284 II. S. NATIONAL MUSEUM BULLETIN 2.17 



of gulls and terns returned to their nests after being transported long 

 distances away. It may be suggested that these maritime, locally 

 undifferentiated birds are not persistently affected by companionship 

 with nesting associates during the nonbreeding season away from their 

 breeding grounds, and that they may be diverted to join individuals 

 bound for other nesting areas sufficiently often to keep the character- 

 istics of the entire species in a mixed state. 



Outside of Alaska and Yukon, examples of long and specialized 

 migration routes can be seen to be associated with taxonomically dis- 

 tinguished populations. Although maritime species have not formed 

 races confined to nesting in the northern interior of Alaska and 

 Yukon, R. C. Murphy (1936) has described many examples of popula- 

 tions of oceanic birds which regularly return to nest on small South 

 American Islands after remarkable long oceanic migrations. Thayer's 

 gull {Larus argentatus thayeri) winters with other herring gulls on 

 the coast of British Columbia and migrates across arctic America to 

 nest in its eastern part. The physical characters of the race are only 

 significant for taxonomic purposes. It shares the strength and hardi- 

 ness of other herring gulls but behavioristic association with a locality 

 isolates the race on an unusual migratory path. 



There are many variations in form among races of some species 

 which are not visibly adaptive and which cannot reasonably be as- 

 sumed to have been established by natural selection. And yet with- 

 out some segregating influence these characters could not be placed 

 in separate order from the common characters of the species. Isola- 

 tion, which in birds is often visibly governed by an acquired pattern 

 of geographic memory, can segregate breeding populations and thus 

 provide conditions suitable for variation. Under the protection of 

 isolating behavior small and nonadaptive differentiation could pro- 

 gress until it became significant for natural selection to perpetuate or 

 destroy. Isolating behavior could protect differentiation until even 

 completely nonadaptive characters became so distinct that the variant 

 population was genetically separated from its former associates in 

 the species. Small variations are common and nonadaptive racial 

 distinctions are numerous. Isolation by behavior seems to be a process 

 which preserves these adaptively insignificant variations until they 

 distinguish populations. 



During great biotic changes in northern lands, and in fact since 

 Pleistocene times, the surviving species of birds have remained essen- 

 tially unaltered (Wetmore, 1931, p. 5) . Although they have probably 

 moved their ranges northward and southward to escape some drastic 

 climatic changes, the northern species have met the fluctuating con- 

 ditions of recent times without modification of their specific char- 



