96 EXTINCTION. [Chap. XL 



of species. When I found in La Plata the tooth of a 

 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, see- 

 ing that the horse, since its introduction by the Span- 

 iards into South America, has run wild over the whole 

 country and has increased in numbers at an unparal- 

 leled rate, I asked myself what could so recently have 

 exterminated the former horse under conditions of life 

 apparently so favourable. But my astonishment was 

 groundless. Professor Owen soon perceived that the 

 tooth, though so like that of the existing horse, be- 

 longed 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 cer- 

 tain, 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 told what the unfavourable condi- 

 tions 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 



