DARWINISM AND MODERN GENETICS 



pears most improbable that any of these recently discovered 

 mutation-like processes can provide models for the kind of 

 directed origin of variation which would be relevant for the 

 theory of evolution. In all the processes described above, the 

 agents to which one might be tempted to attribute the capacity 

 to direct mutation are highly complex products of biological 

 activities. They are such things as DNA from one strain of 

 bacteria which may be introduced into another, or the tissues 

 of one plant or animal which are brought in contact with 

 those from a different species. These are not events of the kind 

 which can form the basis of the general processes of evolution. 

 If we wish to seek for any influence which can direct the ap- 

 pearance of variation of evolving populations in general, we 

 shall have to find it in the environment, that is to say, in the 

 inorganic surroundings in which the animal lives and in such 

 interactions with other species as it is likely to undergo in its 

 normal course of existence. 



During the earlier phase of the development of the geneti- 

 cal theory of evolution, the role of the environment was reduced 

 to that of being a mere filter through which new hereditary 

 variants had to pass; it became merely the agent of natural 

 selection and little else. It was, of course, realized that the en- 

 vironment plays a part in producing the characters which or- 

 ganisms exhibit. Indeed, the whole development of the ge- 

 netics of continously varying characters depended on methods 

 for separating and distinguishing the hereditary contribution 

 and the environmental contribution to the range of variation 

 which is found. For instance, it was shown that one can distin- 

 guish the extent to which variations are hereditarily determined 

 by comparing near relatives, such as parents and offspring, or 

 by measuring the response to selection of given intensity. From 

 such data estimates can be made of the heritability of a given 

 character, that is to say, the portion of the variance actually 

 found which can be attributed to hereditary factors which will 

 pass on to the next generation. The remaining variability can 

 be attributed in the main to the environment. In the early 

 developments of this theory it appeared sufficient to regard the 

 environmentally produced variability merely as a complication 

 which tends to obscure the underlying genetic situation. Hav- 



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