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THE AMERICAN NATURALIST [Vol. LI 



variations is more important in this case, for they appear 

 to have given rise to the geographic subspecies now recog- 

 nized, and, moreover, the specific differences in the genus 

 are merely an exaggeration or intensification of the kinds 

 of difference shown by these subspecies. It seems evi- 

 dent, then, that the differentiation which has gone on in 

 the evolution of the genus Otus is for the most part of the 

 kind exemplified by the small differences now existing be- 

 tween geographic races or subspecies occupying different 



The same thing is true of many other birds and mam- 

 mals, but this condition is by no means universal even in 

 these groups of animals. On the contrary, it is not diffi- 

 cult to find instances in which the discontinuous type of 

 variation, independent of environment or function, has 

 been the main factor in speciation. I will merely mention 

 the case of the North American flickers, Colaptes auratus 

 and C. cafer, set forth by Bateson,^! since the latter is a 

 Californian bird. These species differ remarkably in 

 their color markings, the most conspicuous differences 

 being (1) yellow or red quills, (2) a black or a red malar 

 strip in the males, (3) the presence or absence of a scarlet 

 nuchal crescent in males and females. C. auratus pos- 

 sesses the first of each pair of characters and C. cafer the 

 second. C. auratus extends from Alaska diagonally across 

 Canada and the United States to Texas and eastward to 

 the Atlantic, while C. cafer occurs in its pure form from 

 Oregon through Utah, California and Arizona into 

 Mexico. Each possesses 3 or 4 geographical subspecies. 

 Where the ranges of the species overlap over a large area 

 a mixed population of forms occurs which is usually in- 

 terpreted as a series of complex hybrids, but this will 

 bear further study. It is clear, however, as Bateson 

 points out, that the differences in range of the species 

 can not be associated with any constant environmental 

 difference in the habitats, and that the species can not 

 have differentiated from this mixed population of inter- 



21 Bateson, W., 1913, "Problems of Genetics." Yale Univ. Press, p. 146. 



