1906 ] Stejneger, Isolation vs. Natural Selection. 269 



however, that perhaps ease of living combined with the cold climate 

 of the North and may be some other less important or more obscure 

 conditions, may account for the phenomenon. It must not be 

 imagined that because the northern birds live in a cold climate they 

 have a particularly hard life. Food is very abundant there, per- 

 haps more so than in the southern latitudes, and the large size of 

 these northern birds may be thus accounted for by the combination 

 of plentiful food and vigorous, energetic exercise necessitated by 

 the cold. 



That there is some connection between the more or less heavy 

 spotting and the north-south distribution is very probable. The 

 coincidence of these facts among a large number of species of wood- 

 peckers, especially in Japan, shows this clearly. Let us take a 

 single example from the closely allied genus Yyngipicus. Begin- 

 ning in the Riukiu archipelago with the small dark Y. nigrescens, 

 we find, as we go northwards, Y. kizuki in Kiusiu and Hondo and 

 finally in Yezo the largest and most spotted of them Y. seebohmi, 

 a regular gradation from south to north. 



How are we then to explain that the increase of spotting in North 

 America is essentially west-east and not a south-north phenomenon ? 



We have seen that the present climatic conditions give no clew, 

 but perhaps we may find it in the 'environmental stress ' of a pre- 

 vious geological period. 



The relation of our spotted woodpeckers to the Old World spe- 

 cies of Dryobates and the geographic distribution of the latter is 

 such that we must conclude that ours arrived to this continent from 

 eastern Asia, and various considerations make it probable that this 

 immigration took place not later than Pliocene times. We may 

 then assume that previous to the Glacial period there lived in North 

 America a Hairy Woodpecker small of size. The southern speci- 

 mens probably resembled D. v. jardinii, the northern ones were 

 possibly more like D. v. auduboni. The advance of the glaciation 

 pushed the woodpeckers southward and in combination with the 

 transgression of the Gulf of Mexico affected a separation of the 

 southern Alleghany region from the western portion of our conti- 

 nent which then as now formed the northern continuation of Mex- 

 ico. It is well within the bounds of probability that the Hairy 

 Woodpecker which became isolated in the Alleghanian region 



