THEORY OF MIGRATION. 247 



theory even further than did Darwin. The only 

 scientists of note who advocate this autocracy of 

 the selection theory are Haeckel and O. Schmidt. 

 But Haeckel is always too full of startling theories 

 to be a safe guide, and Schmidt, from certain state- 

 ments recently published, would not be disinclined 

 to admit the inadequacy of the selection principle 

 in some cases. Besides these scientists, there are 

 the extreme materialists, who, reducing every thing 

 to matter, are obliged to find in natural selection 

 and the theory to be mentioned in the next para- 

 graph, the efficient causes of all organic phenomena. 

 But with these exceptions pure Darwinism is no 

 longer accepted. 



Theory of Migration. 



The first additional bit of explanation which we 

 notice is that of Moritz Wagner. We have seen 

 that one of the greatest difficulties with the selec- 

 tion theory is due to the fact that crossing always 

 tends to eliminate variations. An individual with a 

 favorable variation will almost always be obliged to 

 cross with an unfavored individual, and this will 

 cause the variations to diminish rather than increase. 

 Wagner attempts to avoid this difficulty by his 

 theory of isolation by migration. It is a demon- 

 strable fact that when an animal or plant is isolated 

 from other members of the same species, there is an 

 increase in the development of varieties. Pigeons, 

 in the hands of pigeon fanciers, are in a condition 

 of practical isolation, since the varieties are pre- 

 vented from crossing with each other. A glance at 



