EXTINCTION 369 



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 en- 

 dures. 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 mon- 

 sters, 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 Span- 

 iards into South America, has run wild over the whole coun- 

 try 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 favour- 

 able. 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 of all classes, in 

 all countries. If we ask ourselves why this or that species 

 is rare, we answer that something is unfavourable in its 

 conditions of life; but what that something is we can hardly 

 ever tell. On the supposition of the fossil horse still existing 

 as a rare species, we might have felt certain, from the 

 analogy of all other mammals, even of the slow-breeding 



