FERTILITY 643 



by breeding from twin ewes and employing the services of twin rams, 

 have been able permanently to increase the fertility of their stock.^ 



The inheritance of fertility in man and also in thoroughbred 

 horses has been investigated statistically by Karl Pearson and his 

 biometrical collaborators,^ to whose memoir the reader is referred for 

 a full discussion of the mathematical details and the conclusions 

 which are arrived at. It is there shown, among other facts, that the 

 woman inherits fertility equally through the male and female lines. 

 Among thoroughbred race-horses the fecundity was first ascertained 

 (i.e. the ratio of foals surviving to be yearlings to the total number of 

 foals possible under the given conditions), and the following general 

 conclusions were afterwards reached : (1) Fecundity is inherited 

 between dam and daughter, and (2) Fecundity is also inherited 

 through the male line, i.e. the sire hands down to his daughter a 

 portion of the fertility of his dam. Thus fecundity, which is, of 

 course, a latent character in the male, was measured for a horse and 

 for his sire, and was found to be strongly inherited. 



■ More recently Eommel and Phillips ^ have shown mathematically 

 that there is an actual correlation between the size of the litter in 

 two successive generations of Poland China sows, the productiveness 

 being a character which is transmitted from mother and daughter. 



On the other hand, Pearson,* from studying Weldon's records of 

 mice-breeding experiments, failed to find a sensible parental correlation 

 in regard to the size of the litters. Pearl and Surface,' as a result of 

 a statistical investigation on egg-production in Barred Plymouth 

 Eock fowls, found no evidence of the inheritance of fecundity so long 

 as simple mass selection was practised. For this breed the capacity 

 for egg-producing was not increased by this method, but tended 

 rather to be diminished. 



Pearl next started to analyse his results on the assumption that 

 Mendelian factors might exist which were themselves concerned 

 with different degrees of fertility. He found that the winter egg- 

 production bore a direct relation to the total egg - production 



} Marshall, "Fertility in Scottish Sheep," Trans. Highlatid and Agric. Soa., 

 vol. XX., 1908. 



2 Pearson, Lee, and Bramley- Moore, "Mathematical Contributions to the 

 Theory of Evolution : VI , Genetic (Reproductive) Selection, Inheritance of 

 Fertility, etc.," Phil. Travis., A., vol. cxcii., 1899. 



^ Eommel and Phillips, "Inheritance in the Female Line of Size of Litter 

 in Poland China Sows," Proc. Amer. Phil. Soc, vol. xlv., 1907. 



* Pearson, " On Heredity in Mice, from the Records of the late W. F. R. 

 Weldon," Biometrika, vol. v., 1907. 



5 Pearl and Surface, " Data on the Inheritance of Fecundity obtained from 

 the Records of Egg Production, etc," Maine Agric. Exp. Station 'Bulletin 

 No. 166, Maine, 1909. Pearl, " A Biometrical Study of Egg Production in 

 the Domestic Fowl," U.S. Dept. of Agric.,- Bureau of Animal Industry, Bulletin 

 No. 110, Washington, 1909. 



