368 ORIGIN OF SPECIES 



line of varying 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 modification and the pro- 

 duction 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 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 disappear- 

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

 natural selection, the extinction of old forms and the pro- 

 duction 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 successive 

 periods is very generally given up, even by those geologists, 

 as Elie de Beaumont, Murchison, Barrande, &c., whose gen- 

 eral 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 inhabi- 

 tants 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 un- 

 equal periods; some groups, as we have seen, have endured 

 from the earliest known dawn of life to the present day; 



