184 PROCEEDINGS OF THE ACADEMY OF 



more or less arctic present white forms. Among mammals it is only necessary 

 to allude to iJ'rsus, Ccntis, Lepus, &c. Among birds may be cited the Lagopus 

 from the Tetraoninw^ Hieroftilco and Nyctea among raptores, and in the present 

 family of the Frivgillidm certain species of Plectrophanes. Looking to water- 

 birds, it is observed that in the large cosmopolitan genus Larus, nearly if not 

 all the tropical and temperate zone species of which have the back black or 

 blue, and the primaries crossed with black, the glacial species {■:. g., glav.cus, 

 leucopterus, &c.) have not this color on the wings, and the mantle is very pale, 

 or even white, as in the case of L. Imtc/iinsii ; the exclusively boreal Larine 

 genus Pagojihila is all white. It is the same with the most boreal geese, as 

 hyperborean, rossii, &c. Among Procellarndie the most glacial genus, Fulmarus, 

 is the palest ; and an antarctic species, ThahiKsoica antarclica, is similarly paler 

 than most of its allies. The modifications of color that the Greenland ^-Eyiothi 

 have sustained are exactly homologous. This is too evident to call for argu- 

 ment. Moreover, besides the whitening of color, it is a matter of common ob- 

 servation that pelage of mammals and plumage of birds is likely to be increased 

 or otherwise modified in cold regions, or even at colder seasons of the year, for 

 an evident purpose. The covering of the skin is increased in two ways : by 

 thickening over parts already covered, and by extension over parts ordinarily 

 naked. The feet of the Ptarmigan, of the Snowy Owl, of the Northern Hare, 

 are good illustrations of the last. Now in canescens we see both these methods 

 in play. There is a peculiar soft thick mollipitose condition of the plumage, 

 not seen in other species, and the little modified feathers that surround the 

 base of the upper mandible are lengthened and thickened till they form a 

 dense rutf concealing the nostrils. It may be also remarked, by the way, that 

 this ruff is one of the diagnostic characters of a large group, although a true 

 subfamily, oi FringUlidse, many if not most of the species of which are more 

 or less boreal birds. 



One other feature of ca>iesce7is, — the want of enlargement of the feet in cor- 

 respondence with increase in other dimensions, — I shall revert to in another 

 connectioH. 



As the case stands with canescens, few if any ornithologists would deny this 

 fraction of jFgiothus " specific " rank. But if the laws that we have just been 

 noticing have any meaning, — if they are not mere word-formulas, shadowy and 

 insubstantial, — there is no reason to suppose that canescens was not at one 

 time linarius ; nor that, if the physical barriers — the geographical restrictions 

 — that now hedge it about were taken away, and it were permitted free mi- 

 gration and unrestrained commingling with other ^-Egiothi, it would not revert 

 to linarius in the same length of time, or less, that was required for its aberra- 

 tion. 



It seems to me that the special conditions and relations of JEgiothus canescens 

 give it forcible bearing upon the generic question of the origin of species ; and 

 it is evident on which side it stands as witness. 



II. A correlation of species in the matter of size, other than that just spoken 

 of as dependent upon latitude, is frequent among birds. It may be accompa- 

 nied by apparently unimportant, and certainly not very noticeable, differences 

 in color, proportions of parts, &c. ; a correspondent variation of the bill being 

 among the more common This law, so to speak, reaches a maximum, as the 

 writer has shown,* in certain pygopodous birds. In the families Coh/mbidx 

 and Podicipidx, in fact, it is possible to range the species in two parallel se- 

 ries, one of which is the counterpart of the other in case of almost every spe- 

 cies, in nearly everything but size. Thus there is a larger (C. Adamsii) and 

 smaller (6'. for^Ma^Ms) Loon ; a larger («riic««) and smaller [pacijicus) Black- 

 throated Diver; a larger [hoLbvellt) and smaller (griseigena) Red-necked Grebe, 

 and so on. The three North American species, so called, of Accipiter, are ad- 

 mirable illustrations. Examples without number could be advanced, but 



•Proc. Acad. Nat. Sci. Philad. 1862, p. 228. 



[August, 



