100 



THE ORIGIN OF SPECIES 



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 sup- 

 planted 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 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. I am aware that there 

 are some apparent exceptions to this rule, but the ex- 

 ceptions 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. 



