THE "MUTANT" SERIES. 31 



we see no reason for considering them identical as regards that factor. 

 It appears that the mutant race arose from the plus race by a single 

 large plus variation, which seems to have its determiner in some single 

 component of the germ-cell. But the fact that this change came as a 

 large quantitative variation does not show that small variations are 

 impossible in that same cell component. It seems to us quite improb- 

 able that the plus mutation could have arisen in the minus selection 

 series. We believe that the repeated selection which was practised had 

 something to do with inducing this change in the plus direction. If 

 one can increase at will the " modifiers" which make the pigmentation 

 more extensive, it does not seem strange that after a time a readjust- 

 ment should occur within the cell which should incorporate modifiers 

 in that part of the cell which is responsible for the unit-character 

 behavior of the hooded pattern. This would amount to a quantitative 

 change in the unit-character for hooded pigmentation. 



BIBLIOGRAPHY. 



CASTLE, W. E. 



1905. Heredity of coat characters in guinea-pigs and rabbits. Cam. Inst. Wash. 



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1906. The origin of a polydactylous race of guinea-pigs. Cam. Inst. Wash. Pub. 49. 

 1912. The inconstancy of unit-characters. American Naturalist, vol. 46, pp. 352-362. 



CASTLE, W. E., and ALEXANDER FORBES. 



1906. Heredity of hair-length in guinea-pigs and its bearing on the theory of pure 



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 DEVRIES, H. 



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JOHANNSEN, W. 



1909. Elemente der exakten Erblichkeitslehre. G. Fischer, Jena. 

 MAcCuiiDY, H., and W. E. CASTLE. 



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