SEXUAL SELECTION. 235 



glands, it is scarcely possible to decide which 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 sacks of 

 the marsupials. In some few cases also the male possesses 

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

 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 defense of the larvae and the community. 

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

 concern us. There are, however, other sexual differences 

 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 offense or means of defense against rivals, 

 his gaudy coloring aud 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 

 animals differ in structures related to different habits 

 01 life, and not at all, or only indirectly, to the repro- 

 ductive functions. Thus the females of certain flies/ 

 (Culicidae and Tabanidae) are blood-suckers, while the* 

 males, living on flowers, have mouths destitute of mandii 

 bles.* The males of certain moths and of some crustaceans 

 (e. g. Tanais) have imperfect, closed mouths, and cannot 

 feed. The complemental males of certain Cirripedes live 

 like epiphytic plants either on the female or the hermaph- 

 rodite form, and are destitute of a mouth and of prehensile 

 limbs. In these cases it is the male which has been modi- 

 fied 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-worm is des 

 titute of wings, as also are many female moths, some of 

 which never leave their cocoons. Many female parasitic 

 crustaceans have lost their natatory legs. In some weevil- 



* West wood, "Modern Class of Insects," vol. ii, 1840, p. 541. 

 For the statement about Tanais, mentioned below, I am indebted to 

 Fritz Miiller. 



