418 
SEXUAL SELECTION. 
Part II. 
of which is not understood. The sexes, also, often differ 
in their organs of sense or locomotion, so that the males 
may quickly discover or reach the females, and still 
oftener in the males possessing diversified contrivances 
for retaining the females when found. But we are not 
here much concerned with sexual differences of these 
kinds. 
In almost all the Orders, the males of some species, 
even of weak and delicate kinds, are known to be highly 
pugnacious ; and some few are furnished with special 
weapons for fighting with their rivals. But the law of 
battle does not prevail nearly so widely with insects as 
with the higher animals. Hence probably it is that the 
males have not often been rendered larger and stronger 
than the females. On the contrary they are usually 
smaller, in order that they may be developed within a 
shorter time, so as to be ready in large numbers for the 
emergence of the females. 
In two families of the Homoptera the males alone 
possess, in an efficient state, organs which may be called 
vocal ; and in three families of the Orthoptera the males 
alone possess stridulating organs. In both cases these 
organs are incessantly used during the breeding-season, 
not only for calling the females, but for charming or 
exciting them in rivalry with other males. No one 
who admits the agency of natural selection, will dispute 
that these musical instruments have been acquired 
through sexual selection. In four other Orders the 
members of one sex, or more commonly of both sexes, 
. are provided with organs for producing various sounds, 
which apparently serve merely as call-notes. Even 
when both sexes are thus provided, the individuals 
which were able to make the loudest or most continuous 
noise would gain partners before those which were less 
noisy, so that their organs have probably been gained 
