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 strue- 
ture may likewise exist, but only to produce a macroscopic appear- 
ance which is a secondary sexual character, e. @., elongated plumes, 
crests, color effect, ete. 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 eases 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 
ease 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- 
