94 EXTINCTION. [Chap. XL 



general rule being a gradual increase in number, until 

 the group reaches its maximum, and then, sooner or 

 later, a gradual decrease. If the number of the species 

 included within a genus, or the number of the genera 

 within a family, be represented by a vertical line of vary- 

 ing thickness, ascending through the successive geologi- 

 cal 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 gradually 

 thickens upwards, often keeping of equal thickness for, 

 a space, 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 the theory, for the 

 species of the same genus, and the genera of the same 

 family, can increase only slowly and progressively; the 

 process of modification and the production of a number 

 of alUed forms necessarily being a slow and gradual pro- 

 cess, — one species first giving rise to two or three varie- 

 ties, these being slowly converted into species, which in 

 their turn produce by equally slow steps other varieties 

 and species, and so on, hke 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 dis- 

 appearance 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 connected together. The old notion of all 

 the inhabitants of the earth having been swept away by 

 catastrophes at successive periods is very generally given 

 up, even by those geologists, as Eliede Beaumont, Mur- 



