THE AMERICAN NATURALIST [ VOL. LI 



Since all the pedigrees were started from triplets the 

 excess of triplets is so great as unduly to weight the 

 ratios. Inspection of the ratios does not reveal any par- 

 ticular difference in the progeny descended from a par- 

 ticular pair of grandparents, whether the dam is a single, 

 twin or triplet. Since also there seems to be no sex link- 

 age involved it seemed desirable to combine similar 

 matings from the standpoint of birth rank. The totals 

 produced are presented in Table XXXI. 



TABLE XXXI 



Summary of Tables XXIII to XXX with Respect to Birth Eank of 

 No. Offspring 







' i ' 







Both grandparents single... 29 

 One grandparent a twin I 46 



26 

 26 

 12 



60 

 15 





2.11 

 2.05 

 2.43 



0.85851 

 0.88983 

 0.83516 

 0.70855 



Confirmation of the previous view that twins and singles 

 are genetically alike, while triplets differ from either, 

 seems to be found in Table XXXI. However, the differ- 

 ence between triplets and the mating where one grand- 

 parent is a twin is only 0.32 + 0.20. This is not three 

 times the probable error, but by consulting Pearl and 

 Miner's (11) table it is found that the chances that the 

 difference is significant are about two and a half to one. 



Conclusions 



1. In general sheep of a high birth rank tend to produce 

 offspring of a high birth rank. 



2. On the basis of the few data presented, the high- 

 est record of a ewe appears to be a better selection stand- 

 ard for high fertility than a random record. 



3. The frequency of multiple births in sheep varies with 

 the breed. 



4. Physiological factors may exert a marked i 



