MUSEUM OF COMPARATIVE ZOOLOGY. 117 



In respect to the rank and relationships of a great number of forms 

 among North American birds, to which at first was accorded the rank 

 of species, the gradual passage of one form into another, through whole 

 groups of forms that, as it were, cluster about a common type, is an 

 interesting and suggestive fact. Every acquisition of new material from 

 the middle and western portions of the continent but the more fully 

 shows the complete and gradual coalescence of widely differing forms, 

 which reach their typical or maximum development at particular locali- 

 ties, characterized by special climatic conditions, but which intergrade 

 at intermediate points, where the conditions of environment are also of 

 an intermediate character. In illustration of this, the genera Pipilo, 

 Junco, Melospiza, Passerella, Carpodacus, Colaptes, and Picus may be 

 cited from among the numerous and more strongly marked examples 

 of longitudinal variation.* In respect to the Pipilos of the United 

 Siates, the P. erythrophthahnus of the East passes southward into a well- 

 marked form in Florida, differing from the northern race in having the 

 white on the wings and tail much more restricted, in its smaller size, 

 larger claws, and longer tail. To the westward it begins at the Mis- 

 souri River to pass into the P. arcticus, through the occasional accession 

 of white streaks on the scapulars and interscapulars, and its larger 

 claws; these characters — especially the development of white in the 

 dorsal plumage — reach their maximum on the dry plateau of the in- 

 terior ; but westward P. arcticus merges into another form, P. oregonas, 

 towards and on the Pacific coast, in which the white on the wings be- 

 comes again reduced, the white streaks on the back (though generally 

 still retained) become narrower and fewer, and at times are either 

 almost or entirely obsolete, and the claws become considerably smaller. 

 To the southward the two forms, in the interior, run into each other, 

 both culminating in Lower California in the P. megalonyx, in which 

 the claws have become enormously developed, and the white spotting 

 varies from obsoleteness to the large amount that typically characterizes 

 P. arcticus. Further southward, in Mexico, P. megalonyx is well- 

 known to grade through P. macronyx into P. maculatus, which are 

 more or less olivaceous. Junco presents three strongly marked forms 

 or " species," that in a similar manner inosculate ; J. hyemalis being the 

 eastern form, J. oregonus the western, and J. caniceps occupying an in- 

 termediate region at the southward, between the habitats of the others ; 



* See also Bull. Mus. Comp. Zool., Vol. II, p. 237. 



