No. 611] FERTILITY IN SOUTHDOWN SHEEP 671 



triplet bearers, or that ewes recorded as bearing twins 

 may be genetically triplet producers or better. Hence it 

 may be expected that not all single or twin bearers are 

 alike in zygotic make-up with reference to fertility or tliat 

 their breeding performance will fall into sufficiently well- 

 defined categories to permit a rigorous Mendelian group- 

 ing. The relation between a random lambing record and 

 the average record of ewes was shown earlier in this paper 

 to be high, hence a similar relation might be expected to 

 hold for true genetic fertility, were it measurable, and a 

 random record. 



The Data Ixvolved 

 The Relative Influence of Sire and Dam.— Rietz and 

 Roberts (12) found a mathematically significant effect of 

 the sire on the number at birth as adjudged by the corre- 

 lation between offspring and sires, although they do not 

 find a similar relation between dams and maternal grand- 

 sires. While the authors have not secured correlation co- 

 efficients on this point, their averages may be so arranged 

 as to throw some light on the same point. Using pedi- 

 grees which were started from animals of single birth, 

 the following comparison between sires and dams is pos- 

 sible. 



Comparison of the records of single, twin and triplet 

 sires iiiated to single, twin and triplet ewes in the pre- 

 ceding table shows no particular influence of the birth 

 rank of the sire, a fact which is confirmed in Table X, 

 where the average performance of each is given. 



