INTRODUCTION. J 



sisxnificancc in the animal economy we are <niitc if;iiorant. By this ti-rm wo <U'si>^iiate 

 tiiu jioculiaiity in certain sjiccics of Kirds, tiiat in<ii\ iiliials jiresuiit two clil'tViviit slylfs 

 of coloration, or ' phases,' presumably more or less iude|)enilent of guograi)hical dis- 

 tribution, present or past, or, in fact, of any a])parent cause wiiatsoever. Tiie ditliculty 

 in finding a plausible tlieory is much increased by the circumstance that tliero are 

 nearly as many kinds of dichromatism as there are dichromatic species. We shall 

 mention a few exami>Ies. It has been known that the so-called Richardson's jajger 

 {Stercorarius parasiticus) appears in two different styles, one unif(jrnily sooty all 

 over, tlie other with tlic wliole under side white. At one time they were reganled as 

 different species, while some observers tliought that the difference was a se.xual one ; 

 but it is now demonstrated beyond doubt that the white and the dark bird are only 

 individual jihascs of the same si)ecies, irrespective of sex or locality. It is interesting 

 to remark liiat the closely allied sjiecies S. loinjicaudas has only one, the light i)hase. 

 The relation between the common and the spectacled murre {Uria troile and ringvia) 

 seems to be somewhat similar, the latter having a white ring round the eye and a post- 

 ocular strijie which is wanting in the former, a strong argument being the relative 

 paucity of the spectacled form, in connection with the fact that it does not occur in 

 any locality where the ]ilain-colored one is not found. A more striking and also more 

 puzzling exam]>le of dichromatism is exhibited by several members of the heron 

 family, a (piestion Mliich has been particularly studied by 'Sir. \{. Hidgway. Already 

 Peale's egret and Wiirdenian's heron have disappeared, as separate si)ecies, from the 

 lists of North American birds. It is regarded as proven that the former is only a 

 white phase of the reddish egret {Dichromanassa rufa, the generic name of which 

 has been given according to tliis view) ; for, according to Hidgway, in Floriila, where 

 they breed abundantly, both forms have been found in the same nest, attended by 

 parents either both reddish, both white, or one in each of these stages of ])lumage, 

 other circumstances at the same time leading to the conclusion th.at the two jiliases 

 are not only not sjiecifically distinct, but that they have nothing to do with either sex, 

 age, or season. In the little blue heron (Florida ca;rulea) the facts are still more con- 

 vincing; for here the white phase is seldom, if ever, i)erfectly developed in the ailults, 

 while intermediate specimens are much more numerous. The question is considerably 

 more complicated when we come to the great white and the great blue herons of this 

 country. We shall state the facts briefly, first giving a clue to tlie different forms, 

 which may be distinguished thus: — 



I Ardea occuJentnlis, white all over. 

 Legs olive; size l.-jrger, ^ Anlea iciirdemanni, parti-colored; occiput .inJ plumes white. 



,, , . „ ' r eawar t, | pjrti-colorcd ; occipital streak and plumes black. 



Legs black; size smaller, Ardea iierodia-t, ) 



Kn white phase of hcrodias is as yet known, which seems rather strange when wo 

 consider that Ardea wardi, which is almost an exact c<umter|iart of .1. /urodias, 

 excejit in the coloration of tin' legs ;md the size, is matched so absolutely by A. occi- 

 deiifalis, as far as structure is concerned, that the two could not possibly be told apart 

 if the colored bird be bleached so as to become pure white. The same may be said of 

 A. tFilrdemanni, and we might be led to suppose a kind of trichromatism, the white 

 occidental i.<i with two different colored phases, were it not for the fact that the type 

 specimen of A. irilrdcmaiini is still uuiiiue, and therefore most probably nothing more 

 than an individual variety, or an adolescent bird not having yet lost the last Inices of 



