Sexual Selection 45 
Many lower Crustaceans (Daphnidae) have better developed organs 
of smell in the male sex. The difference is often slight and amounts 
only to one or two olfactory filaments, but certain species show a 
difference of nearly a hundred of these filaments (Leptodora). The 
same thing occurs among insects. 
We must briefly consider the clasping or grasping organs which 
have developed in the males among many lower Crustaceans, but- 
here natural selection plays its part along with sexual selection, for 
the union of the sexes is an indispensable condition for the main- 
tenance of the species, and as Darwin himself pointed out, in many 
cases the two forms of selection merge into each other. This fact. 
has always seemed to me to be a proof of natural selection, for, in 
regard to sexual selection, it is quite obvious that the victory of the 
best-equipped could have brought about the improvement only of 
the organs concerned, the factors in the struggle, such as the eye and 
the olfactory organ. 
We come now to the excitants; that is, to the group of sexual 
characters whose origin through processes of selection has been most 
frequently called in question. We may cite the love-calls produced 
by many male insects, such as crickets and cicadas. These could only 
have arisen in animal groups in which the female did not rapidly flee 
from the male, but was inclined to accept his wooing from the first. 
Thus, notes like the chirping of the male cricket serve to entice the 
females. At first they were merely the signal which showed the 
presence of a male in the neighbourhood, and the female was 
gradually enticed nearer and nearer by the continued chirping. The 
male that could make himself heard to the greatest distance would 
obtain the largest following, and would transmit the beginnings, 
and, later, the improvement of his voice to the greatest number of 
descendants. But sexual excitement in the female became associated, 
with the hearing of the love-call, and then the sound-producing organ : 
of the male began to improve, until it attained to the emission of the 
long-drawn-out soft notes of the mole-cricket or the maenad-like cry 
of the cicadas. I cannot here follow the process of development in 
detail, but will call attention to the fact that the original purpose of 
the voice, the announcing of the male’s presence, became subsidiary, 
and the exciting of the female became the chief goal to be aimed 
at. The loudest singers awakened the strongest excitement, and the 
improvement resulted as a matter of course. I conceive of the origin 
of bird-song in a somewhat similar manner, first as a means of en- 
ticing, then of exciting the female. 
One more kind of secondary sexual character must here be 
mentioned: the odour, which emanates from so many animals at the 
breeding season. It is possible that this odour also served at first 
