Chap. xi. of Organic Beings. 293 



Woodward (though all strongly opposed to such views as I maintain) 

 admit its truth ; and the rule strictly accords with the theory. For 

 all the species of the same group, however long it may have lasted, 

 are the modified descendants, one from the other, and all from a 

 common progenitor. In the genus Lingula, for instance, the species 

 which have successively appeared at all ages must have been con- 

 nected by an unbroken series of generations, from the lowest 

 Silurian stratum to the present day. 



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 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 spoken only incidentally of the disappearance 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 naturally lead them to this conclusion. On 

 the contrary, we have every reason to believe, from the study of the 



