CHAPTER XXIII 

 OTHER THEORIES OF GERMINAL SELECTION 



In the preceding chapter we have seen that Darwin's theory of 

 natural selection, while it appeals strongly to th(^ logical mind as 

 having a definite bearing upon the process of evolution, is by no 

 means a complete explanation, as has sometimes been urged. In 

 recognition of its hmitations various scientists have proposed 

 other theories to account for those processes of evolution which 

 are not within the scope of natural selection. Some of these 

 theories have returned to the Lamarckian concept of environ- 

 mental influence; these will be treated in the next chapter. Others 

 have assumed the heritability of the factors involved and conse- 

 quently stand on the same basis as natural selection; these are 

 our present concern, and they may be known as theories of germinal 

 evolution since they look upon changes in the hereditary germ 

 plasm as causes and not results of evolution. 



The failure of natural selection to account for the origin of 

 variations is shared in part by these other theories but this subject 

 involves factors which can better be treated in a succeeding chap- 

 ter. 



The Limitations of Selection. Darwin's use of fluctuating 

 variations as a basis for evolution has been a source of difficulty, 

 for in artificial selection and the experiments of geneticists it has 

 been shown that the possibility of change through selection of 

 such characters is probably limited. Artificial selection was 

 emphasized by Darwin as an example of the possibility of rapid 

 change in organisms through selective processes. It is now recog- 

 nized that the aggregations of individuals making up a species 

 vary not only in actual range of characters, but also in the latitude 

 of variation which they are able to impart to successive generations, 

 and that the limits of artificial selection are the separation of the 

 various pure lines. Short and tall races may be produced, for 

 example, from a normal population, but the maximum and 

 minimum cannot be extended indefinitely by selection alone. 



This question is closely related to Galton's law of filial regres- 



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