Vol. XXIII 

 1906 



Stejneger, Isolation vs. Natural Selection. 267 



It is plain then, that of the present climatic factors determining 

 the formation and the distribution of the forms of D. villosus, the 

 rainfall is by far more important than the temperature. 



If we examine Mr. Jenkins's map further we discover a doubly 

 curved, solid black line running from N. W. to S. E. "separating 

 heavily and lightly spotted forms." East and north of this line we 

 find forms which have the wing feathers spotted with white to the 

 tips, while the races living west and south of the line have these 

 feathers only spotted at the base. Intermediate specimens are 

 found occasionally along this line. 



Mr. Jenkins has tried to explain the variations in these birds 

 "through natural selection and the survival of those best fitted to 

 escape from enemies or to obtain food in the particular region that 

 they inhabit," but he does not seem to have much faith in the expla- 

 nation. As for the spotting, he reasons that the spots "were origi- 

 nally developed for some good service, as for directive markings, 

 etc." The loss of them at the tip he then suggests as due to the 

 protection it would be to the bird to have the folded wings uniformly 

 black so as "to make the bird less conspicuous while climbing up 

 dark colored tree trunks." But he brings down the whole con- 

 struction very effectively with the following remark: 



"Could this theory be absolutely proven we would still have to 

 explain why all of the Hairy Woodpeckers have not developed in 

 like manner. If we draw a geographical line between light and 

 heavily spotted forms this line would in part coincide with the geo- 

 graphical line between the Arid West and Humid East. But I 

 greatly doubt if aridity and humidity have anything to do with the 

 wing spotting. 1 We might say that the more highly developed 

 form has had more enemies and more competition, but such a 

 statement needs yet to be proven." 



Mr. Jenkins in offering his explanation labors under one funda- 

 mentally erroneous notion, viz., "that the western bird is more 

 highly developed" (p. 169). He speaks also of "the spots on the 

 inner webs and proximal ends of the feathers" as "more persistent.'' 

 Evidently he regards the eastern forms with the wings spotted to the 



1 He might have added that plainly it has not in this case, since D. v. harrisi, 

 which inhabits the most humid region, is also one of the least spotted forms. — L. S. 



