270 Stejneger, Isolation vs. Natural Selection. [July 



belonged essentially to the form which above I have compared to 

 D. v. auduboni, and that on the other hand, the one which was 

 forced south in the Sonoran region mostly consisted of scantily 

 spotted southern specimens comparable to D. v. jardinii. I feel 

 convinced of the correctness of the hypothesis that it was during 

 this long period of separation that the two main forms, the western 

 and the eastern, finally got established. I am even strongly in- 

 clined to believe that by the timfc of the last retreat of the glacial 

 covering the two forms were so well differentiated that they might 

 be called species rather than subspecies, as these terms are now 

 commonly used. The two now distinct species of Hairy Wood- 

 peckers following the melting ice cap and occupying the forests 

 which gradually covered the reclaimed land finally met along the 

 solid line of Mr. Jenkins's map, assuming a distribution such as 

 we see it at the present day. That the two species which were 

 established through some such separation should intermingle to 

 a considerable extent along their mutual boundary line is quite 

 natural. I leave it to those ornithologists who are more familiar 

 with the forms in question to decide whether the 'intermediate' 

 specimens, where the eastern and western birds come into contact, 

 may not properly be regarded as the result of hybridization. The 

 formation of the three eastern and four western races, chiefly 

 characterized by size, amount of darking of the underside and 

 extent of the white spots, is then subsequent to the primary segrega- 

 tion of the eastern and western species. There is nothing in the 

 quantity or quality of their specialization which contradicts the 

 assumption that they were formed since the glacial period. 



The above seems to me to be a much more satisfactory theory 

 as to the origin of the various forms of the Hairy Woodpecker, 

 than the one suggested by Mr. Jenkins. That natural selection 

 at some stage or another in the evolution of these forms, may have 

 exercised an auxiliary influence it is not necessary to deny, but I 

 think there can be but little doubt that 'environmental stress' work- 

 ing on a material possessing considerable plasticity in a certain 

 direction accomplished the 'speciation' of these forms because of 

 their complete isolation during an earlier geological period. 



