318 GEOLOGICAL SUCCESSION. Chap. X. 



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 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 



