CHAPTER VII 



VARIOUS THEORIES OF SPECIES-FORMING 

 AND DESCENT CONTROL 



The four factors named, variation, inheritance, selection, and sepa- 

 ration, must work together in order to form different species. It is 

 impossible to think that one of these should work by itself or that 

 one could be left aside. ORTMANN. 



As mentioned in the introductory chapter on the factors of 

 evolution (Chapter IV) , and as referred to several times in the 

 chapter on natural selection, the factor of the segregation or 

 isolation of groups of individuals must be taken into account in 

 any discussion of species-forming causes. This factor has long 

 been recognized by biologists, that phase of it, and undoubtedly 

 the most important of its several phases, called geographic 

 or topographic isolation or segregation being very clearly 

 stated and its importance emphasized by Moritz Wagner in 

 1868. Alfred Russel Wallace gave much attention, in his years 

 of active investigation, to the general subject of geographical 

 distribution, and was a pioneer in calling the attention of natu- 

 ralists to the great significance, in the light of the evolution 

 theory, of the facts of the geographical distribution of both 

 animals and plants. To-day, especially among American 

 biologists, the factor of topographic segregation is recognized 

 as one of the most important of species-molding influences. 

 Indeed it seems self-evident to many naturalists that natural 

 selection is impotent as an actual cause of species-forming with- 

 out some effective sort of isolation factor to assist it. Because 

 of the importance in the eyes of present-day naturalists of the 

 geographic isolation factor we have given (Chapter VIII), a 

 brief special discussion of this factor. In addition, in Chapter 

 XIV, will be found a discussion of the more general subject 

 of geographical distribution. 



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