326 EXTINCTION. 



other varieties and species, and so on, like the branching of 

 a great tree from a single stem, till the group becomes large. 



ON EXTINCTION. 



We have as yet only spoken incidentally of the disap- 

 pearance of species and of groups of species. On the theory 

 of natural selection, the extinction of old forms and the 

 production of new and improved forms are intimately con- 

 nected together. The old notion of all the inhabitants of 

 the earth having been swept away by catastrophes at succes- 

 sive periods is very generally given up, even by those geolo- 

 gists, as Elie de Beaumont, Murchison, Barrande, 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 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 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 endui__>. 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 extermination 

 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 



