Darwin's Theory of Sexual Selection 203 
being savage, quarrelsome, and noisy, the males gentle and 
good. The case is very different with the African ostrich, 
for the male is somewhat larger than the female and has 
finer plumes with more strongly contrasted colors; neverthe- 
less he undertakes the whole duty of incubation.” 
Darwin attempts to explain these reversals of instincts on 
the assumption that the males have turned the tables on the 
females, and have themselves done the selecting; and inci- 
dentally, it may be pointed out in passing, they have had to 
pay the penalty by incubating the eggs. 
In the group of mammals, Darwin thinks that the male wins 
the female by conquering other males rather than by charming 
her through his display. The males, even when unarmed, 
engage in desperate conflicts with each other, and sometimes 
kill, but more often only wound, their fellows. The second- 
ary sexual characters of the males have been acquired, 
therefore, by natural selection applied to one sex, and less 
frequently through the choice of the female. Since we are 
here more especially concerned with the latter class of 
phenomena, we may examine only a few cases under the 
first head. 
The horns of stags are used by them in their conflicts 
with each other; the tusks of the elephant make this animal 
the most dangerous in the world, when in must. The horns 
of bulls, the canine teeth of many mammals, the tusks of the 
walrus, are further examples of organs which have been, 
according to Darwin, acquired through the competitions of 
the males with each other. 
The voices of mammals are used for various purposes, “as 
a signal of danger, as a call from one member of the troup to 
another, and from the mother to her lost offspring, or from 
the latter for protection.” 
« Almost all male animals use their voices much more dur- 
ing the rutting season than at any other time ; and some, as 
the giraffe and porcupine, are said to be completely mute 
