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THE AMERICAN NATURALIST [Vol. XLIX 



statistics. The constitution of a population does not di- 

 rectly affect the individual. My expectation of life will 

 not be materially increased if I chance to move into a 

 community in which all the other inhabitants are of ad- 

 vanced age. To this everyone will agree. But the ex- 

 treme selectionist appears to believe that in some mys- 

 terious way the act of continued selection, which means 

 concretely only the transference of each selected individ- 

 ual from one cage or pen to another to breed, will in and 

 of itself change the germ-plasm, so that after the act it is 

 different from what it was before ! It does not seem that 

 the evidence that such is in fact the case is critically valid. 

 A careful study of the very interesting and valuable work 

 of Castle and Phillips 11 with rats leaves the writer with 

 the feeling that those experiments prove no more than do 

 the experiments here reported: namely that the compo- 

 sition of a population may be altered by selection. It 

 does not appear to be proven that selection has essentially 

 altered the constitution of the germ-plasm of any partic- 

 ular individual as compared with the germ-plasm of that 

 individual's ancestors, making due allowance of course 

 for the phenomenon of segregation. That selection can 

 alter the composition of a population with respect to 

 genetic determiners, by a process of sorting over what is 

 already there and rejecting some portion, no one can 

 doubt. But it still appears to the writer to be true that : 

 " It has never yet been demonstrated, so far as I know, 

 that the absolute somatic value of a particular hereditary 

 factor or determinant {i. e., its power to cause a quanti- 

 tatively definite degree of somatic development of a char- 

 acter) can be changed by selection on a somatic basis, 

 however long continued." 12 



'1 Castle, W. E., and Phillips, J. C, "Piebald Rats and Selection," 

 Carnegie Institution of Washington Publication Xo. 195, 1914. 

 12 R. Pearl, Jour. Exp. Zool., Vol. 13, p. 264, 1912. 



