Chap. X. EXTINCTION. 317 



number of the species of a genus, or the number of 

 the genera of a family, be represented by a vertical 

 line of varying thickness, crossing the successive geo- 

 logical formations in which the species are found, the 

 line will sometimes falsely appear to begin at its lower 

 end, not in a sharp point, but abruptly ; it then gradu- 

 ally thickens upwards, sometimes keeping for a space 

 of equal thickness, and ultimately thins out in the 

 upper beds, marking the decrease and final extinction 

 of the species. This gradual increase in number of the 

 species of a group is strictly conformable with my 

 theory ; as the species of the same genus, and the 

 genera of the same family, can increase only slowly and 

 progressively ; for the process of modification and the 

 production of a number of allied forms must be slow 

 and gradual, — one species giving rise first to two or 

 three varieties, these being slowly converted into species, 

 which in their turn produce by equally slow steps other 

 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 spoken only inci- 

 dentally of the disappearance of species and of groups 

 of species. On the theory of natural selection the ex- 

 tinction of old forms and the production of new and im- 

 proved forms are intimately connected together. The 

 old notion of all the inhabitants of the earth having 

 been swept away at successive periods by catastrophes, 

 is very generally given up, even by those geologists, 

 U_^ as Elie de Beaumont, Murchison, Barrande, &c, whose 

 general views would naturally lead them to this conclu- 

 sion. 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 



