° 1912 J TnoTTER, Relation of Genera to Faunal Areas. 161 



during the Middle Pleistocene in a more or less restricted area 

 south of the glaciers, and that in spreading northward after the 

 melting of the ice sheet, certain individuals reached farther to the 

 north than others, establishing breeding grounds in comparatively 

 high latitudes. This ancestral type was undoubtedly one of the 

 many species that characterized the forest fauna of the Pleistocene 

 and spread northward with the spread of this forest during what 

 geologists term the Glacio-Lacustrine sub-stage. The instinct 

 to return to this northerly nesting area at each succeeding spring 

 would become a fixed habit through inheritance, and this group of 

 individuals, removed by its position from the swamping effects of 

 intercrossing, would tend to hold any variations that developed by 

 segregation, the inherent quality of resistance against disruption 

 determining the degree of change. The specific forms we recognize 

 as Hylocichla alicice, H. ustulata, and H. guttata, with their several 

 varieties, are thus the more closely related northerly-breeding 

 species, the nesting grounds of which now overlap one another, 

 though the Gray-cheeked Thrush has advanced beyond the limits 

 of the other two, quite to the tree-line, while the Olive-backed 

 Thrush has spread somewhat beyond the breeding range of the 

 Hermit. In these two last species I have observed in Nova Scotia 

 a marked difference in habitat. The Olive-backed Thrush was 

 invariably found during the breeding season in the tall and heavy 

 growth of coniferous woods, while the Hermit Thrush frequented, 

 almost entirely, the lighter, scattered growth, being especially 

 abundant in burned-over tracts and in sprout-lands of birch. In 

 limited areas their nesting sites did not coincide and this habitat 

 difference may have been responsible in the fixing of their specific 

 characteristics by segregation. 



These remarks on the Genus Hylocichla apply mainly to the 

 eastern phase of distribution. In the Cordilleran region and on 

 the Pacific slope the varied conditions of mountain topography 

 have more profoundly disturbed the several types which have 

 broken up into a number of varietal forms. On the eastern side 

 of the continent we find a variety of Hylocichla alicice — the 

 Bicknell's Thrush — occupying quite isolated areas during the 

 breeding season. Hylocichla fusccscens, the Veery, is of a more 

 southerly breeding range than any of the foregoing species, while 



