CHAPTER IX. 



OTHER THEORIES OF SPECIES-FORMING AND 

 DESCENT (CONTINUED): AUXILIARY THE- 

 ORIES (CONTINUED). 



Isolation Theories. The varying importance attributed 



by different biologists to the theories explaining means 



and results of isolation is notable. While by 



Importance of ., r n r 1 j.- 



the isolation some the species-forming influence of isolation 

 factor in species- i s held to be as effective as selection itself, 

 some deem it more effective, others attach but 

 little importance to it, indeed see no effects of consequence. 

 These latter men are likely to be morphologists, cytologists, 

 and laboratory men generally ; the former are systematists, 

 students of distribution, and so-called field naturalists. Thus 

 Delage, who gives much attention in his general discussion 

 of the theories of heredity, variation, and species-forming 

 to many purely speculative theories of the ultimate structure 

 and behaviour of protoplasm, and of the mechanism of 

 heredity, dismisses the whole subject of geographic and 

 topographic isolation with a couple of superficial para- 

 graphs, in which he presents a singularly fallacious state- 

 ment of what the effects of isolation should be. On the 

 other hand the veteran German world-voyager and exploring 

 naturalist, Moritz Wagner, established long ago, on the basis 

 of his observations and deductions, a "law" of species-form- 

 ing by migration and consequent isolation, which in his mind 

 makes the natural selection theory superfluous. And Henry 

 Seebohm in a discussion of Romanes's * formulation of the 

 principle of physiological selection, says: "So far as is. 



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