26o READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



which to select ; that individual variation is a qualitative factor, giving 

 the differences which make a selection possible; and that heredity is 

 a conservative factor, holding fast those characters which better fit 

 the organism to its environment. 



Now it seems to me that there is no possible outcome of the 

 necessary action and interaction of these three factors that would 

 not be a selection of some sort. Darwin thought it comparable in a 

 large way to the selection by which the stock-breeder improves his 

 herd, and therefore called it ''natural selection," carefully guarding 

 the phrase from misinterpretation from the teleological angle as well 

 as from a too close parallelism between artificial and natural selection. 

 And I believe no one has suggested a more acceptable term for the 

 process of selection resulting from the interplay of natural laws. 



Three outstanding theories have been advanced since the publica- 

 tion of the Origin, each involving an advance in our knowledge of 

 the mechanism of heredity on the one hand and the origin of varia- 

 tions on the other. 



Weismann's theory of the continuity and stability of the germ 

 plasm was of immense importance in its discussion of the mechanism 

 of heredity, and his amphimixis gave a plausible explanation of the 

 origin of variations. His results were almost universally regarded as 

 confirming and greatly extending the scope of natural selection. 



Mendel's theory regarding the purity of the gametes, their segre- 

 gation in the sex cells, and the whole complex Mendelian mechanism 

 so admirably described by Morgan; all of these, fascinating and 

 important as they are, deal with the mechanism rather than the fact 

 of heredity. In my opinion their acceptance or rejection does not 

 affect the status of natural selection as a theory of organic evolution. 



But it is the theory of mutation that has furnished most of the 

 ammunition for the opponents of natural selection; and this in spite 

 of the fact that De Vries, the originator of the mutation theory, 

 expresses himself with great clarity as follows: 



" My work claims to be in full accord with the principles laid down 

 by Darwin and to give a thorough and sharp analysis to some of the 

 ideas of variability, inheritance, selection, and mutation which were 

 necessarily vague in his time." 



In 1904, when these words were published, there did seem to be 

 a sharp distinction between the ideas of Darwin and those of De Vries. 

 The former believed that natural selection acted upon many small 

 variations and accumulated them until the differences were sufficient 



