90 
GENERAL ORNITHOLOGY, 
seinbling more or less closely the adult males, and the young females more or less closely the 
adult females." — (Darwin, Desc. of Man, new ed., 18S1, p. 466.) 
Summary of Secondary Sexual Characters of Birds. — The temptation to give the 
conclusion of the whole matter in Darwin's own words, summary of his views of Sexual 
Selection as so important a factor in Natural Selection, need not be resisted. 1 therefore quote 
again from the work last cited, pp. 496-499. 
" Most male birds are highly pugnacious during the breeding-season, and some possess weapons adapted for 
fighting with their rivals. But the most pugnacious and the best armed males rarely or never depend for success 
solely upon their power to drive away or kill their rivals, but have special means for charming the female. With 
some it is the power of song, or of giving forth strange cries, or instrumental music, and the males in consequence 
dift'er in their vocal organs, or in the structure of certain feathers. From the curiously diversified means for pro- 
ducing various sounds, we gain a high idea of the importance of this means of courtship. Many birds endeavor to 
charm the female by love-dances or antics, performed on the ground or in the air, and sometimes at prepared places. 
But ornaments of many kinds, the most brilliant tints, combs, and wattles, beautiful plumes, elongated feathers, 
top-knots, and so forth, are by far the commonest means. In some cases mere novelty appears to have acted as a 
charm. The ornaments of the males must be highly important to them, for they have been acquired in not a few 
cases at the cost of increased danger from enemies, and even at some loss of power in fighting with their rivals. 
The males of very many species do not assume their ornamental dress until they arrive at maturity, or they assume 
it only during the breeding season, or the tints then become more vivid. Certain ornamental appendages become 
enlarged, turgid, and brightly colored during the act of courtship. The males display their charms with elaborate 
care and to the best effect ; and this is done in the presence of the females. The courtship is sometimes a pro- 
longed affair, and many males and females congregate at an appointed place. To suppose that the females do not 
appreciate the beauty of the males, is to admit that their splendid decorations, all their pomp and display, are 
useless ; and this is incredible. Birds have fine powers of discrimination, and in some few cases it can be shewn 
that they have a taste for the beautiful. The females, moreover, are known occasionally to exhibit a marked 
preference or antipathy for certain individual males. 
"If it be admitted that the females prefer, or are unconsciously excited by the more beautiful males, then 
the males would slowly but surely be rendered more and more attractive through sexual selection. That it is 
this sex which has been chiefly modified, we may infer from the fact that, in almost every genus where the sexes 
differ, the males differ much more from one another than do the females ; this is well shown in certain closely-allied 
representative species, in which the females can hardly be distinguished, whilst the males are quite distinct. Birds 
in a state of nature offer individual differences which would amply sufiice for the work of sexual selection ; but we 
have seen that they occasionally present more strongly-marked variations which recur so frequently that they 
would immediately be fixed, if they served to allure the female. The laws of variation must determine the nature 
of the initial changes and will have largely influenced the final result. The gradations, which may be observed 
between the males of allied species, indicate the nature of the steps through which they have passed. They 
explain also in the most interesting manner how certain characters have originated, such as the indented ocelli 
on the tail-feathers of the peacock and the ball and socket ocelli on the wing-feathers of the Argus pheasant. It is 
evident that the brilliant colors, top-knots, fine plumes, &c., of many male birds cannot have been acquired 
as a protection ; indeed, they sometimes lead to danger. That they are not due to the direct and definite action 
of the conditions of life, we may feel assured, because the females have been exposed to the same conditions, 
and yet often differ from the males to an extreme degree. Although it is probable that changed conditions acting 
during a lengthened period have in some cases produced a definite effect on both sexes, or sometimes on one sex 
alone, the more important result will have been an increased tendency to vary or to present more strongly marked 
individual differences : and such differences will have afforded an excellent ground-work for the action of sexual 
selection. 
" The laws of inheritance, irrespectively of selection, appear to have determined whether the characters 
acquired by the males for the sake of ornament, for producing various sounds, and for fighting together, have been 
transmitted to the males alone or to both sexes, either permanently, or periodically during certain seasons of the 
year. Why various characters should have been transmitted sometimes in one way and sometimes in another, is not 
in most cases known ; but the period of variability seems often to have been the determining cause. When the 
two sexes have inherited all characters in common, they necessarily resemble each other; but as the successive 
variations may be differently transmitted, every possible gradation may be found, even within the same genus, 
from the closest similarity to the widest dissimilarity between the sexes. With many closely-allied species, follow- 
ing nearly the same habits of life, the males have come to differ from each other chiefly through the action of 
sexual selection; whilst the females have come to differ chiefly from partaking more or less of the characters thus 
acquired by the males. The effects, moreover, of the definite action of the conditions of life, will not have been 
masked in the females, as in the males, by the accumulation through sexual selection of strongly-pronounced colors 
and other ornaments. The individuals of both sexes, however affected, will have been kept at each successive 
period nearly uniform by the free intercrossing of many individuals. 
" With species, in which the sexes differ in color, it is possible or probable that some of the successive varia- 
tions often tended to be transmitted equally to both sexes ; but that when this occurred the females were pre- 
