No. 534] FECUNDITY IN THE DOMESTIC FOWL 341 



genotypes, some high, some low, and some intermediate 

 or mediocre. In an ordinary flock these genotypes will 

 be greatly mixed and intermingled. Further, the facts in 

 hand indicate that the range of variation in fecundity 

 within the genotype is relatively very large, nearly as 

 great, in fact, as in the general population. Thus while 

 fecundity genotype means may be and usually are per- 

 fectly distinct, there is much overlapping of individuals 

 in the different lines. In consequence it results that the 

 egg record of an individual bird is of almost no value in 

 helping to tell in advance of the breeding test to what 

 fecundity genotype it belongs. Essentially this same 

 fact has been brought out in all of the work which lias 

 been done with pure lines. The only difference in the 

 present case lies in the fact that the range and degree of 

 variation within the line appears to be relatively greater 

 in the case of fecundity than in the case of most char- 

 acters hitherto studied, as, for example, size relations in 

 beans or Paramecium. 



The most serious difficulty which confronts one in the 

 attempt to analyze the inheritance of a character like fe- 

 cundity lies in the almost inextricable mingling of geno- 

 types in the great majority of individuals. This, of 

 course, is a direct consequence of the manner of repro- 

 duction. The germ plasm of two separate individuals 

 must unite to form a new individual. By prolonging in- 

 cestuous mating one may in theory come indefinitely 

 close to reproductive purity, but in practise even this is 

 extremely difficult, if not impossible of accomplishment 

 on any large scale or through any long period of time. 

 The fact simply is that a "pure line" in the strict sense 

 of Johannsen 15 can not by definition exist in an organism 

 reproducing as the domestic fowl does. This, however, 

 by no means indicates that the inheritance of fecundity 

 does not rest on a genotype basis, or, in other words, that 



