INHERITANCE OF FECUNDITY 221 



production depends upon two separately inherited physiological 

 factors. The presence of both of these factors {Lx and L2) is 

 essential to a high fecundity record. The second factor La, with- 

 out whiph high fecundity never appears is inherited in a sex-cor- 

 related manner, such that it is never borne in the same gamete 

 that carries the female sex-factor F. 



2. That the things segregated are perfectly definite and dis- 

 tinct. This is shown by the mean or average production records 

 of the birds falling into the several fecundity classes. The birds 

 bearing the factors for high fecundity have mean winter produc- 

 tion records ranging from two to five or six times as great as the 

 mean production records of birds lacking these high fecundity 

 factors. Such differences as these do not depend upon refined 

 statistical analysis for their detection and appreciation. 



While by no means all the possible gametic combinations in 

 respect to fecundity within the Barred Rock breed have yet been 

 made, still the rangecovered by the data given above is fairly wide. 

 All classes of females except the zero producers (class 5) have been 

 repeatedly tested in the breeding pens in various different com- 

 binations. The zero winter producing females have been fairly 

 often bred, but the difficulties of getting chickens hatched within 

 the necessary time limits and in sufficient number to get adult 

 daughters for fecundity work have been too great for the available 

 resources. Of the nine possible types of males six have been tested 

 in various combinations. 



It may fairly be said, I think, that in its range, its qoiality and 

 its amount, the evidence from the pure Barred Rock matings, as 

 set forth in the preceding sections, is sufficient alone to demon- 

 strate the Mendelian inheritance of fecundity in the breed of 

 fowls. If, however, the principles set forth above for Plymouth 

 Rocks are true, they ought to apply, in general at least, to other 

 breeds of fowls and to crosses, with, of course, possible limitations 

 and modifications in particular instances. It is desirable, there- 

 fore, to examine the results regarding the inheritance of fecundity 

 in other breeds and crosses. This we may proceed to do. 



