THE BREEDING SEASON 31 



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. 1 The 

 secretions of these glands, like the musk glands of Mammals, 

 no doubt serve the purpose of enabling the sexes to detect one 

 another's presence more easily. (See p. 241.) 



The periodicity which is such a marked feature of animal 

 life in temperate climates has been discussed at some length 

 by Semper. 2 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 in- 

 dividual development from the egg to sexual maturity and full 

 growth ; the length of time requisite for this is very various, 

 and, above all, bears no proportion to the size attained. . . . 

 This length of time, which we may generally designate as the 

 period of individual growth, is not alike even for all the in- 

 dividuals of the same species ; on the contrary, it depends on 

 the co-operation of so many different factors that it must 

 necessarily vary considerably. Now, if from any cause the 

 period of individual growth, say of the salmon, became changed 

 in consequence of the slower development of the embryo in the 

 egg or of the young larvae, most or all the young salmon thus 

 affected would die in our climate, because the greater heat of 

 spring is injurious to them at that stage/' In a similar way 

 it may be argued that the periodicity of the breeding season, 

 no less than the rate of growth, is governed by the necessities of 

 the youftg. No doubt this is true to a large extent, yet at the 

 same time it is equally evident, as has been shown above in 

 numerous instances, that this periodicity is greatly affected by 



1 Owen, Anatomy of Vertebrates, vol. i., London, 1866. Laycock, Nervous 

 Diseases of Women, London, 1840. 



2 Semper, Animal Life, London. 1881 



