462 The American Naturalist. [June, 



breeding areas are extraordinarily great, so as to include the 

 whole of the arctic region, or northern America together with 

 northern Eurasia, for them to subdivide into two geographical 

 varieties, occupying respectively the eastern and western hem- 

 ispheres. Thus, the eurasiatic Colymbus nigricoUis is repre- 

 sented by a variety (californicus) in western North America ; 

 and to give other examples where an eurasiatic form, which 

 undertakes long periodic migrations, is represented by a geo- 

 graphical variety in North America, may be mentioned one 

 species of Fratercula, 1 Uria, 1 Larus, 1 Hydrochelidon, 2 Aythya, 

 1 Glaucionetta, 1 Somateria, 1 Anser, 1 Tringa, 1 Limosa, 1 Char- 

 adrius, 2 Falco, 1 Pandion, and others. But no species with ex- 

 tensive migration ranges shows any tendency to geographical 

 variation, unless its breeding areas are also very large in 

 extent. And the species with the least demonstrable tendency 

 to produce local races, are those in which the wing power is 

 greater, and the range of migration more extensive, than in 

 any other species of birds, namely, those enumerated in List I. 

 Further, we find it to be the rule, that in those avian families 

 most of the species of which undertake long migrations, if spe- 

 cies are present which are divisible into geographical varieties, 

 that these latter are more restricted in their migrations than 

 the former; examples are Uriatroile, Rissa tridadyJa, Fulmar w 

 glacialis (only North American species of the family presenting 

 geographical varieties), Rallus longirostrus, Porzanaj'i 

 AegiatiHa wilsonia and Ae. meloda, and others. After the con- 

 sideration of these facts it is certainly permissible to conclude 

 that, as a rule, species which undertake annual migrations of 

 comparatively great extent, through distances of 30° lat., or 

 more, evince no tendency to give rise to geographical varie- 

 ties, unless their breeding areas are very extensive ; and, con- 

 versely, that species which do not undertake extensive migra- 

 tions, owing to insufficient wing power or to some other cause, 

 and which occupy broad breeding areas, have the tendency to 

 produce geographical varieties. Consequently, also, extended 

 migration acts as a check upon the production of varieties ; 

 and the extent of the range of migration will, therefore, stand 

 in inverse ratio to the amount of geographical variation 



