386 castle: role of selection in evolution 



dividuals have later flowering offspring and vice versa. In other 

 words selection within this supposed "pure line" is evidently 

 effective. Accordingly either the gene here involved is plastic 

 or the supposed pure line is not pure. 



From the various lines of evidence which have been cited (and 

 I might have cited many more) it is clear that the pure line prin- 

 ciple, valid as a working hypothesis for seed size in beans and 

 for certain morphological characters in self-fertilized cereals, 

 does not fit in with the observed facts as regards the effects 

 of selection in the majority of the domesticated animals and 

 cultivated plants, nor even with the behavior of certain characters 

 in self-fertilized plants and asexually propagated animals. In 

 the case of such characters as white spotting in mammals, it is 

 evident that a change in the mean of the character in a partic- 

 ular direction in consequence of selection actually displaces in 

 the direction of selection the center of gravity of variation, so 

 that in a very true sense selection makes possible further varia- 

 tion in that same direction. The same is probably true as re- 

 gards protein content and oil content in the Illinois corn experi- 

 ment. It is doubtful whether, outside of that particular experi- 

 ment, maize with as high a protein content as 15 per cent has 

 ever been observed, or maize with as high an oil content as 8.5 

 per cent. It is not then a misuse of terms to say that the selec- 

 tion has in this case been the cause of further variation in the 

 direction of selection and so an agency in the progressive evolu- 

 tion of a new type. 



If this is true concerning a single character under experimental 

 study for a period of twenty generations, may it not also be 

 true of entire organisms and groups of organisms subjected to 

 keen competition with all other organisms in a struggle for 

 existence which has continued for millions of generations? If 

 there are characters which are plastic under artificial selection, 

 why need we be skeptical about the plasticity of organisms sub- 

 jected to natural selection? If artificial selection can, in the 

 brief span of a man's life time, mould a character steadily in a 

 particular direction, why may not natural selection in unlimited 

 time also cause progressive evolution in directions useful to the 



