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THE AMERICAN NATURALIST [Vol. XLVIII 



there also can be no doubt. 1 1 We can only escape the 

 conclusion that some species are fully fertile when 

 crossed," wrote Darwin, 10 ' 'by determining to designate 

 as varieties all the forms that are quite fertile," and he 

 added that some plants exposed to unnatural conditions 

 are so modified " that they are much more fertile when 

 crossed by a distinct species than when fertilized by their 

 own pollen." 



The rareness of these crosses between unlike strains or 

 species and the partial sterility of the offspring are not 

 obstacles in the way of regarding occasional hybridiza- 

 tion as one of the chief sources of mutation and hence 

 eventually of new species, for, as my preliminary experi- 

 ments in hybridizing species of Colias have already 

 shown, there may exist within a strain of species-hybrids 

 certain individuals that are fertile, though the most of 

 their brothers and sisters, mated, respectively, in a similar 

 way, are sterile. Nature probably makes more random 

 experiments in hybridization than we imagine ; many fail ; 

 some succeed; and in especially favorable stock like 

 Colias, judging from the numbers of closely allied but 

 different types (species) occurring in the same localities 

 in western Asia or in northwestern United States and 

 British America, probably many succeed. 



In seeking to determine how mutation, whether the re- 

 sult of hybridization or of possible climatic influences, 

 acts in the production of new species, it is possible from 

 cases already at hand to suggest possible steps in the evo- 

 lution of distinct, mutually infertile, types from one com- 

 paratively simple polymorphic species. 



The well-known dimorphic European currant moth, 

 Abraxas grossulariata, in which the light-colored (reces- 

 sive) variety, lacticolor, is found in nature only in the 

 female sex, will serve as an example of an elementary 

 condition. Lacticolor males, as Doncaster 17 has shown, 



i« "Animals and Plants under Domestication," Vol. II, Chap. 19, p. 179. 

 it "Report of the Evolution Committee," 4, 1908. 



