THE BREEDING SEASON 19 



accumulated in them. It should be noted, however, " that the 

 gain of solids by the genitalia is small as compared with the loss 

 of sohds by the muscle, that in fact the greater part of the 

 sohds lost from the muscles are used up for some other purpose 

 than the building up of the genitaha." ^ Pa ton concludes that 

 the state of nutrition is the main factor determining migration 

 towards the river, and that, when the sahnon has accumulated 

 a sufficiently large store of material, it returns to the rivers 

 which were its original habitat. It does not seem possible, 

 however, to maintain that nutrition is a determining influence 

 in the growth of the genital glands, since these are undeveloped 

 when the fish begin to migrate and enter upon their period of 

 starvation. 



Wiltshire ^ states that in some fishes, at the period of ovi- 

 position, the lips of the genital orifice swell and become congested. 

 This condition he regards as comparable to that which occurs 

 during the " heat " period of a mammal. 



Amphibia 



The intimate connection between sexual periodicity and 

 chmatic variation exhibited by many Amphibia and Reptiha, 

 especially in temperate climates, was commented on by Spallan- 

 zani.^ This close dependence upon environmental conditions is 

 evidently due largely to the habits of hfe of these animals, many 

 of which hibernate or show great sluggishness in cold weather ; 

 while among Amphibia it must be associated with the further 

 fact that, whereas most members of the group hve to a great 

 extent upon land, it is necessary for them to deposit their eggs 

 in water. Spallanzani concludes that the reason why Amphibia 

 are subject to a variation which is not observable in birds and 

 Mammals is because the former, hke insects, are cold-blooded, 

 and have a comparatively small supply of internal heat to 



' Paton, loc. cit. Milroy (" Chemical Changes in the Muscles of the Herring 

 daring Eeproduotive Activity," Seventh International Congress of Physiolo- 

 gists, Heidelberg, 1907; abstract in Zeit. f. Phys., vol. xxi., 1907; and 

 Biochcm. Jour., vol. iii., 1908) has recently shown that similar changes take 

 place in the herring, in which, however, the starvation period is briefer. 



' Wiltshire, " The Comparative Physiology of Menstruation," Brit. Med. 

 Jour., 1883. 



' Spallanzani, Dissertations, vol. ii., London, 1784. 



