670 



THE AMERICAN NATURALIST 



[Vol. LI 



No 'difficulty was experienced in obtaining ewes that 

 produced milk from six nipples. These multi-nippled 

 sheep, however, did not prove to be more fertile than 

 normally nippled sheep. In his 1912 paper (2) he states 

 that the indications are that the six-nippled stock will 

 ultimately prove to be twin bearers, as a rule, at ma- 

 turity. 



Method of obtaining Data 

 The source of the data in the present study was the 

 American Southdown Kecord, the first twelve volumes 

 being used to obtain cases of triplets, and volumes nine 

 to twelve for twins and singles. The pedigree of each 

 animal was reported into the third generation, recording 

 the numbers of offspring at the birth of each animal. 

 Some records on triplets were also taken from the Amer- 

 ican Shropshire Record, while Volume 25 was used to 

 determine the ratio of singles, twins and triplets, Vol- 

 umes 9-12 of the Southdown Flock Book for the same 

 purpose. Volumes 12, 13, and 14 of the Continental Dorset 

 Club Record, and Volumes 11 and 12 of the American 

 Cotswold Record. 



Reliability of Flock Book Data 

 Records of the number at a birth in sheep are probably 

 highly accurate for sneli material, since there is no ob- 

 servable tendency to discriminate in favor of, or against, 

 recording offspring of multiple births, except the indirect 

 one of lesser development in offspring from multiple 

 births. This would not affect the reliability of the figures 

 presented by the flockmaster, except perhaps to reduce 

 slightly the proportion of multiple births registered. It 

 may be safely assumed that the bulk of the records are 

 accurate, barring clerical error. 



The Number at a Birth as a Genetic Index 

 Due to the physiological causes limiting the full expres- 

 sion of the genetic fertility of an animal it is obvious that 

 animals recorded as singles may be potentially twin or 



