Darwin's Theory of Sexual Selection 207 
is not odoriferous.” Darwin believes in these cases that the 
odor serves to attract the females. He admits that here, 
“active and long-continued use cannot have come into 
play as in the case of the vocal organs.” He concludes, 
therefore, that “the odor emitted must be of considerable 
importance to the male, inasmuch as large and complex 
glands, furnished with muscles for everting the sac, and for 
closing or opening the orifice, have in some cases been 
developed. The development of these organs is intelligible 
through sexual selection, if the most odoriferous males are 
the most successful in winning the females, and in leaving 
offspring to inherit their gradually perfected glands and 
colors.” 
There is sometimes a difference in the mammals in the 
hair of the two sexes both in amount and in color. In some 
species of goats the males have a beard, in others it is 
present in both sexes. The bull, but not the cow, has curly 
hair on the forehead.’ In some monkeys the beard is con- 
fined to the male, as in the orang; in other species it is only 
larger in the males. 
“The males of various members of the ox family (Bovida), 
and of certain antelopes, are furnished with a dewlap, or 
great fold of skin on the neck, which is much less developed 
in the female. 
“Now, what must we conclude with respect to such sexual 
differences as these? No one will pretend that the beards 
of certain male goats, or the dewlap of the bull, or the crests 
of hair along the backs of certain male antelopes, are of any 
use to them in their ordinary habits. 
“Must we attribute all these appendages of hair or skin to 
mere purposeless variability in the male? It cannot be 
denied that this is possible ; for in many domesticated quad- 
rupeds, certain characters, apparently not derived through 
reversion from any wild parent form, are confined to the 
males, or are more developed in them than in the females 
