346 THE OEOLOGIGAL SUCCESSION 



tinct 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 estab- 

 lished race of the domestic pigeon, for the successive varia- 

 tions 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 this, 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 disappeared, never reappears; that is, its existence, as 

 long as it lasts, is continuous. 1 am aware that there are 

 some apparent exceptions to this rule, but the exceptions 

 are surprisinglv 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 descend- 

 ants one from the other, and all from a common progeni- 

 tor. 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 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 



