INFLUENCE OF ISOLATION. 369 



with the primary form is prevented, and the isolation of 

 the emigrant form, which becomes a new species by adapta- 

 tion, prevents its breeding with the old stock, and hence 

 prevents its return in this way to the original form. 



The importance of migration for the isolation of newly- 

 originating species and the prevention of a speedy return to 

 the primary form has been especially pointed out by the 

 philosophic traveller, Moritz Wagner, of Munich. In a 

 special treatise on " Darwin's Theory and the Law of the 

 Migration of Organisms," ^^ Wagner gives from his own 

 rich experience a great number of striking examples which 

 confirm the theory of migration set forth by Darwin in 

 the eleventh and twelfth chapters of his book, where he es- 

 pecially discusses the effect of the complete isolation of emi- 

 grant organisms in the origin of new species. Wagner sets 

 forth the simple causes which have " locally bounded the 

 form and founded its typical difference," in the following 

 three propositions : — 1. The greater the total amount of 

 change in the hitherto existing conditions of life which the 

 emigrating individuals find on entering a new territory, the 

 more intensely must the innate variability of every organ- 

 ism manifest itself. 2. The less this increased individual 

 variability of organisms is disturbed in the peaceful process 

 of reproduction by the mingling of numerous subsequent 

 immigrants of the same species, the more frequently will 

 nature succeed, by intensification and transmission of the 

 new characteristics, in forming a new variety or race, that is, 

 a commencing species. 3. The more advantageous the 

 changes experienced by the individual organs are to the 

 variety, the more readily will it be able to adapt itself 

 to the surrounding conditions; and the longer the undis- 



