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 



