318 
GEOLOGICAL SUCCESSION. 
Chap. X. 
groups of species last for very unequal periods; some 
groups, as we have seen, having endured from the ear¬ 
liest known dawn of life to the present day; some having 
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 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 in¬ 
crease in numbers of the species. In some cases, how¬ 
ever, 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 dura¬ 
tion. 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 intro¬ 
duction 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! Pro- 
