ingencies, and at what period of the horse's life, and in
what degree, they severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly should not have
perceived the fact, yet the fossil horse would certainly have become rarer
and 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 {320} 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 specific 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 ex