No. 534] FECUNDITY IN THE DOMESTIC FOWL 343 



cundity is not inherited on a genotypic basis. We have 

 to consider the problem of genetic or gametic purity. 

 Do we have homozygote lines in such cases as those dis- 

 cussed in this paper? It plainly is the fact that one can 

 get lines of birds which, broadly speaking, will breed 

 true (perhaps throwing occasionally a few individuals 

 not true to the type of the line) to definite degrees of 

 fecundity. The same thing is true of milk production in 

 dairy cattle, speed in race horses, etc. What are these 

 lines gametically? Theoretically the formation of 

 gametically pure (homozygote) lines with respect to 

 definite degrees of fecundity is simple. Practically it is 

 exceedingly difficult to do this, owing to the fact that 

 (a) the character studied is not expressed in the male, 

 and (b) it is subject to a wide fluctuating variability 

 caused by environmental conditions. The question as to 

 the gametic constitution of the fecundity lines here dis- 

 cussed obviously can not be answered finally now. It is 

 a matter for much further research. One may, however, 

 form a general conception of the probable gametic con- 

 stitution of such lines, which lias much evidence in its 

 support. The essential points in such a conception are: 



1. Probably no line yet obtained is absolutely pure 

 gametically in respect to fecundity. It represents a mix- 

 ture of a greater or less number of fecundity genes. 



2. Lines which breed reasonably true to a definite de- 

 gree of fecundity may in most cases be taken to be made 

 up of individuals bearing a preponderant number of 

 genes of the particular degree of fecundity to which the 

 line breeds true, so that in gametogenesis a great major- 

 ity of the gametes formed carry only these genes. They 

 also carry some genes of higher, or lower fecundity, 

 or both kinds. When individuals of a definite (e. g., 

 "high") line thus constituted are bred together the ma- 

 jority of the offspring will, purely as a matter of chance, 

 be produced by the union of two high fecundity gametes. 

 It is quite possible that with families of the size obtained 

 with poultry nearly or quite every individual produced 



