THE BREEDING SEASON 31 



the sexual phenomena in the lower animals is essential, while, as 

 already remarked, sufficient data for a comprehensive treatment of 

 this subject do not at present exist. 



That the breeding season occurs in some animals " as the result 

 of a stimulus which may be effected through the alimentary canal is 

 demonstrated by the effect upon ewes of certain stimulating foods." 



"That it is associated with a stimulus which is manifested by 

 exceptional vigour and exceptional bodily ' condition ' is demonstrated 

 by the pugnacity of the males at such times, by the restless activity 

 of the females, by the brilliant colouring of such widely divergent 

 animals as, for instance, annelids, amphibia, birds, and mammals, by 

 the condition of the plumage of birds, and of the pelage or skin of 

 mammals." 



" That it is [frequently] associated with nutrition, and that it is 

 a stimulus gradually collected is indicated by the increased frequency 

 of the [breeding] season among domesticated mammals as compared 

 with nearly allied species in .the wild state. 



" That it is manifested by hypertrophy and by congestion of the 

 mucous tissue of the generative organs, and of various other organs, 

 such as the wattles and combs of birds, the crest of the newt, and by 

 the activity of special glands, the affection of all of which may be 

 exceedingly severe, is true. 



" These, and many other similar facts, are well known, but they 

 do not assist in the elucidation of the origin of the function. 



" The most they do is to show that the sexual instinct is seasonal, 

 and that nutrition, whether affected by external or internal factors, 

 plays an important part in its manifestation." l 



The last proposition may be expressed even more generally in the 

 statement, already formulated, that generative activity in animals 

 occurs only as a result of definite stimuli, which are partly external 

 and partly internal ; while the precise nature of the necessary 

 stimuli varies considerably in the different kinds of animals, 

 according to the species, and still more according to the group to 

 which the species belong. 2 



1 Heape, loc. cit. It should be remembered, however, that many animals, 

 such as the salmon, have their breeding season after prolonged fasts. See 

 above. Cf. also the fur-seal, p. 54. 



2 For a number of illustrations of periodicity in generative activity among 

 animals inhabiting the sea-shore, and the tendency to modify chapters of the 

 normal life-history in accordance with special needs, or in response to peculiar 

 environmental conditions, see Flattely and Walton, The Biology of the Sea-Shore, 

 London, 1922. 



