NATURAL SCIENCES OF PHILADELPHIA. 183 



stricted a certain per cent, of ^Effiotlnis is, the more decidedly and extremely 

 does such per cent, differ from the rest. 



I. To take up this point next; A certain T[)avi of jEgioihuK — perhaps only one 

 or two per cent.— now live, and probably always have lived, in Greenland. A 

 few years ago this fraction of the genus received from Mr. Gould the name of 

 " canesc«ns," in reference to a certain condition of plumage. By this word we 

 may summarily imply the fact that these birds differ from linarius in the fol- 

 lowing particulars: 1, larger size, averaging six inches in length, with wings 

 and tail to correspond ; 2, a less compressed, less acute, more regularly conic 

 bill, differently colored, with heavier nasal plumules; 3, not correspondingly 

 enlarged feet, tiie toes, especially, being relatively shorter; 4, marked de- 

 ticieucy in coloring matter of the feathers, which makes the whole plumage a 

 more hoary-whitish, leaves the rump pure white, reduces the streaking of the 

 under parts, and lightens the red of the adult (^ to a pale rosy. These are simply 

 observed /ac/5, not open to cavil. It is also a fact that these physical condi- 

 tion of Greenland yEffiot/nis represent the extreme of differentiation that ^Egio- 

 thus has yet attained ; for no known bird of the genus differs so much from the 

 common standard, Imarius as canescens does. These facts are to be reasoned 

 upon in connection with the following considerations : 



As just slated, canescens is the most local, as well as the most boreal, demon- 

 stration of the genus. It is confined to Greenland ; at least, it only reaches 

 its characteristic manifestation in that country. Though quoted from North 

 America, the citation is held, upon the best of grounds, to be erroneous. It is 

 recorded from Northern Europe ; I have seen no typical specimens from such 

 localities; ifrenlly occurring, they can scarcely be regarded as more than ad- 

 ventitious. All other styles of jEffiolhvs range over more ground than this 

 one ; if then, according to any laws whatsoever, extraneous influences 

 effect permutation of individual characters after a sufficient nuiuber of genera- 

 tions, we should expect such causes to be more efficiently operative in the 

 case of the Greenland birds than in any other. Such is found to have been 

 the case. Canescens has for an indefinite time been subjected to certain spe- 

 cial, if not exceptional, conditions, with the witnessed result as above detailed ; 

 which is tantamount to a demonstration of the assertion already made, that 

 the most local ^'Eyiolhi are the most specialized ones. 



The fact of this greatest differentiation settled, we have next to inquire how 

 far the particular Amc/ of modification that has been brought about is amenable 

 to certain laws that have been found of extensive applicability. I think that, 

 with perhaps one exception, all the distinctive features that canescens presents 

 are explainable by reference to some of these known laws. 



An increase of size, coincidently with increase of latitude, has been shown 

 by Baird* and others to be of wide application in the cases of species ranging 

 over many degrees of latitude. It is unnecessary to cite examples. Th c case 

 of canescens vs. linarius is a parallel one. If it be objected that in this case we 

 are dealing with two distinct " species,'' instead *)f variations in a single spe- 

 cies, it is to be replied that the "specific" distinctness of canescens is precisely 

 the point at issue, not a proven theorem. Moreover, it is to be observed that 

 this distinction in size is one especially marking, not birds that migrate over 

 a great extent of country-, but those resident species, individuals of which are 

 comparatively stationary, some living north, others south. It is here that the 

 law comes most clearly into play, and canescens is the only set of individuals 

 that conforms to this requirement ; the others (with possibly one exception) 

 are more or less migratory. So we can see whj' linarius taken at Hudson's 

 Bay and in Carolina should be of the same size, while canescens surpasses their 

 dimensions. 



Whether as effect of climate, adaptative modification, or pure incidence (but 

 more reasonably the former), the fact remains that a large number of genera 



' Baird, B. N. A., passim, and Am. Journ. Sci, and Arts, 180G, p. 20. 



1869.] 



