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THE AMEUK AX XATURALIST [Vol. LI 



individuals of a higher order of genetic worth, this will have its 

 due effect in impeding progress; but since the essential point 

 is, that, collectively considered, members of different pure lines 

 do, in general, rather definitely reflect their different germinal 

 constitution, advance may undoubtedly be made. However, it is 

 unnecessary to labor the point, as it is not only admitted, but is 

 urged by geneticists themselves in explanation of such results, for 

 ex.iinple, as those Castle and Phillips- obtained in selection ex- 

 i)ri lin(Mits with hooded rats. 



lu'tVrence to the genetic behavior of such characters as side- 

 .s])rig of the comb in poultry may also be included in the quota- 

 tion above. If, in this case, emphasis is laid upon the fact 

 that selection might api)arently be exercised indefinitely without 

 the least tendency toward evolution of a side-sprigged race, it 

 needs only be pointed out that the argument is directed against 

 the contention tliat selection is capable of modifying the germ 

 plasm. It has no bearing whatever upon the possibility of its 

 occurrence in nature, nor, aside from the point indicated, upon 

 its influence in evolution. 



Upon the other hand it seems quite impossible that even if 

 characters of which side-sprig is representative were highly 

 useful, they should fundamentally modify the course of evolution 

 under selection, or place any notjiblc (ihstach^ in its way. for they 

 are not germinal variations and iitv ;is likrly to o.-nn- in one line 



as probable that they should assmv the survival of a suimormal 

 representative of a superior genotype, as that they should tide 

 over a superior representative of an inferior one ; and the chance 

 would be no greater that the attribute in question should appear 

 in the offspring of one rather than in that of the other. 



A second of the three general groups of facts to which refer- 

 ence has been made is summarized in the statement : 



come about hy rchuivcly Inrcre. diseontimious steps, the new form being 

 not merely fully <lirfcn'!!t iated at its first appearance, but also fully 

 able to survive. 



In another conneet i(»n it is stated forcefully by Dr. Pearl that 

 if the game of survival is actually played by the quoted rules 

 he fornuilates (and no others are necessary) the conclusion is 

 logically irresistible that progress is bound to occur in the direc- 

 tion of those differences which distinguish the survivors. But 



2 Castlp, W. E., and Phillips, John C, 1914, Carnegie Institution of Wash- 

 ington, Publication 49. Castle, W. E., 1916, "Genetics and Eugenics," Har- 

 vard IJniversity Press, Cambridge, 8vo, pp. vi + 353. 



