EIDGWAY ON AMERICAN HERODIONES. 233 



In the case of other sorts of dichromatism (that is, where other colors 

 than white are substitated for the uormal dress), it is well known that 

 the difference between the extreme phases varies greatly in degree 

 among species of the same genus, or genera of the same family. Thus, 

 among Owls (in which family the condition known as "erythrism"* is 

 most developed), Glaucidium gnoma varies from brownish-gray to deep 

 sepia- or umber-brown, the pattern remaining distinct, while G. fer- 

 rugineum, with a very similar normal dress, has its rufescent extreme 

 a very bright brick-red color, with the markings almost if not quite ob- 

 literated; 8yrriium aluco of Europe has also its grayish (normal) and 

 rufescent (erythrismal) phases about equally marked, as has also the 

 American 8co]^s asio; but none of the American species of Syrnium (of 

 which there is a considerable number) tend to erythrism, nor does the 

 European Scoj^s [8. zorca). The same is also the case with the American 

 Falconine genus Micrastur (one of the very few FalconidcB iu which this 

 variation presents itself), one species {M. rujicollis) having the two ex- 

 tremes almost as strikingly different as in the Owls above named, while 

 in another (M. concentricus) there is not the slightest tendency to ery- 

 thrism, — other species being variously intermediate. From what is 

 known of Scoops asio and other Owls, it is also evident that the presence 

 or absence of erythrism has more or less of a geographical significance, 

 this species being never rufous, so far as known, in any part of the West- 

 ern Province of the United States, while this bright rufous plumage is the 

 rule in the Eastern States, particularly to the southward.! It is also a 

 fact to be borne in mind that although the extreme phases characterize 

 a very large majority of the individuals of a species, intermediate speci- 

 mens are by no means wanting; they are, however, the rare exception.!: 



It may be further stated that, as the condition of melanotic dichromat- 

 ism § is subject to precisely the same rules as that of erythrism, it is 

 unnecessary to further extend the discussion of that subject. But, as a 

 matter exceedingly pertinent to the relationship between Ardea occiden- 



* Conf. August von Pelzeln in '■ Novara-Expedition, Zoologisclier Theil, Bd.I: 

 Vogel", pp. 14-25, where various color-variations are discussed under bead of " tJber 

 Farbenabiinderungen bei den Falconiden ". 



tin the Austro-riparian region (including, besides the Gulf States, the lower Missis- 

 sippi Valley to Southern Illinois and Indiana), the proportion of red to gray individuals 

 of this species is at least as 90 to 100; or, in other words, ninety of every one hundred 

 specimens represent the rufous phase ; taking into account with this fact the apparent 

 total absence of this plumage among the Western birds, the geographical signitication 

 becomes very evident. 



X Dr. Brewer has, therefore, erred slightly in saying that "There are no mixtures. 

 They are either entirely the one or the other." 



§ In birds, the conditions which I propose to term melanotic, alb notic, and ery- 

 thrismal dichromatism are of rather limited applicaion ; the first being especially 

 characteristic of the Falconidce and Frocellariidoe, the second of the Ardeida, and the 

 third of the Sirigidce. Both of the former are to be distinguished from those accidental 

 abnormalities, true melanism and albinism, which are of only occasional occurrence, 

 and connected with some physiological derangement. 



