of the species of a group is generally a slower
process than their production: if the appearance and disappearance of a
group of species be represented, as before, by a vertical line of varying
thickness, the line is found to taper more gradually at its upper end,
which marks 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! {319} 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 cont