634 



THE AMERICAN NATURALIST 



[Vol. L 



that r a is actually though perhaps not significantly lower than 

 r S! , whereas on the female side r Dd r > r Dl . Obviously there is no 

 genetic reason for a correlation between the size of the litters in 

 which the grandsires, 8, and the grandams, D, were farrowed. 

 But columns 1-3 of Table II of Wentworth and Aubel actually 

 give: 



a value quite as large as those recorded above. 



Such a correlation might arise (a) through the existences in 

 the pens of different breeders of strains slightly differentiated 

 with respect to fertility, (6) through differences in the condi- 

 tions in which different breeders maintained their pens, provid- 

 ing such conditions affect litter size, or (c) through actual dis- 

 honesty of certain breeders in reporting the size of litters for 

 herd-book publication. 



Such differentiation, if it exists, would also account in part at 

 least for the correlations hitherto regarded as due to inheritance. 

 The whole problem is evidently one of great complexity and re- 

 quiring far more detailed investigation than it has yet received. 



The problem of the inheritance 'of the production of twins in 

 sheep which has been studied experimentally by Alexander 

 Graham Bell for the past several years, has recently been in- 

 vestigated statistically by Rietz and Roberts. 32 



There seems to be unmistakable evidence of inheritance, or at 

 least of ascendant influence, 33 upon descendant characteristics. 

 This may be most clearly seen by comparing the average number 

 per litter resulting from certain matings. 



Thus for the parental relationship the results are: 



When sire and dams are singles 1.3452 ± 0.0059 



When sire is single and dam is twin 1.4171 ± 0.0067 



1.3979 ± 0.0035 



.3946 ± 0.0073 

 .4548 ± 0.0088 



3a Rietz, H. L. and E. Roberts, "1 

 Offspring with Reference to Birth 



Offspring with Reference to Birth 

 Sheep," Jour. Agr. Bes., 4: 479-510, 



Degree of Resemblance of Parents and 

 i as Twins for Registered Shropshire 

 ), 1915. 



