96 EXTINCTION. [Chap. XL 



horse embedded with the remains of Mastodon, Mega- 

 therium, Toxodon, and other extinct monsters, which 

 all co-existed with still living shells at a very late geo- 

 logical 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 exter- 

 minated the former horse under conditions of life ap- 

 parently so favourable. But my astonishment was 

 groundless. Professor Owen soon perceived that the 

 tooth, though so like that of the existing horse, belonged 

 to an extinct sj^ecies. 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 

 elephant, and from the history of the naturalisation 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 

 tdld what the unfavourable conditions were which 

 checked its increase, whether some one or several con- 

 tingencies, 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 favour- 

 able, we assuredly should not have perceived the fact, 

 yet the fossil horse would certainly have become rarer 



