tell. On the supposition of the fossil horse still existing as a rare
species, we might have felt certain from the analogy of all other
mammals, even of the slow-breeding elephant, and from the history of
the naturalisation of the domestic horse in South America, that under
more favourable conditions it would in a very few years have stocked
the whole continent. But we could not have told what the unfavourable
conditions were which checked its increase, whether some one or several
contingencies, 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 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
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