THEORY OF MIGRATION, 249 



to breed with the original unmodified stock, 

 the variations were transmitted and augmented. 

 This finally resulted in the development of man, 

 the possibility of his origin arising, according to 

 Wagner, from the early isolation of a few indi- 

 viduals. 



Considering the series of facts already discussed, 

 it is impossible to doubt that isolation would have 

 something of the effect which Wagner supposes. 

 Not only do the results of breeding domestic ani- 

 mals show this, but it is more conclusively shown 

 by the fauna of oceanic islands. In many of these 

 islands, as we have seen in Chapter V., there is almost 

 a complete isolation of the inhabitants from the main 

 land. These islands have been originally peopled 

 by migration, and the individuals have been sub- 

 sequently isolated, exactly as Wagner's theory as- 

 sumes. We have already seen that the result of 

 this isolation is the production of new species. In 

 all cases where the isolation has been tolerably com- 

 plete there has been a remarkable production of new 

 species, and, if the time has been long enough, of 

 new genera. In those cases, however, where the iso- 

 lation has been less complete and there has been 

 a chance of continual crossing with continental spe- 

 cies, new forms have not arisen. Without doubt 

 isolation has had its influence in the past, and we 

 must believe that Wagner's theory expresses a truth. 

 But to ascribe to it any thing more than a subordi- 

 nate position is not possible. There are some 

 300,000 known species of animals living to-day, and 

 to suppose that they all could have arisen by such 



