the progress of extermination,
than at its lower end, which marks the first appearance and increase
in numbers of the species. In some cases, however, the extermination
of whole groups of beings, as of ammonites towards the close of the
secondary period, has been wonderfully sudden.

The whole subject of the extinction of species has been involved in the
most gratuitous mystery. Some authors have even supposed that as the
individual has a definite length of life, so have species a definite
duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a horse
embedded with the remains of Mastodon, Megatherium, Toxodon, and other
extinct monsters, which all co-existed with still living shells at a
very late geological period, I was filled with astonishment; for seeing
that the horse, since its introduction by the Spaniards into South
America, has run wild over the whole country and has increased in
numbers at an unparalleled rate, I asked myself what could so recently
have exterminated the former horse under conditions of life apparently
so favourable. 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