SEXUAL SELECTION 275 



glands, it is scarcely possible to decide whicli ought to be 

 called primary and which secondary. 



The female often differs from the male in having organs 

 for the nourishment or protection of her young, such as the 

 mammary glands of mammals, and the abdominal sacs of 

 the marsupials. In some few cases also the male possesses 

 similar organs, which are wantiag in the female, such as the 

 receptacles for the ova in certain male fishes, and those 

 temporarily developed in certain male frogs. The females 

 of most bees are provided with a special apparatus for col- 

 lecting and carrying pollen, and their ovipositor is modified 

 into a sting for the defence of the larvss and the community. 

 Many similar cases could be given, but they do not here 

 concern us. There are, however, other sexual difiEerences 

 quite unconnected with the primary reproductive organs, 

 and it is with these that we are more especially concerned — 

 such as the greater size, strength, and pugnacity of the male, 

 his weapons of offence or means of defence against rivals, 

 his gaudy coloring and various ornaments, his power of 

 song, and other such characters. 



Besides the primary and secondary sexual differences, 

 such as the foregoing, the males and females of some ani- 

 mals differ in structures related to different habits of life, and 

 not at all, or only indirectly, to the reproductive functions. 

 Thus the females of certain flies (Culicidse and Tabanidse) 

 are blood-suckers, while the males, living on flowers, have 

 mouths destitute of mandibles.' The males of certain moths 

 and of some crustaceans {e.g., Tanais) have imperfect, closed 

 mouths, and cannot feed. The complemental males of cer- 

 tain Cirripeds live like epiphytic plants either on the female 

 or the hermaphrodite form, and are destitute of a mouth and 

 of prehensile limbs. In these cases it is the male which 

 has been modified, and has lost certain important organs 

 which the females possess. In other cases it is the female 

 which has lost such parts; for instance, the female glow- 



• Westwood, "Modem Class, of Insects," vol. ii., 1840, p. 541. For tha 

 Btatement about Tanais, mentioned below, I am indebted to Frita MuUer. 



