232 COOK 



nants " that we can avoid the natural result of perpetual hybrid 

 diversity. 



EVOLUTIONARY IMPLICATIONS OF MENDELISM. 



The general evolutionary significance of Mendelism as a 

 method of descent does not lie in the assumptions of character- 

 units and pure germ-cells, but in the facts of polarized inheri- 

 tance, which show how new variations may be preserved without 

 isolation and become the means of evolutionary progress of 

 species. There are, as we have seen, all degrees of accentua- 

 tion of the phenomena of dominance and polarity. The nar- 

 row-bred varieties and mutations show them in the most extreme 

 form, but sex-inheritance is very widespread, and the other less 

 accentuated forms of specialized diversity will probably be 

 found to be quite as generally distributed. 



Hitherto we have been expecting species to be uniform, and 

 have considered the presence of definite diversities in the same 

 species as something exceptional and peculiar. The possibility 

 that new variations may be preserved without isolation and thus 

 contribute to the evolutionary progress of species, was recog- 

 nized some months ago by Professor Davenport,' but was then 

 associated with the mutation theory of De Vries, with which it 

 has no true relation. Professor De Vries is particularly ex- 

 plicit in denying any such process of gradual transformation, 

 his theory being that the fact of mutation by itself constitutes 

 the evolution of a new species, which remains unchanged until 

 another period of mutation ensues. 



" There is neither a gradual modification nor a common 

 change of all the individuals. On the contrary, the main group 

 remains wholly unaffected by the production of new species. 

 After eighteen years it is absolutely the same as at the begin- 

 ning, and even the same as is found elsewhere in localities 

 where no mutability has been observed. It neither disappears 

 nor dies out, nor is it ever diminished or changed in the slightest 

 degree. . . . 



^" Such facts as I have cited above [melanic sports, etc.] . . . seem to lead 

 me to the conclusion that some new characters may arise in nature suddenly 

 . . . and persist as specific characteristics. The Mutation Theory in Animal 

 Evolution, Science, N. S., 24: 558. 



