54 



THE AMERICAN NATURALIST 



[Vol. XLII 



Castle and Forbes, showing that " Everywhere unit char- 

 acters are changed by hybridizing." This testimony is 

 chiefly from those who have experimented with the cross- 

 ing of animal forms, but even in the case of plants, there 

 is reason to believe that when free-crossing continues di- 

 vergence is checked. There is reason to believe that even 

 with plants the controlling factor in each case of con- 

 tinuously divergent evolution is either some form of 

 heteronomic isolation or some form of variation intro- 

 ducing autonomic isolation. 



Moritz Wagner's Theory and My Theory were Iso- 

 lated and Divergent from the Beginning 

 Moritz Wagner, in his "Law of the Migration of Or- 

 ganisms," was the first to insist on the importance of geo- 

 graphical isolation as a factor in evolution, but when he 

 asserted that without geographical isolation natural selec- 

 tion could have no effect in producing new species he 

 went beyond what could be sustained by facts. My own 

 theory, though it did not take form till four years later, 

 was reached without any knowledge of his, and therefore 

 in complete isolation from his; and when they came to- 

 gether for comparison they were found to be quite di- 

 vergent. 



In .March, 1S(JS, Moritz Wagner read a paper before the 

 Royal Academy of Sciences at Munich on "The Law of 

 the Migration of Organisms," and in 1873 an English 

 translation of a fuller paper by him entitled "The Dar- 

 winian Theory and the Law of the Migration of Organ- 

 isms" was published by Edward Stanford, of London. 

 It was through this pamphlet that I became acquainted 

 with his theory concerning the impossibility of the pro- 

 duction of new species except when and where migration 

 establishes a colony geographically isolated from the 

 original stock. In this paper we read: "The constant 

 tendency of individuals to wander from the station of 

 their species is absolutely necessary for the formation of 

 races and species" (p. 4). "Where there is no migra- 

 tion, that is, where no isolated colony is founded, natural 



