RELATION TO GENERAL BIOLOGICAL PROBLEMS 121 



with, the extra-nipple character. Bell had in mind 

 such a procedure when he wrote his 191 2 communica- 

 tion, but has not, so far as I am aware, carried it out. 

 He noted, however, several interesting facts that might 

 increase the hereditary tendency to produce twins. 

 ^'Twin-bearing ewes are on the average much heavier 

 than single-bearing ewes." The condition of the 

 mother at time of mating is also important, for when 

 the mother is fat and in prime physical concHtion the 

 percentage of twins is larger than when the mother is 

 lean. Ewes mated in October when the pasturage is 

 at its best have a much larger proportion of twins than 

 those mated later in the breeding season when the 

 pasturage is on the wane. Very few ewes mated in 

 December have twins. A lowered nutrition of the 

 mother after mating in October favors carrying twins 

 successfully to birth, as it keeps the size of fetuses 

 rather small and thus obviates undue crowding. 



We may conclude from Bell's experiments that 

 twinning is distinctly a hereditary character in sheep 

 which is not merely sporadic but more or less racial; 

 a fairly large percentage of twins appears to be specific 

 for sheep, but this percentage may be greatly enhanced 

 by selective breeding from twinning strains. The 

 economic importance of establishing a twinning race 

 of sheep is obvious, for, as Bell says, ''if the farmers 

 could raise two lambs instead of one for e\er\' ewe 

 wintered, sheep breeding in Nova Scotia might become 

 a profitable industry of great importance." 



Although there is a widespread belief among breeders 

 that twinning in cattle is hereditary, there is, so far 

 as I am aware, no direct evidence that such is the case. 



