28 THE PHYSIOLOGY OF REPRODUCTION 



not directly associated with excessive sexual vigour. There is only 

 one moult annually and no pre-nuptial moult, but a progressive 

 increase in coloration up to and beyond the breeding season. 1 



In some animals certain glandular organs, apart from those 

 concerned in the reproductive processes, show a special activity at 

 the breeding season. For example, in the swiftlets (Collocalia) the 

 salivary glands become peculiarly active, and secrete a substance 

 which is allied to mucin, and is employed in building the edible 

 birds' nests of Chinese epicures. 2 



A somewhat similar peculiarity exists in the male of the sea- 

 stickleback (Gusterosteus spinachia), which binds together the weeds 

 forming its nest by means of a whitish thread, secreted by the 

 kidneys, and produced only during the breeding season. According 

 to Mobius, as quoted by Geddes and Thomson, 3 the secretion is 

 semi- pathological in nature, being caused by the mechanical pressure 

 of the enlarged testes upon the kidneys. The male gets rid of the 

 thread-like secretion by rubbing itself against objects, and thus, by 

 an almost mechanical process, the weaving habit is supposed to have 

 become evolved. 



During the breeding season the anal scent-glands of snakes are 

 said to be actively functional, but not at other times. A similar 

 fact is stated about the submaxillary glands of crocodiles, and the 

 cloacal glands of tortoises and other reptiles. 4 The secretions of 

 these glands, like the musk glands of Mammals, no doubt serve the 

 purpose of enabHng the sexes to detect one another's presence more 

 easily. (See p. 253.) 



PEEIODICITY 



The periodicity which is such a marked feature of animal life in 

 temperate climates has been discussed at some length by Semper. 5 

 This author concludes that the phenomenon in question is dependent 

 on the severe extremes of summer and winter temperature to which 

 the animals are exposed. "Every individual requires a certain 

 duration of life to achieve its individual development from the egg 

 to sexual maturity and full growth ; the length of time requisite for 



1 The assumption of plumage by birds (ducks, bullfinches, etc.) in the 

 autumn and long before the breeding season may be due to the time of moult. 

 Patten has shown that in the sanderling there is a pre-nuptial plumage closely 

 resembling the plumage of the sexually mature bird but preceding the 

 enlargement of the gonads. (" The Vernal-Plumage Changes in the Adolescent 

 Blackbird and their Correlation with Sexual Maturity," Brit. Assoc. Report, 

 1911.) 



2 Geddes and Thomson, Evolution of Sex, Revised Edition, London, 1901. 



3 Geddes and Thomson, loc. cit. 



4 Owen, Anatomy of Vertebrates, vol. i., London, 1866. Lay cock, Nervous 

 Diseases of Women, London, 1840. 



6 Semper, Animal Life, London, 1881. 





