EVOLUTION OP THE COLORS OF BIRDS. 249 



In the Aleutian Islands a distinct species of soiig- 

 sparrow (M. cinerea) is found, which is larger and grayer 

 than any of the others. Mr. Ridgway thinks that if 

 material could be had from the intervening district it- 

 would prove to intergrade with M. fasciata rufina, but 

 whether this be so or not, it is perfectly evident that the 

 form is a comparatively recent off-shoot from the same 

 stock as the others. On comparing the extreme forms 

 of this species it is difficult to believe them so closely 

 related, yet every stage of intergradation between them 

 can be shown. 



On Plates IX to XII, inclusive, I have represented in 

 a conventional manner the distribution of a number of 

 forms, which are of interest as showing the effect of en- 

 vironmental influences. As a general rule, a closely 

 related group, resident over the North American con- 

 tinent, divides into two species, and then subdivides 

 into several races. Mr. Seebohm, in his " Geographical 

 Distribution of the Charadriidae," makes the isolation 

 incident to the successive glacial epochs an important 

 factor in the establishment of generic, specific and 

 varietal characters. The fact that in a number of in- 

 stances two tolerably distinct species of the same genus 

 are found upon this continent, one to the east and the 

 other to the west of the Rocky Mountains, would seem 

 to argue that the genus was originally one homogeneous 

 species found in the northern part of the continent, 

 which was pushed south by the invasion of the ice, and 

 kept apart by an ice cap following south along the highest 

 mountain range of the continent. This may be the 

 explanation of the two species of Cyanocitta and of 

 Pipilo, for instance. The western country is much more 

 diversified in topography and climate than the eastern, 

 so it is but natural that the western species would be- 

 come modified into more races than the eastern. 



