52 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



with locality than that afforded by the flying squirrels, 

 in which the northern race is more than one-half larger 

 than the southern ; yet the two extremes are found to 

 pass so gradually the one into the other, that it is 

 hardly possible to define even a southern and a north- 

 ern geographical race, except on the almost wholly 

 arbitrary ground of difference in size. The species, 

 moreover, is one of the most widely distributed, rang- 

 ing from the Arctic regions (the northern limit of for- 

 ests) to Central America. 



"Among birds the Iqcal differences in size are al- 

 most as strongly marked as among mammals, and in 

 the main, follow the same general law. A decided 

 increase in size southward, however, or toward the 

 warmer latitudes, occurs more rarely than in mam- 

 mals, although several well-marked instances are 

 known. The increase is generally northward, and is 

 often very strongly marked. The greatest difference 

 between northern and southern races occurs, as in 

 mammals, in the species whose breeding-stations em- 

 brace a wide range of latitude. In species which breed 

 from Northern New England to Florida, the southern 

 forms are not only smaller, but are also quite different 

 in color and in other features. This is eminently the 

 case in the common quail {^Ortyx virginianus), the 

 meadow-lark {Sturnella magna), the purple grackle 

 {^Quiscalus purpureus), the red- winged blackbird {Age- 

 laeus phaeniceus), the golden-winged woodpecker {Co- 

 laptes auratus), the towhee {Pipilo erythrophthalmus), 

 the Carolina dove {Zenadura macrura), and in nu- 

 merous other species ; and is quite appreciable in the 

 blue-jay {Cyanurus cristatus), the crow {Corvus ameri- 

 canus), in most of the woodpeckers, in the titmice, 

 numerous sparrows, and several thrushes and war- 



