No. 588] EXPERIMENTS IN MASS SELECTION 715 



a theory he must choose material suited to the purpose. 

 No adequate test of the efficacy of mass selection can be 

 obtained from material which can not be accurately 

 judged in the mass. 



Pearl points out further limitations of his material in 

 the statement 1 1 that phsenotypic variation of the character 

 fecundity, in fowls, markedly transcends, in extent and 

 degree, genotypic variation." That is, non-heritable 

 causes of fecundity are in excess of heritable causes and 

 serve to obscure the occurrence of the latter. Further, 

 Pearl says: 



It is quite impossible in the great majority of eases to determine with 

 precision what is a hen's genetic constitution with respect to fecundity 

 from an examination of her egg record alone. 

 If then one has reared his pullets to the age of one year, 

 has kept them free from disease and parasites, has fed 

 and housed them properly and has even trap-nested them 

 and recorded their eggs all winter, still he has no suffi- 

 cient basis on which to base a selection. He must first 

 rear and test their progeny in the same way. Pearl's 

 statements on this point, the accuracy of which I do not 

 question, are sufficient to show the entire unsuitability of 

 his material for testing the efficacy of mass selection. 



One might with propriety even question whether such 

 a thing as inherited capacity for winter egg production 

 exists in fowls, but on this point, I think, another inves- 

 tigation 3 made by Pearl is conclusive, in which he crossed 

 Cornish Indian game fowls, which are poor winter layers, 

 with Barred Plymouth Rocks which are fairly good winter 

 layers. Reciprocal crosses were made in both of which 

 the daughters showed resemblance to the racial winter 

 egg productiveness of the sire 's race. This result indicates 

 that a sex-linked genetic factor of some sort exists which 

 affects winter egg production in fowls. But since the fe- 

 cundity of the offspring was obviously influenced by the 

 mothers' race as well as by the father's race, Pearl was 



a "The Mode of Inheritance of Fecundity in the Domestic Fowl," Jour. 

 Exp. Zool., Vol. 13, p. 153, 1912. 



