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THE AMERICAN NATURALIST [Vol. LTV 



but this is no proof that such has actually been the historic 

 method by which the series lias arisen. It may actually have 

 started in the middle and worked both ways, or in several direc- 

 tions. Only a study of contemporaneous genetic variation can 

 show what the method of evolution is. Color variation in mam- 

 mals is not unlike that of birds. We might arrange the color 

 varieties of any species of mammal or group of mammals in a 

 linear series and assume logically enough that evolution had 

 progressed from the darkest to the lightest form in orderly man- 

 ner, or vice versa, yet the study of contemporaneous variation 

 shows that this is not the case. A wild species, like the gray 

 rabbit or the brown rat, undergoes sporadically genetic variations 

 (" mutations") some of which are lighter, some darker than the 

 parental form. They have no relation to each other as to the 

 order, time, or place of their appearance, so far as we can dis- 

 cover. Breeding evidence shows that they are genetically inde- 

 pendent one of another. 



As an alternative to the hypothesis of orthogenesis in varia- 

 tion, the mutation theory of DeVries received much critical con- 

 sideration in Whitman's writings. The lateness of publication 

 of much of this is to be regretted. Discussions which miuhi have 

 been helpful a few years ago are now quite superfluous and out 

 of date in the light of critical experimental evidence since pro- 

 Mutation has practically ceased to he considered as a hypothet- 

 ical method of the immediate and direct origin of species. Even 

 as regards the origin of characters, mutation is no longer sup- 

 posed to be a simple process. Whitman maintains with entire 

 correctness that " unit-characters" often have small beginnings 

 and may later be gradually increased by systematic selection. 

 Frizzling of the feathers in pigeons and fowls is an example cited 

 by him. He says, p. 151 : 



