270 EVOLUTION AND GENETICS 



Modern emphasis upon the inseparability of heritage and 

 environment in the existence of organisms seems to dispose of 

 the question rather effectively. Although it does not explain in 

 absolute terms, it at least makes possible a logical interpretation 

 in place of pure opinion. If this emphasis is well placed, the 

 cooperating factors in existence can hardly fail to be jointly 

 responsible for the resulting expression of organic characters. 

 Any organism is infinitely complex in its inherited qualities, and 

 its environment is no less complex and variable. Either through 

 changes emanating from the organism, such as locomotion, or 

 through such factors as climatic fluctuations, the balance between 

 organism and surroundings is likely to be in a state of very delicate 

 and constantly shifting adjustment. It seems reasonable to sup- 

 pose that the result would be variation. 



To this extent the interpretation is a combination of the opinions 

 of Lamarck and Darwin. With variation before us in its com- 

 plexity, however, we are forced to the belief of Weismann as 

 well, who saw in the mingling of diverse parental qualities in 

 each generation a provision for the diversification of the species. 

 It is necessary to bear in mind that an organism as complex as a 

 vertebrate or an arthropod is independent of the outer world to 

 the same degree that it possesses within itself the proper mechanism 

 for the maintenance of normal living conditions. In such a state, 

 with a constant shifting of stimuli through any of the changes 

 which have been considered as causes of adaptation, differences 

 may readily be expected to arise from conditions wholly within 

 the organism or partly without. There is sound reason in both 

 Lamarck's and Weismann's views, and Darwin expressed the 

 gist of the matter in his terse analysis. If we seek exact relations 

 between cause and effect in this field, however, we can only follow 

 Bateson's view; we do not know, and at present cannot expect to 

 know, exactly what condition will cause a given variation, nor is 

 it necessary for the purposes of genetics. 



Importance of Heritability. Since the science of genetics deals 

 with the resemblance of different generations, it is obvious that 

 variations in any of the above categories must be heritable if 

 they are to furnish material for the study of problems of genetics. 

 Only combinations and mutations, therefore, are available sources 

 of information in this science. The latter, when they appear, can 

 be manipulated for the production of new combinations so as to 



