24 THE PHYSIOLOGY OF REPRODUCTION 



petrel and the raven are single-brooded.^ Other birds, such as the 

 sparrow, are probably often treble-brooded. It is, of course, well 

 known that domestication tends to increase the number of broods 

 which a bird may produce {e.g. in pigeons and poultry). In some 

 cases, however, domestication has had the opposite effect, e.g. those 

 breeds of poultry such as the non-sitting breeds which produce large 

 numbers of eggs but have lost the power of brooding them. Broodi- 

 ness in fowls is most frequent in spring and summer, the time of 

 greatest egg-production, and is associated with warmth.^ 



Mammalia 



The breeding season in the Mammalia, and the variations in its 

 periodicity, are discussed at some length in the next chapter. Here 

 it will suffice to point out that whereas the occurrence of breeding in 

 any one country or locality is closely connected with the climatic 

 conditions and the periodicity of the seasons in that country, this 

 rule does not hold invariably. For while the sheep in Soiith Africa 

 breeds in April and May (the South African autumn), thus following 

 the seasons (since sheep breed ordinarily in autumn in this country), 

 the camels in the Zoological Gardens in London experience rut in 

 early spring, or at approximately the same time as the breeding 

 season of the wild camels in Mongolia.^ It has been already noted 

 that some Mammals refuse to breed in captivity, while in many 

 others the occurrence of breeding can be regulated to some extent by 

 such factors as accommodation, heating, and feeding. Also in certain 

 domestic animals, such as the sheep, the condition of " heat " can be 

 induced more readily by the supply of additional or special kinds 

 of food.* 



Associated Phenomena 



The approach of the breeding season in many animals, if not in 

 most, is marked by a display of greater vitality, as manifested by an 

 increased activity, which relates not merely to the sexual organs but 

 to the \yhole metabolism of the body. This enhanced vitality is, as 

 a rule, maintained throughout the breeding season. Thus male birds 

 at the time of pairing are in a state of the most perfect development, 

 and possess an enormous store of superabundant energy. Under the 



' I am indebted to Dr. Eagle Clarke for certain of this information. 



^ See Gordon, Jour. Amer. Assoc. Inst., and Invest. Poultry Hushandru, 

 No. 3, 1915. 



3 Heape, "The Sexual Season of Mammals," Quar. Jour. Micr. Science, 

 vol. xliv., 1900. The black swans in the Zoological Gardens breed at the 

 same time as those in Australia. {Gf. also Timor pony, p. 46, and goose, p. 22.) 



* Gf. birds, p. 21, and insects, p. 12. This point is referred to more 

 fully in Chapter XIV., where the causes which influence fertility are discussed. 



