GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 299 



degree. A group, when it has once disappeared, never reappears; 

 that is, its existence, as long as it lasts, is continuous. I am aware 

 that there are some apparent exceptions to this rule, but the ex- 

 ceptions are surprisingly few, so few that E. Forbes, Pictet, and 

 Woodward (though all strongly opposed to such views as I main- 

 tain) 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 connected 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 ex- 

 ceptional; 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 repre- 

 sented 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 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 only spoken incidentally of the disappearance 

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

 lection, 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 



