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
creature is constantly being checked by unperceived hostile agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. So little is this subject understood, that I have
heard surprise repeatedly expressed at such great monsters as the Mastodon
and the more ancient Dinosaurians having become extinct; as if mere bodily
strength gave victory in the battle of life. Mere size, on the contrary,
would in some cases determine, as has been remarked by Owen, quicker
extermination, from the greater amount of requisite food.
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