s of species gradually disappear, one after another, first from one
spot, then from another, and finally from the world. In some few cases,
however, as by the breaking of an isthmus and the consequent irruption
of a multitude of new inhabitants into an adjoining sea, or by the final
subsidence of an island, the process of extinction may have been rapid.
Both single species and whole groups of species last for very unequal
periods; some groups, as we have seen, have endured from the earliest
known dawn of life to the present day; some have disappeared before
the close of the palaeozoic 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 extinction of a whole group
of species is generally a slower process than their production: if their
appearance and disappearance 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 the early increase in
number of the species. 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