No. 611] FERTILITY IN SOUTHDOWN SHEEP 663 



worth-Davis and Turner (1) reported a preliminary in- 

 vestigation on this subject, but their numbers are too 

 small and results too contradictory, as published, even to 

 be indicative of the method of inheritance. Arkell and 

 Jones (7) at the New Hampshire station also instituted 

 investigations along this line, but have published no re- 

 sults. 



Due to the environmental and physiological factors in- 

 volved in multiple births, as well as to the economic im- 

 practicability of maintaining large flocks under rigid ex- 

 perimental conditions, there are at hand no considerable 

 masses of experimental data which yield evidence on this 

 point, nor are the probabilities great that such exper- 

 iments will ever be conducted on an adequate scale ; hence 

 the bulk of evidence on the inheritance of fertility must 

 come from breeders' flocks or from breed registry records. 



The Fertility Problem 



High fertility obviously depends on three factors— the 

 number at a birth, the frequency of reproduction, and the 

 total number of successful gestations an animal may 

 undergo. Unfortunately flock book records give available 

 data on the first point only, although for specific cases 

 some evidence on the second point (barring abortions and 

 unregistered progeny) exists. 



For breeding purposes the number of successful gesta- 

 tions is not a practical selective index, since the breeder 

 can not afford to withhold progeny from breeding until 

 their dams or sires shall have completed their breeding 

 cycles. Frequency of reproduction or regularity of breed- 

 ing as termed ])y the breeder is a more practicable trait 

 for ])nrpo>ps of solpptiou, lint since 1)arron reproductive 

 periods arc so imicl! moi'c fi-oqnently due to ]^athological 

 or ]^]iysiologi('al causos tlinii to geiiotic, most sheepmen 

 lay priiici])al emphasis on the number of offspring at the 

 given birth. 



There are two ways in which selection on this basis 

 may be applied. The ewe may be selected on the basis of 



