EVOLUTION BY HYBRIDIZATION 



EDWARD C. JEFFREY 



Harvard University 



Not long ago we were told that the investigation of the problems 

 of evolution had left behind the inexact if broad phase of study in the 

 field and had now entered upon the more accurate and satisfactory 

 stage of quantitative elaboration under laboratory conditions. Leav- 

 ing aside the question whether whatever exactitude in connection 

 with this tendency has not been more than offset by a corresponding 

 narrowness of outlook, it is now quite apposite to inquire if the experi- 

 mental methods of the physiologist are in reality in the position to 

 supply final light upon the fundamental problems of evolution. The 

 judging of living beings by what they do rather than by what they 

 are, has made notable progress in recent years. We are often told for 

 example that an organ is the tool of a function and consequently 

 should be defined by its performance rather than by its organization. 

 I need not point out the essential fallacy of the physiological definition 

 of an organ cited above. It obviously breaks down the moment it is 

 used on any wide range of facts. 



Perhaps the most striking illustration of depending overmuch 

 upon physiological data is supplied in connection with present investi- 

 gations upon the all important question of the origin of species. It 

 is practically universally assumed in genetical studies, that the capacity 

 to breed true under exacting experimental conditions is the most 

 reliable criterion of good species. It has for example assumed that 

 breeding results obtained with Oenothera and Drosophila are of funda- 

 mental importance for the science of biology. By those of us who 

 have neither been intoxicated with the cult of the evening primrose 

 nor bowed the knee in the temple of the god of flies, this conclusion 

 will in general be held undemonstrated. We must obviously know a 

 good deal more about the antecedents of those forms which have 

 been raised in recent years to the dignity of veritable biological touch- 

 stones, before we can admit the validity of the far-reaching con- 

 clusions drawn from their genetical behavior. 



The question of the origin of the species is as much with us today 

 as it was at the time of the publication of Darwin's epoch-making 

 work. Darwin himself ultimately ventured no explanation of the 

 causes of the changes concerned in the formation of new species, but 



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