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:—1l. 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- 
VOL. I, 2B 
