THE BREEDING SEASON 23 



of the shoulder-girdle bearing the four limbs, together with the 

 connected skin and muscles, and the three upper segments of 

 the spinal cord, are cut out from the male frog during the breed- 

 ing season (but not at other times), the irritation of the skin 

 wiU cause a reflex, clasping movement, similar to that char- 

 acteristic of the normal male at this period. In spring and 

 early summer, after reviving from their winter sleep, frogs tend 

 to be irregular in certain other of their reflex responses. MacLean 

 has shown that in the heart of the frog, newt, and salamander, 

 and also the eel, vagus inhibition is absent or markedly diminished 

 at certain periods corresponding roughly to the seasons of sexual 

 activity,* but the significance of the changes is not very apparent. 



Reptilia 



Reptiles which hibernate usually begin to breed shortly 

 after the commencement of the warm weather which terminates 

 the hibernating period, just as in the case of Amphibia. Other 

 reptiles, which live in warm or tropical climates, also have 

 regularly recurrent breeding seasons, in some cases extending 

 over many months, generally in the spring and summer.^ It 

 would seem that in reptiles also, breeding only occurs in 

 response to certain external stimuli, and that temperature is 

 the main factor, as supposed by Spallanzani. 



AVES 



It would appear almost superfluous to cite examples of 

 sexual periodicity from among birds. That spring and summer 

 are the seasons when most birds pair, build their nests, and 

 incubate their eggs, and that these processes are wont to vary 

 slightly with the character of the season, are facts that are 

 famihar to all. Bird-fanciers know also that the capacity of 

 certain birds for egg-laying may be influenced by diet, and that 

 this capacity can sometimes be increased (e.g. in the common 

 fowl ') by the supply of suitable food. 



' MacLean, "The Action of Mnsoarin and Pilooarpin on the Heart of 

 certain Vertebrates, with Observations on Sexual Changes," Biochem. Journal, 

 vol. iii., 1908. 



^ See Gadow, loc. cit. 



" Wright, TJie Kew Book of PouUry, London, 1902. 



