FERTILITY 627 



obviously many individual exceptions. Minot^ observed that in 

 guinea-pigs the size of the litters increased with age during the first 

 sixteen months of their lives ; Hammond/ on the authority of the 

 late Major P. G. Bailey, makes a similar statement for the rabbit; 

 'and Jones and Eouse,^ who give a review of the literature, besides 

 recording their own observations on various domestic animals, show 

 that a similar general rule holds good for cattle, sheep, and pigs. 

 Furthermore, King* found that rats reach the height of their 

 reproductive capacity at about seven months, and that this age 

 represents also the median point in the animal's breeding career. 



Geyelin^ gives the following table showing the fertility of the 

 domestic fowl at different ages : — 



Pearl,® however, states that the greatest egg production is in 

 the first year. With Barred Plymouth Eocks he found that repro- 

 ductive ability diminished with advancing age, being at its maximum 

 at ten to fourteen months. Eobinson,'' writing of fowls and ducks, 

 says that few birds are as good breeders the third year as the 

 second, and fewer still are good after the third year. "It is largely 

 a question of condition ; the older a bird grows, the more difficult it 

 is to keep in good breeding condition." 



Effects of Environment and Nuteition 



That the generative system in animals is peculiarly susceptible to 

 changed conditions of existence has been recognised from early days. 

 Thus Aristotle * commented on the increased fertility of sheep in a 

 favourable environment. In more recent times Buffon,^ among 

 others, remarked on the fact that domestic animals breed oftener 



' Minot, " Senescence and Rejuvenation," Amer. Jour, of phydol., vol. xii., 

 1891. 



^ Hammond, " On some Factors controlling Fertility in Domestic Animals," 

 Jaw. of Agric. Science, vol. vi., 1914. 



^ Jones and Eouse, " The Eelation of Age of Dam to observed Fecundity," 

 ' Jour, of Dairy Science, vol. iii., 1920. See also article by Harrison, Amer. 

 Natvralist, vol. 1., 1916. 



* King, " The Eelation of Age to Fertility in the Eat," Anat. Record, vol. xi., 

 1916. 



^ Geyelin, Poultry -Breeding in a Commercial Point of View, London, 1865. 



^ Pearl, "The Mode of Inheritance of Fecundity in the Domestic Fowl," 

 Jour. Exp. ZooL, vol. xiii., 1912 ; " Studies in the Physiology of Reproduction 

 in the Domestic Fowl," XVIT., Genetics, vol. ii., 1917. 



' Eobinson (J. H.), Principles and Practice of Poultry Culture, Boston, 1912. 



* Aristotle, History of Animals, Bohn's Library, London. 

 ^ Buflfon, Histoire Naturelle, Paris, 1802. 



