NATURAL SCIENCES OF PHILADELPHIA, 185 



these will suffice. In all these instances the foregoing law of increase in size 

 with latitude has no part ; for these birds always may be, and, as a matter of 

 fact, usually are, more or less associated. The genus ^Egiothus is also obnoxious 

 to this special kind of variation, which remains as yet unexplained. 



There are in Euroi)e, and probably also in America, two races, or varieties, 

 of Redpolls, that differ from the ordinary style of ^-Effiothus in little except size. 

 One is smaller, the olher larger. The latter has, in addition, a somewhat (but 

 barely appreciably) larger and more yellowish bill than average linarius ; but 

 none of these points, even that of size, are sufficiently marked to be obvious 

 and unmistakeable except in the extreme they have as yet reached ; that is, 

 intermediate individuals now living complete a gradation back to linarius. 

 This form is called ^-'Egiothus holhoeUi, after Boehm ; it has been on record but 

 a few years. It is found in northern and western Europe, associated with li- 

 narius ; and I have seen identical samples from Canada. The smaller race 

 has been longer known. Though usually credited, as Fringilla rufescens, to 

 Vieillot, somewhere about 1817, it was an old entry in the books at that date. 

 Brisson describes it with his usual accuracj-; Miiller has it in his Supplement, 

 of 1776, as F. cabaret. Although authors speak of a notable amount of rufous 

 in the plumage, over and above that commonly exhibited by linarius^ there is 

 reason to suspect that this is exaggerated. Very young linarius is largely 

 rufous ; and it is credible that, age for age, and season for season, the differ- 

 ence in the colors of rnfescens and linarius is not very tangible. Difference in 

 size, then, is the main if not the onlj' point in this, as in the former case ; and 

 rufescens, as surely as holboelli, grades in this respect with linarius. 



Were these larger and smaller birds separated from each other and from li- 

 narius by geographical range, and particularly by a difference in latitude, we 

 could argue more plausibly concerning them. In such case, even the slight 

 difference that exists might be traced to some cause, and be really of more 

 consequence, in a classificatory point of view, than it now appears to be. In 

 the larger bill of holboelli we might see the operation of tlie same class of causes 

 (however obscure their special determination maj- be) as those that, e. y.^ have 

 sharpened the bills of all the White-bellied Nuthatches west of a certain me- 

 ridian, enlarged the Florida Crows' bills, turned the California Magpies' bills 

 yellow, strengthened the claws of the Arizona black Pipilos, drawn out the 

 tails of western Mocking Thrushes, put warts on the bill oi Anser rossii, VlX\^ 

 produced a thousand modifications corresponding in degree, though so differ- 

 ent in kind. As the case stands, we are totally in the dark. We had best, 

 perhaps, content ourselves with bare statement of the fact that a certain per 

 cent, oi M'jiothi have proven susceptible to some special unknown influences 

 and have consequently undergone certain modifications that the rest, though 

 similarly exposed, have successfully resisted. If we go further, it must be 

 upon speculative grounds. We may conjecture that these two races are form- 

 ing species : that they began to be differentiated within the last few thousand 

 years, more or less; and that in the process of time they will either become 

 jiermanently distinct, the differences that they have become possessed of being 

 of advantage to them ; or that they will eventually revert to an original stand- 

 ard, such differences proving useless. A supposition as little likely to be sub- 

 stantiated as refuted, except by analogical reasoning. 



At present, ornithologists are very properly indisposed to look upon rufescens 

 and holboelli as anything more than " varieties " of linarius, and as not even 

 very satisfactorily defined varieties. But I hold it to be demonstrable that 

 the characters that separate these birds from linarius are of the same kind 

 (though of different degree, or intensity, so to speak) as those distinguishing 

 the Greenland Red-polls from linarius. The characters of all the ^'Egiothi, once 

 reduced, as they can be, to the same level, and shown to differ only by varying 

 force of expression, I see no means of distinguishing any set of the birds from 

 the rest, as species. Intermediate links of the chain are easily found, — links 

 that bind the whole so firmly that there is no break in the series between the 



1869.J 13 



