rarer, and finally extinct;--its place being seized on by some
more successful competitor.

It is most difficult always to remember that the increase of every
living being is constantly being checked by unperceived injurious
agencies; and that these same unperceived agencies are amply sufficient
to cause rarity, and finally extinction. We see in many cases in the
more recent tertiary formations, that rarity precedes extinction; and we
know that this has been the progress of events with those animals which
have been exterminated, either locally or wholly, through man's agency.
I may repeat what I published in 1845, namely, that to admit that
species generally become rare before they become extinct--to feel no
surprise at the rarity of a species, and yet to marvel greatly when
it ceases to exist, is much the same as to admit that sickness in the
individual is the forerunner of death--to feel no surprise at sickness,
but when the sick man dies, to wonder and to suspect that he died by
some unknown deed of violence.

The theory of natural selection is grounded on the belief that each new
variety, and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition;
and the consequent extinction of less-favoured forms almost inevitably
follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the
less improved varieties in the same neighbourhood; when much improved it
is transported far and near, like our short-horn cattle, and takes the
place of other breeds in other countries. Thus the appearance of new
forms and the disappearance of old forms, both natural and artificial,
are bound together. In certain flourishing groups, the number of new
specific forms which have been produced within a given time is probably
greater than that of the old forms which have been exterminated; but we
know that the number of species has not gone on indefinitely increasing,
at least during the later geological periods, so that looking to later
times we may believe that the production of new forms has caused the
extinction of about the same number of old forms.

The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like each
other in all respects. Hence the improved and modified descendants of
a species will generally cause the