316 GEOLOGICAL SUCCESSION. Chap. X. 



exterminated by its improved offspring, it is quite in- 

 credible that a fantail, identical with the existing breed, 

 could be raised from any other species of pigeon, or 

 even from the other well-established races of the do- 

 mestic pigeon, for the newly-formed fantail would be 

 almost sure to inherit from its new progenitor some 

 slight characteristic differences. 



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

 the same general rules in their appearance and disap- 

 pearance as do single species, changing more or less 

 quickly, and in a greater or lesser degree. A group 

 does not reappear after it has once disappeared; or 

 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 surprisingly 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 my theory. 

 For as all the species of the same group have descended 

 from some one species, it is clear that as long as any 

 species of the group have appeared in the long succession 

 of ages, so long must its members have continuously 

 existed, in order to have generated either new and 

 modified or the same old and unmodified forms. Species 

 of the genus Lingula, for instance, must have continu- 

 ously existed by an unbroken succession of generations, 

 from the lowest Silurian stratum to the present day. 



We have seen in the last chapter that the species 

 of a group sometimes falsely appear to have come in 

 abruptly ; and I have attempted to give an explana- 

 tion of this fact, winch if true would have been fatal 

 to my views. But such cases are certainly excep- 

 tional; the general rule being a gradual increase in 

 number, till the group reaches its maximum, and 

 then, sooner or later, it gradually decreases. If the 



