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.
Pages:
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384