CHAP. XL] OF ORGANIC BEINGS. 275 



We have seen in the last chapter that whole groups of species 

 sometimes falsely appear to have been abruptly developed ; and I 

 have attempted to give an explanation of this fact, which if true 

 would be fatal to my views. But such cases are certainly excep- 

 tional ; the 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 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 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 pro- 

 duce 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 disappearance of 

 species and of groups of species. On the theory of natural selec- 

 tion, 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, 

 Ac., whose general views would naturally lead them to this con- 

 clusion. 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 



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