THE AMERICAN NATURALIST [Vol. XLVII1 



With many birds the characteristic coloration may not 

 be at all that of its pigment. Thus the blue of the male 

 indigo bird (Passerina cyanea) is due solely to the phys- 

 ical structure of its feathers which though pigmented 

 with brown, appear blue by reflected light. If, however, 

 a blue feather be immersed in oil and viewed under a 

 microscope by transmitted light, it is seen to be brown- 

 pigmented. The physical feather-structure of the adult 

 male is thus in this species a secondary sexual character 

 chiefly developed during the breeding period. 



The important point at present is, however, that the 

 color effects just described are none the less due to pig- 

 ment, quite apart from the fact that the apparent color 

 of the pigmented area may be different from the actual 

 color of the pigment (except that iridescence may some- 

 times be faintly seen in an unpigmented feather). 



The use of pigmentation to its possessor is a matter 

 still under discussion and investigation. In many cases 

 it is doubtless the result of purely physical causes and 

 it is quite without the power of the animal to make use 

 of its coloration for outward effect. Thus the beautiful 

 colors inside the shells of some molluscs are never appar- 

 ent from an exterior view, and are supposed by some to 

 be in part a waste product, the result of metabolism 

 within the organism. 



The present discussion has to do only with the external 

 pigmentation of the hair and feathers, respectively, in 

 mammals and birds. 



The simplest cases of coloration are those in which the 

 body or its covering is everywhere of the same hue, or 

 nearly so — as in the elephant, the wild buffalo, or the 

 house mouse in which the hairy covering (or hide in the 

 elephant) is of a nearly uniform tone everywhere. So 

 too, the crow, the apteryx, and the nestlings of many 

 birds whose parents show a more highly differentiated 

 style of markings. Such mammals and birds, so far as 

 the development of pattern is concerned, I would con- 

 sider unspecialized, yet it does not follow that in this 



