OF ORGANIC BEINGS. 325 



some degree different, and the newly-formed variety would 

 probably inherit from its progenitor some characteristic 

 differences. 



Groups of species, that is, genera and families, follow 

 the same general rules in their appearance and disappear- 

 ance as do single species, changing more or less quickly, and 

 in a greater or lesser degree. A group, when it has once dis- 

 appeared, 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 exceptions are sur- 

 prisingly few, so few that E. Forbes, Pictet, and 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 connected by an unbroken series 

 of generations, from the lowest Silurian stratum to the pres- 

 ent 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 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 grad- 

 ually thickens upward, 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 grad- 

 ual 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 



