298 GEOLOGICAL SUCCESSION, Chap. X. 



of the domestic pigeon, for the successive variations would al- 

 most certainly be in 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 disappearance as 

 do single s))ecies, changing more or less qviickly, and in a 

 greater or lesser degree. A group, when it has once disap- 

 peared, never reappears ; that is, its existence, as long as it 

 lasts, is contiruious. I am aware that there are some apparent 

 excei:)tions 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 the theor}'. For all the 

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

 are the modified descendants of each other, and of some com- 

 mon progenitor. In the genus Lingula, for instance, the 

 species which have successively appeared at all ages nmst have 

 been connected by an unbroken series of generations, from the 

 loAvest Silurian stratum to the present day. 



We have seen in the last chapter that many species of a 

 group sometimes falsely appear to have come in abruptly in a 

 body ; and I liavc attempted to give an explanation of tliis 

 fact, which if true would be fatal to my views. But such cases 

 are certainly exceptional ; the general rule being a gradual in- 

 crease 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 gen- 

 era Avithin a family, be represented by a vertical line of vary- 

 ing thickness, ascending through the successive geological for- 

 mations in which the species are found, the line will sometimes 

 falsely appear to begin at its lower end, not in a sharp point, 

 but al^ruptly ; it then gradually thickens upward, often keep- 

 ing for a space of equal thickness, and xdtimately 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 Avith the theory, for the species 

 of the same genus, and the genera of the same family, can in- 

 crease only slowly and progressively ; the process of modifica- 

 tion 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 in- 

 to specie?, Avhich in their turn produce by equally slow steps 



