26 THE PHYSIOLOGY OF REPRODUCTION 



period of incubation. This period in wading birds and sea-birds 

 is approximately double that of passerine birds ; but, within the 

 limits of the group to which they belong, it is closely related to 

 the size of the birds. Thus the incubation-period of the stormy 

 petrel is thirty days ; that of the starling is fifteen or sixteen 

 days ; while that of the raven (the largest passerine bird) is 

 about nineteen days. The starling is, as a rule, almost certainly 

 double-brooded, while the 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). 



Mammalia 



The breeding season in the MammaUa, 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 South 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.^ 



' I am indebted to Mr. Eagle Clarke for certain o£ this information. 



' 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. (Cf. also Timor pony, p. 51.) 



3 Cf. birds, p. 23, and insects, p. 13. This point is referred to more 

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

 cussed. 



