tertiary formations, that species and groups of species gradually
disappear, one after another, first from one spot, then from another,
and
finally from the world. Both single species and whole groups of species
last for very unequal periods; some groups, as we have seen, having
endured from the earliest known dawn of life to the present day; some
having disappeared before the close of the pal ozoic period. No fixed
law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that
the complete extinction 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! 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
240