280 U. S. NATIONAL MUSEUM BULLETIN 217 



of life on most of the northern part of America must have been 

 similarly reduced, but in the interglacial periods climatic conditions 

 may have been suitable for repopulation. 



The assumption that differentiation of the migratory species of birds 

 has taken place since the ice cover melted sets a time when populations 

 migrating to nest in western arctic America could have started their 

 isolation from more southern members of the species. But although 

 melting of the ice removed one barrier against the migration of birds 

 into the Arctic, it did not at once make a path fit for their travel 

 and nesting, for the changes were gradual and did not progress 

 steadily toward the conditions we now see. Eelics of vegetation under 

 sediments and in Alaskan bogs provide evidence that marked climatic 

 fluctuations have occurred there as elsewhere during the last few 

 thousand years. The warmest period since the Wisconsin ice, for 

 example, is indicated to have been widespread in North America 

 about 6,000 years ago (Karlstrom, 1957), and many species could 

 then have extended populations far north of their present limits, for 

 a warmer climate seems likely to favor increase of the variety of living 

 forms. Only a few hoary redpolls (Acanihis hornemanni) now 

 winter in the Arctic, and snow buntings {Plectrofhenax) and com- 

 mon redpolls {Acanthis flaminea) sparsely winter along the margin 

 of the Arctic, but a little moderation of climate would allow large 

 populations of these species to remain as residents and the extent of 

 many migrations would be reduced. 



The larger species of northern mammals are visibly adapted to 

 cold by their insulation. Although the physiological adaptability of 

 small mammalian species is limited, large populations of lenmiings, 

 mice, and shrews live in the Arctic sheltered from the cold by their 

 adaptive behavior. Arctic human populations are adapted to their 

 cold environment mainly by each individual's learning of the culti- 

 vated practices of an arctic society. Birds seem to have devised few 

 methods of behavior which are obviously protective against cold; 

 the larger species are adapted by their insulation, but the obviously 

 successful adjustment of small birds to arctic cold is not now satis- 

 factorily explainable in terms of physiology and behavior. 



Following Bergmann's (1847) important discussion of animals' 

 economy of heat in relation to size, it has become popular to say that 

 the northward increase in dimensions of clines within some species 

 demonstrates natural selection of a climatically adaptive trend in 

 variation. P. F. Scholander (1955) has shown that such small di- 

 mensional changes as are reported in clines are insignificant for the 

 economy of heat. Therefore they could not be selected for adaptive 

 value in relation to climate. 



