No. 1.] ORGANIC VARIATION. 267 
Together with the degrees of variation, will be given for each 
species also, as briefly as possible, the range of migration and 
breeding area. These facts have been extracted principally from 
Ridgway’s work (/.c.), and from the recent work of Witmer Stone.! 
Birds with a migration range of 30° lat. north and south, or a 
corresponding distance east and west across the continent, I have 
classed as extensive migrants; birds with a smaller migration 
range, as migrants; those which do not undertake regular peri- 
odic migrations, but occasionally accomplish wanderings of 
considerable extent, as zrregular migrants ; and finally, those 
which migrate not at all, or, as the meadow lark and crow, 
migrate through only short distances, as ves¢dents. Such a 
classification according to the range of migration is necessarily 
an arbitrary one ; as is shown e.g. by the migration of certain 
species from high mountain ranges to the adjoining valleys in 
the winter season, a case which could not be classed as an 
extensive migration, although each such species meets with a 
considerable change of environment. This classification has 
been used merely as a convenience for computing the relation 
of the amount of variation to the extent of migration of the 
species. In other words, the extent of migration and the 
breeding area have been given for each species in order to 
learn the laws of the amount of individual variation in its 
relation to the environment.? 
1 Witmer Stone: The Birds of Eastern Pennsylvania and New Jersey, etc. 
Philadelphia, 1894. I would here express my gratitude to my friend Mr. Stone for 
his valuable aid in helping me to determine the migration and breeding ranges 
of certain species; and also for the facilities offered me to study the bird collec- 
tions of the Philadelphia Academy of Natural Sciences. 
2 The nomenclature adopted here for the species of birds is that employed by 
the American Ornithologists’ Union, with the emendations contained in its 
supplementary lists. I was unable to consult the second edition of this work, 
which appeared after this paper had been sent to press. 
