^;/Oi' j - ;Aioi 



when mated with a jack. Such a physiological aversion 

 is not easy to explain, but is nevertheless so frequent as 

 to be a well-recognized fact among breeders. Prolonged 

 lactation must be regarded as unfavorable to fecundity 

 in some species. This is especially true in the case of 

 swine, where early weaning of the litter will certainly 

 encourage an earlier return of the heat period and thus 

 make possible a larger number of litters during the natural 

 breeding life of the mother. 



99. Relation of number of mammae in swine to fertility. 

 An interesting study of the mammae in swine was 

 reported by Wentworth. 1 From these researches there 

 is little evidence in favor of the popularly accepted opinion 

 that there is a relation between the fertility of swine and 

 the number of mammae. The normal type of mammary 

 pattern in swine consists of regularly placed pairs on the 

 ventral side of the body. The first pair lie immediately 

 behind the juncture of the ribs and sternum. The 

 greatest variation occurs in the second pair. The last 

 pair are closer together and thus nearer the median line 

 in an inguinal position. Variations occur in the number 

 of pairs and also in the suppression of one nipple of a 

 pair. These variations are often inherited. The normal 

 number of mammae in the Tamworth and Berkshire 

 breeds is 13, 14 and 15, in the Duroc-Jersey breed 10, 

 11 and 12. The tendency to vary is greater when the 

 number of pairs exceeds five. 



100. Twins. The normal number of young in several 

 of the larger breeds of the domestic animals and in man 

 is one. The production of a larger number at a single 

 birth is exceptional. It happens, however, that twins 

 are frequently born, while triplets and even four and 



1 Wentworth, Amer. Naturalist, vol. 47, p. 257. 



