MUSEUM OF COMPARATIVE ZOOLOGY. 423 



the bobolink, the one having an almost cosmopolitan breeding range, 

 while the breeding range of the other is nearly or quite restricted 

 to the Alleghanian Fauna. Several of the Sylvicolidce have a breeding 

 range as restricted as the bobolink, while a few other species of the 

 same family breed throughout nearly the whole of North America. 

 One of the species of Dendrceca (D. cestiva) has this wide breeding 

 range, while other species of the same genus appear to breed only in 

 the Canadian Fauna. 



IX. Species which have a wide breeding range usually present 

 a greater or less number of easily distinguishable local forms, which 

 merge generally the one into the other in the regions lying be- 

 tween the localities at which these several forms are most fully 

 developed. A part of these local forms have received distinctive 

 names, and have of late been quite commonly regarded as distinct 

 species, while many are as yet not so regarded. Every year additional 

 races of this character are discovered, and doubtless many still remain 

 unknown. Much time will probably elapse before naturalists will gen- 

 erally agree as to their true character and relations, though evidence 

 indicative of their being the result of general and uniformly acting 

 laws of geographical variation is apparently by no means wanting. 

 The difference in color, size, form of the bill, length of the tail, etc., 

 that appear almost universally to obtain between the northern and 

 southern representatives of the same species, have already been suffi- 

 ciently dwelt upon in the preceding pages ; but the insertion of a few 

 species in the list of those alleged above (Class IV of the preceding 

 tables) to range across the North American continent calls for an ad- 

 ditional word in respect to the differences which have led to the specific 

 separation of the western representatives of these species from their east- 

 ern representatives, or to suggestions that they might prove to be spe- 

 cifically distinct. Most of the cases of this kind have been distinguished 

 in the tables under Class IV by the prefix of a [?] before their names. 

 In all these cases almost the sole difference alleged for the separation 

 of the western forms is that of either the darker or brighter or, in other 

 words, the more intense colors of those from the Pacific coast ; this char- 

 acter being always the one most strongly urged as distinguishing them, 

 and not unfrequently the Only one, especially in those species that breed 

 wholly to the northward of the latitude of San Francisco. The fre- 

 quency of this difference seems to be a strong reason for regarding 



