286 ON THE ORIGIN OF SPECIES 



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 

 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. But 

 how utterly groundless was my astonishment ! Pro- 

 fessor 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 sur- 

 prise 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 exist- 

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

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

 breeding elephant, and from the history of the natural- 

 isation of the domestic horse in South America, that 

 under more favourable conditions it would in a very few 

 years have stocked the whole continent. But we could 

 not have told what the unfavourable conditions were 

 which checked its increase, whether some one or several 

 contingencies, and at what period of the horse's life, 

 and in what degree, they severally acted. If the 

 conditions had gone on, however slowly, becoming 

 less and less favourable, we assuredly should not have 

 perceived the fact, yet the fossil horse would certainly 

 have become rarer and rarer, and finally extinct ; — its 

 place being seized on by some more successful competitor. 



