294 EXTINCTION. [Chap. XI. 



tertiary formations, that species and groups of species gradually 

 disappear, one after another, first from one spot, then from another, 

 and finally from the world. In some few cases, however, as by the 

 breaking of an isthmus and the consequent irruption of a multitude 

 of new inhabitants into an adjoining sea, or by the final subsidence 

 of an island, the process of extinction may have been rapid. Both 

 single species and whole groups of species last for very unequal 

 periods; some groups, as we have seen, have endured from the 

 earliest known dawn of life to the present day ; some have dis- 

 before the close of the palaeozoic period. No fixed law 

 to determine the length of 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 pro- 

 cess 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 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 con- 

 ditions 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 

 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 



