Chap. XL] OF ORGANIC BEINGS. 93 



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 characteristic differences. 



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

 the same general rules in their appearance and dis- 

 appearance as do single species, changing more or less 

 quickly, and in a greater or lesser 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 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 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 explana- 

 tion 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 



