276 EXTINCTION. [CHAP. XL 



time during which any single species or any single genus endures. 

 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 extermina- 

 tion, than at its lower end, which marks the first appearance and 

 the early increase in number of the species. In some cases, how- 

 ever, 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 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 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 attri- 

 bute 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 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. 



