FERTILITY 87 



It does not seem to be generally true among domestic 

 animals that the smaller breeds are more fertile than the 

 larger breeds of the same species. Among sheep, the 

 American Merino and Southdown are relatively small 

 breeds, but are less prolific than the larger Shropshire, 

 Cotswold or Lincoln. The smaller breeds of swine, 

 like the Essex, Cheshire and Small Yorkshire, are in 

 general less prolific than the larger Berkshire, Duroc- 

 Jersey and Large Yorkshire. In this instance, it is 

 probable that selection by man has intervened to counter- 

 act the general law that the smaller animals are more 

 fecund. 



83. Period of gestation and fertility. The length of 

 the period of gestation seems to be some index of the 

 number of young produced at a birth. In all animals 

 requiring a longer period of gestation than six months, 

 one is the normal number of young at a birth. 1 Among 

 the animals in which the period is less than six months 

 are the hog, sheep, goat, rabbit, dog, cat, rat and mouse. 

 In all of these, except the sheep and goat, the normal 

 number is greater than one. The sheep and goat under 

 domestication have so increased in fertility, in the cases 

 of some breeds at least, that fecundity is much greater 

 than in the horse or cow, where single births are the rule. 

 Milch goats usually drop twins, and triplets are not rare. 2 



The Border Leicester breed in England has -produced 

 150 to 160 per cent of lambs under ordinary conditions. 

 When the same breed has been specially fed before breed- 

 ing, the number of lambs has been increased to 200 per 

 cent. 



1 Marshall, "The Physiology of Reproduction." 

 2 Clos, vol. Ill, p. 410, "Cyclopedia of American Agri- 

 culture." 



