300 THE ORIGIN OF SPECIES 



by catastrophes at successive periods is very generally given up, 

 even by those geologists, as Elie de Beaumont, Murchison, Bar- 

 rande, etc., whose general views would naturally lead them to 

 this conclusion. On the contrary, we have every reason to believe, 

 from the study of the 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 conse- 

 quent irruption of a multitude of new inhabitants into an ad- 

 joining 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 disappeared before the close of 

 the palaeozoic period. No fixed law seems to determine the length 

 of time during which any single species or any single genus en- 

 dures. There is reason to believe that the extinction of a whole 

 group of species is generally a slower process than their produc- 

 tion: 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 ap- 

 pearance and the early increase in number of the species. In some 

 cases, however, the extermination of whole groups, as of am- 

 monites, toward the close of the secondary period, has been won- 

 derfully sudden. 



The extinction of species has been involved in the most gra- 

 tuitous 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 of 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 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; 



