282 U. S. NATIONAL MUSEUM BULLETIN 217 



leads us to consider the possibility that the species involved, by virtue 

 of their behavior, are their own active isolating agent rather than 

 that they are the more or less passive recipients of molding influences 

 of external factors operating on undiscernible variational values. 



We have seen that in neither resident nor migratory species of arctic 

 birds are the taxonomic distinctions of their races adaptive. Natural 

 selection cannot operate directly upon nonadaptive characters, but 

 yet in 9 resident species and 14 migratory species, the populations nest- 

 ing mainly in northern Alaska and Yukon have become differentiated 

 to the point where they are taxonomically distinguishable as races. 

 It would appear that some influence other than natural selection must 

 operate to bring about the segregation of their special although non- 

 adaptive variations from the common attributes of the species. 



Small inheritable variations mark individuals of a species, and any 

 sample of individuals contains a genetic assortment which is some- 

 what different from the whole species. The smaller the sample of a 

 population, the less complete is its representation of the genetic com- 

 plement of the species and the greater is the probability that the genes 

 will vary during their hereditary transmission. If a population is re- 

 productively isolated it may well be different in its total genetic 

 range from the remainder of the species and that difference may well 

 increase with reproduction in isolation. These conditions for varia- 

 tion of a population are fulfilled by the isolation of the breeding 

 populations which are restricted to northern Alaska and Yukon. 

 The breeding isolation conforms with geographic limits, but for strong 

 and mobile birds like ptarmigan the restriction by inert geographic 

 obstacles appears to be not so much a factor as their active coherence 

 in a population that selects its own range and associations. 



Birds of many species return to the nesting places from which they 

 have been experimentally removed, and they recognize their own nests 

 and their young even after only brief acquaintance. Birds of some 

 species persistently return in spring to nest in the locality where they 

 were raised, evidently attracted by some pattern imprinted on their 

 memories by early experience in the nesting locality. This particular 

 impression upon memory appears not to be inheritable and is acquired 

 by each individual. 



Some races breeding mainly in Alaska and Yukon migrate to remote 

 wintering grounds different from those of other races of the same 

 species. Western solitary sandpipers ( Tringa solitaHa cinnamomea) 

 winter mainly south of the equator in South America, while the east- 

 em race {T. s. solitaria) winters from Texas and Louisiana south to 

 the equator. Near the time of autumn departure and spring return 

 the appearance of organized flocks of some species on their nesting 



