In some cases, however, the
extermination of whole groups, as of ammonites, towards the close of the
secondary period, has been wonderfully sudden.
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 can have
marvelled more than I have done at the extinction of species. 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 my astonishment was groundless. 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.
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