GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 367 



inherit different characters from their distinct progenitors; 

 and organisms already differing would vary in a different 

 manner. For instance, it is possible, if all our fantail 

 pigeons were destroyed, that fanciers might make a new 

 breed hardly distinguishable from the present breed ; but 

 if the parent rock-pigeon were likewise destroyed, and under 

 nature we have every reason to believe that parent-forms 

 are generally supplanted and exterminated by their improved 

 offspring, it is incredible that a fantail, identical with the 

 existing breed, could be raised from any other species of 

 pigeon, or even from any other well-established race of the 

 domestic pigeon, for the successive variations would almost 

 certainly be in some degree different, and the newly-formed 

 variety would probably inherit from its progenitor some char- 

 acteristic differences. 



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

 same general rules in their appearance and disappearance 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 ap- 

 parent exceptions to this rule, but the exceptions are surpris- 

 ingly 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 gen- 

 erations, 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 devel- 

 oped; 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 maxi- 

 mum, 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 



