94 EXTINCTION. [Chap. XI. 



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 geological 

 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 modifica- 

 tion and the production of a number of allied forms 

 necessarily being a slow and gradual process, — one 

 species first giving rise to two or three varieties, these 

 being slowly converted into species, which in their turn 

 produce by equally slow steps other varieties and species, 

 and so on, like the brandling 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 Elie de Beaumont, 

 Murchison, Barrande, &c, whose general views would 



