But how utterly groundless was my
astonishment! Professor Owen soon perceived that the tooth, though so
like that of the existing horse, belonged to an extinct species. Had
this horse been still living, but in some degree rare, no naturalist
would have felt the least surprise at its rarity; for rarity is the
attribute of a vast number of species of all classes, in all
countries. If we ask ourselves why this or that species is rare, we
answer that something is unfavourable in its conditions of life; but
what that something is, we can hardly ever 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.
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