
from the original type; and which also produces, in domesticated
animals, the tendency of varieties to return to the parent form.

_The Struggle for Existence._

The life of wild animals is a struggle for existence. The full exertion
of all their faculties and all their energies is required to preserve
their own existence and provide for that of their infant offspring. The
possibility of procuring food during the least favourable seasons, and
of escaping the attacks of their most dangerous enemies, are the primary
conditions which determine the existence both of individuals and of
entire species. These conditions will also determine the population of a
species; and by a careful consideration of all the circumstances we may
be enabled to comprehend, and in some degree to explain, what at first
sight appears so inexplicable--the excessive abundance of some species,
while others closely allied to them are very rare.

_The Law of Population of Species._

The general proportion that must obtain between certain groups of
animals is readily seen. Large animals cannot be so abundant as small
ones; the carnivora must be less numerous than the herbivora; eagles and
lions can never be so plentiful as pigeons and antelopes; and the wild
asses of the Tartarian deserts cannot equal in numbers the horses of the
more luxuriant prairies and pampas of America. The greater or less
fecundity of an animal is often considered to be one of the chief causes
of its abundance or scarcity; but a consideration of the facts will show
us that it really has little or nothing to do with the matter. Even the
least prolific of animals would increase rapidly if unchecked, whereas
it is evident that the animal population of the globe must be
stationary, or perhaps, through the influence of man, decreasing.
Fluctuations there may be; but permanent increase, except in restricted
localities, is