348 EXTINCTION. 



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 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 extermi- 

 nation of whole groups, as of ammonites, toward 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 in- 

 troduction 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 re- 

 cently have exterminated the former horse under conditions 

 of life apparently so favorable. 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 unfavorable 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 naturalization of the domestic horse in 

 South America, that under more favorable conditions it 

 would in a very 'few years have stocked the whole con- 

 tinent. But we could not have told what the unfavorable 



