OTHER THEORIES OF GERMINAL SELECTION 409 



"It has been ol)joctcd to this thcoiy, that since the individually 

 acquired modifications possess the main selective value in these 

 instances, there is no reason why the corresponding germinal 

 variations should be fostered at all. The individuals with the 

 right, but slight, congenital variations would have no special 

 advantage over their fellows who show no such coincident varia- 

 tions. Nor is there any ground to assume that the individuals 

 with the greatest amount of plastic modification in a given direc- 

 tion will tend to exhibit similar innate variations to a greater 

 degree than those individuals not possessing this plasticity." 

 (Herbert.) 



The Theory of Isolation. In the discussion of natural selection 

 Wagner's theory of isolation was briefly mentioned. It is without 

 doubt one of the valuable theories of species-formation, although, 

 as previously indicated, its fundamental principle of isolation may 

 also be an initial stimulus for natural selection. 



Isolation may be due to any factor that prevents the mingling 

 of individuals for purposes of reproduction. Such factors may be 

 purely biological, and they may be purely physical. The writer 

 once noted the flight of a small l^utterfly, Pleheius melissa, over a 

 period of several years, and found that the males were abundant 

 six weeks before the appearance of the first females. The fortuitous 

 destruction of such delicate insects must be great, and the chance 

 of the earliest males to mate correspondingly slight. Such seasonal 

 distribution amounts to isolation through biological factors alone, 

 and must have some influence on the development of the species. 



The best example of physical isolation is found in the separation 

 of land areas by barriers. In oceanic islands isolation of this kind 

 is often extreme^ and in such a group as the Galapagos Islands the 

 animals may show definite correlation of characters and distribu- 

 tion which is apparently the result of isolation. Beebe's account 

 of the mocking birds, in Galapagos, World's End, is an excellent 

 exposition of this correlation. It is as follows: 



"... Without doubt we have here birds which were once living 

 on a single land mass — connected long ago with the mainland, 

 probably along the Costa Rican-Panamanian latitude, and later 

 insulated on a single large Galapagos island. This in turn has, 

 through subsidence, been reduced to a few volcanic peaks, on all 

 of which Nesomimus still survive, and which have begun to differ 

 slightly among themselves. This is due probably to causes far 



