GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 101 



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 exceptional; 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 upward, 

 often keeping of equal thickness for a space, and ulti- 

 mately 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 conform- 

 able 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 only spoken incidentally of the dis- 

 appearance of species and of groups of species. On the 



