20 THE PHYSIOLOGY OF REPRODUCTION 



animate them when it is cold. " As therefore the exercise of 

 their functions depends on the heat of the atmosphere, their 

 amours will also depend upon this cause, and will, of course, be 

 later in cold than in hot climates, and in both will vary with 

 the season." 



Spallanzani illustrates the truth of this fact by pointing out 

 that various species of frogs and toads begin to propagate 

 earlier in Italy than in Germany or Switzerland. 1 On the other 

 hand he records the observation that the tree-frog and the fetid 

 terrestrial toad were copulating in the ponds and reservoirs of 

 Geneva in March, at a time when in Lombardy they had not 

 yet quitted their subterranean abodes. 



It is interesting to note that in the frog and other Amphibia 

 the ova are produced in winter, when the animals eat little or 

 nothing, just as the genital organs of the salmon develop during 

 the period of migration, when the fish have practically ceased 

 to feed. 



Bles 2 has discussed at some length the conditions under 

 which it is possible to induce various species of Amphibia to 

 breed in captivity. He states that the most necessary con- 

 dition is that the animals should be allowed to hibernate at 

 the proper season, and in order to accomplish this they must 

 be in thoroughly good health when the winter sets in, having 

 passed the summer in the best circumstances in regard to light, 

 heat, and supply of food. Bles's observations relate more 

 especially to the African frog, Xenopus kevis, but he believes 

 his conclusions to apply in a large degree to many other species 

 of Amphibia. 



The frogs in question were kept in a " tropical aquarium " 

 (that is to say, an aquarium which could be kept at a tropical 

 temperature by regulating a heating apparatus). In the summer 

 the temperature was maintained at about 25 C. ; in December 



1 In the common frog (Rana temporaria) the usual time for spawning in 

 Middle Europe is March, earlier in warm, later in cold seasons; in southern 

 countries, January or February, but in Norway not until May. Vide Gadow, 

 Cninlirldfjc Natural History, vol. viii., London, 1901. This book contains a 

 quantity of valuable information concerning the breeding habits of many 

 Amphibia and reptiles. 



* Bles, "The Life-History of Xenopus la-vis," Trans. Roy. Sot. Edin., 

 vol. xli., 1905. 



