1916] Chandler: Structure of Feathers 281 



come under the head of pathogenic conditions, and need no further 

 consideration here. 



•Age, seasonal, and sexual variations in feathers occur only when 

 needed for the production of a special result. Just as the greater 

 coverts of a bird may differ in microscopic structure from the middle 

 coverts in order to produce a different macroscopic effect, so in some 

 birds certain of the feathers in the spring plumage may differ from 

 the corresponding ones in the fall plumage, in order to bring about 

 a different total effect. Sexual differences in minute feather struc- 

 ture may likewise exist, but only to produce a macroscopic appear- 

 ance which is a secondary sexual character, e. g., elongated plumes, 

 crests, color effect, etc. Differences in microscopic feather structure 

 are not, in themselves, secondary sexual characters, but are merely 

 employed in the production of more obvious secondary characters. 

 In a few cases variations in feather structure are employed to pro- 

 duce different effects in different ages, even though in the same 

 seasonal dress, as for instance in many of the orioles, whose plumage 

 pattern in the spring of the second year differs from that of the 

 third year. Such changes are rare and occur only in the first few 

 years; when the ultimate adult plumage is attained, no further age 

 variations occur. 



As a rule, there are no considerable variations in the feather 

 structure of different species of the same genus, except, as in the 

 case of intraspecific variations, when instrumental in the production 

 of some larger specific difference. Species, of course, are by no 

 means of equal rank, and in subspecies or in slightly differentiated 

 species feather structure, per se, cannot be used as a taxonomic char- 

 acter, although very slight differences in similar feathers do some- 

 times exist in widely different species of a single genus. Moreover, 

 generic differences in feather structure may usually be passed over, 

 since they are ordinarily so slight that they cannot positively be 

 distinguished at all, or only with intensive study, and then only in 

 case it is certain that the portion examined comes from an exactly 

 similar part of a corresponding feather. 



In all groups higher than genera, however, epiphyologic differ- 

 ences may almost always be detected. In other words, it is usually 

 possible to distinguish, by details of feather structure, any feather 

 of a specimen of a given family from any approximately similar 

 feather of a specimen of another family, even if in the same sub- 

 order. The amount of differentiation, however, is extremely vari- 



