INTRODUCTION. 7 



significance in the animal economy we are quite ignorant. By this term we designate 

 the peculiarity in certain species of birds, that individuals present t\vo different styles 

 of coloration, or ' phases,' presumably more or less independent of geographical dis- 

 tribution, present or past, or, in fact, of any apparent cause whatsoever. The difficulty 

 in finding a plausible theory is much increased by the circumstance that there are 

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

 mention a few examples. It has been known that the so-called Richardson's jjcger 

 (Stercorarius parasiticus) appears in two different styles, one uniformly sooty all 

 over, the other with the whole under side white. At one time they were regarded as 

 different species, while some observers thought that the difference was a sexual one ; 

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

 individual phases of the same species, irrespective of sex or locality. It is interesting 

 to remark that the closely allied species 8. longicaudus has only one, the light phase. 

 The relation between the common and the spectacled murre ( Uria troile and ringvici) 

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

 ocular stripe 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 plain-colored one is not found. A more striking and also more 

 puzzling example of dichromatism is exhibited by several members of the heron 

 family, a question which has been particularly studied by Mr. R. Ridgway. Already 

 Peale's egret and Wtirdeman's heron have disappeared, as separate species, 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 ritfa, the generic name of which 

 has been given according to this view) ; for, according to Ridgway, in Florida, 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 plumage, 

 other circumstances at the same time leading to the conclusion that the two phases 

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

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

 vincing; for here the white phase is seldom, if ever, perfectly developed in the adults, 

 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 the different forms, 

 which may be distinguished thus: 



/ Ardea occidentalis, white all over. 

 Legs olive; size larger, ) Ardea wurdemanni, parti-colored; occiput and plumes white. 



( Ardea wardi. ) 



T 11 i 11 f parti-colored; occipital streak and plumes black. 



Legs black; size smaller, Ardea herodias, ) * 



No white phase of herodias is as yet known, which seems rather strange when we 

 consider that Ardea wardi, which is almost an exact counterpart of A. herodias, 

 except in the coloration of the legs and the size, is matched so absolutely by A. occi- 

 dentalis, 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. wurdemanni, and we might be led to suppose a kind of trichromatism, the white 

 occidentalis with two different colored phases, were it not for the fact that the type 

 specimen of A. wurdemanni is still unique, and therefore most probably nothing more 

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



