232 THE EVOLUTION OF SEX. 



Green, consists chiefly of a substance akin to mucin, is used to form 

 the snow-white fibrous nest. 



Take only one other instance of peculiar secretion curiously linked 

 to the above by one of those profound physiological unities which 

 show how superficial after all are the utmost contrasts of organic form, 

 — we refer to the viscid threads with which the male stickleback 

 weaves his nest. Mobius has shown that the kidneys are greatly 

 affected by the mature testes; that they produce, by a now normal 

 pathological process, special waste or katabolic elements, in the form 

 of mucous threads. The male gets rid of this uneasy encumbrance 

 (which has a somewhat parallel pathological equivalent in higher ani- 

 mals), by rubbing itself against objects, and thus almost mechanically 

 has been evolved the familiar weaving of the aquatic nest. 



1 ig 88— The nest of the Stickleback (Gasterosteus).— From Thomas Bolton. 



IX. Incubation. — The physiological sacrifice of the female birds 

 does not end with providing the large capital of nutritive material with 

 which the germ is endowed, but is continued in all the patience of 

 brooding. In passerine birds the male relieves the female in her task 

 of love, and in the ostrich tribe takes the duty usually upon himself. 

 In the cuckoos and cow-birds the parental care is shirked, and with 

 varying degrees of deliberateness the eggs are foisted into foster-nests, 

 and the young thus put out to nurse. After the fatigue of reproduc- 

 tion it is perhaps natural enough that the female should rest a while 

 upon the eggs in the shelter of the nest; and since there is observed to 

 be an increased circulation in the skin of the abdominal region at this 

 time, it has been argued that the bird merely sits to cool itself! This 

 view has been supported by the cruel experiment of singeing off the 

 feathers from the same region in a cock, which then sat to cool the 



