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.