gency. 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 the species 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 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 those naturally and
artificially produced, are bound together. In flourishing groups, the
number of new specific forms which have been produced within a given
time has at some periods probably been greater than the number of
the old specific forms which have been exterminated; but we know that
species have not gone on indefinitely increasing, at least during the
later geological epochs, 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 extermination of the parent-species;
and if many new forms have been developed from any one species, the
nearest allies of that species, i.e. the species of the same genus, will
be the most liable to extermination. Thus, as I believe, a number of
new species descended from one species, that is a new genus, comes to
supplant an old genus, belonging to the same family. But it must often
have happened that a new species belonging to some one group has seized
on the place occupied by a species belonging to a distinct group, and
thus have