34 THE PHYSIOLOGY OF REPRODUCTION 



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. 



:c 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." 1 

 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. 



1 Heape, loc. cit. It should be remembei'ed, however, that many animals, 

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

 above. (Cf. also the fur-seal, p. 60.) 



