THE BREEDING SEASON 21 



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, 1 but the significance of the changes 

 is not very apparent. 



KEPTILIA 



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 recur- 

 rent breeding seasons, in some cases extending over many months, 

 generally in the spring and summer. 2 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 familiar 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 3 ) by the supply of suitable food. 

 However, in the domestic fowl the production of eggs is mainly 

 influenced by the season of the year, the maximum production 

 taking place in the- months of March and April, and the minimum 

 in October and November. In very high producing strains Pearl 

 and Surface 4 found that there is an additional egg-laying season 

 in the autumn months. The time of maximum production is 

 influenced by climatic conditions. Thus Buckley 5 found in the 

 South of England that this occurred in March and April, whereas 

 in places where the spring is later the maximum is not attained 



1 MacLean, "The Action of Muscarin and Pilocarpin on the Heart of 

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

 vol. iii., 1908. 



2 See Gadow, loc. cit. 



3 Wright, The New Book of Poultry, London, 1902. 



4 Pearl and Surface, 7.$. Dep. Agric. Bureau, Animal Industry, Bull. 110, 

 1911. 



5 Buckley, Farm Records and the Production of Clean Milk at Monadsmere. 



