230 THE EVOLUTION THEORY 



■Gloii and Potamanthus, which were long ago described by Pictet, the 

 monographer of this family. These are large turban-shaped com- 

 pound eyes, occurring beside the ordinary eyes in the males alone, 

 which in these genera are in a majority of sixty to one. Whole 

 swarms of these males fly about over the water on the search for 

 females, and their highly developed organ of vision seems to decide 

 matters for them just as the organ of smell does for Leptodora. 

 Neither of these sense-organs can have any other advantage than 

 that of making their possessors aware of the female, for the whole 

 activity of the short-lived adult Ephemerides is limited to repro- 

 duction ; they take no food, and have nothing whatever to do except 

 to reproduce. 



Finally, when in an enormous number of eases we find in 

 addition to one or the other of the already mentioned male dis- 

 tinguishing characters some which do not directly lead to gaining 

 possession of the female, but do so only by sexually exciting her, can 

 we doubt that the same principle has been operative, that here too 

 processes of selection are fundamental, depending on the fact that 

 in the wooing of the female the successful male is the one who most 

 strongly excites her ? There is no question of sesthetic pleasure in 

 this, as the opponents of the theory of sexual selection have often 

 urged, but only of sexual excitement, which may be aroused by very 

 different means, by colours and shapes, but also by love-calls, songs, 

 or odours. There are a few tropical birds {Chasmorhynchus) which 

 have as the only distinguishing character of the male sex a hollow 

 and soft appendage several inches long borne on the head. Usually 

 it hangs down limply at the side of the head, but during the breeding 

 season it is inflated from the mouth-cavity, and then stands erect like 

 a spur. One species of this genus has as many as three of these 

 horns, one of which is upright, while the other, two stand out laterally 

 from the head. Can it be supposed that these remarkable horns 

 satisfy the female's ' sense of beauty ' ? To human beings they 

 appear rather ugly than beautiful, both when limp and when inflated, 

 but at any rate they are striking, and will be regarded by the female 

 bird as something out of the common, and, since they are only 

 fully displayed during the breeding season, that is, when the male 

 is sexually excited, they will have an exciting effect on the female 

 too. These inflated horns are symptoms of excitement, and they arouse 

 it in the female. In exactly the same way the decorative feathers, the 

 ruby -red and emerald-green feather collars of the humming-birds and 

 birds of Paradise, are only erected and displayed when the males are 

 wooing, and they, too, act as signs of excitement. This is not to say 



