BIRDS OF MIDDLE AND NORTH AMERICA. 351 



in everj' case coinciding strictlj^ with uniformity or continuity of 

 physical cond!itions. Thus the form having the widest distribution is 

 that inhabiting the Atlantic watershed, or the entire region from the 

 wooded vallej's of the Great Plains eastward, while those of most 

 limited range belong to the Pacific slope, where the topographic and 

 resultant climatic features are so varied and complicated. In Califor- 

 nia, for example, practically each distinct drainage area has its own 

 peculiar form, one being strictly limited to the salt marshes fringing 

 San Francisco Bay. From the last-mentioned point, inhabited by 

 decidedlj'^ the smallest of all the subspecific forms, northward along 

 the coast there is a gradual change, the size steadily increasing, the 

 plumage becoming first more rusty, then more sooty, and finally 

 more grayish, until the extreme limit of variation is reached in the 

 gigantic M. c. cinerea of the Shumagin and Aleutian Islands. 



In preparing the following "key" great diflScult}' has been experi- 

 enced in the attempt to characterize satisfactorily the different sub- 

 species of M. mier'ea, since however distinct these may appear from 

 one another when specimens are actually compared (and the differences 

 are perfectl}^ obvious to an unprejudiced eye) the differences are in 

 every instance comparative and therefore most difficult to formulate. 

 When size is involved the measurements of contiguous forms inosculate 

 on account of a decided average sexual difference in size, some males 

 of a smaller form exceeding, in some of their measurements at least, 

 some females of a larger form. It therefore becomes necessary to 

 compare specimens of the same sex. The chief difficulty, however, is 

 to decide as to where the line between recognizable forms should be 

 drawn, a matter requiring most careful study of the largest possible 

 amount of material and entire absence of personal bias as to whether 

 the forms recognized be few or maiiy. 



While intermediates connecting some of the forms have not actually 

 been seen by me, there can not be the slightest doubt as to their exist- 

 ence, their absence being due in everj^ case to lack of specimens from 

 intermediate localities. Between the following forms intergradation 

 may be considered as thoroughly established: M. c. rnontana and M. 

 c. fallax; M. c. rnontana and M. c. heermanni; M. c. rnontana and 

 M. c. morphna; M. c. heermunni and M. c. cooperi; 2L c. heermanni 

 and M. c. samueUs; M. c. samuelis and 3f: c. jnwilhila; M. c. 

 samuelis and M. c. cleonensis; M. c- inorphna and M. c. rvfina; M. 

 c. rvfina and M. c. caurina; M. c. caurina and M. c. l-enaie)isis; 

 M. c. kenaiensis and M. c. insignis; M. c. insignis and J/i c. cinerea. 

 Respecting the northern Pacific coast series (beginning with M. c. 

 morphia and ending with M. c. cwierea) it necessarily follows, from 

 the nature of the case, that all the forms between the two extremes 

 are themselves intermediates; M. o. rufina connecting M. c. morphna 

 with M. c. cauri7ia, the latter connecting 3r. c. nifina with 2L c. 



