BY RAYMOND PEARL 49 



Whether a given degree of fecundity is to be regarded as a single unit 

 character, in the Mendelian sense, or, on the other hand, as a complex 

 dependent upon a particular combination of separately segregable unit 

 characters, can not yet be determined. Every one must recognize the 

 fundamental importance of the investigations of Nilsson-Ehle, Baur and 

 East, which have shown that many characters which at first glance do not 

 appear to conform to any determinate law of inheritance are really com- 

 plexes, formed by the combination of a number of unit characters, each of 

 which segregates and otherwise behaves in a perfectly regular and lawful 

 manner. There are some facts which indicate that high fecundity is a 

 character of this kind, but it will require prolonged analysis to decide this, 

 because of the numerous practical difficulties which attend the study of 

 fecundity. 



A great help in this analysis, as well as a contributory line of evidence of 

 much weight in supporting the general conception of the manner of inheri- 

 tance of fecundity set forth above, is derived from the study of crosses 

 between breeds of poultry in which high and low degrees of fecundity are 

 definite breed characters. Studies of this sort carried out at the Maine 

 station indicate that the relatively high fecundity characteristic of the 

 Barred Plymouth Rock breed is inherited as a sex-limited character. In 

 this respect it behaves like a simple unit character, but this does not 

 necessarily prove that it is not a complex. More data are needed to 

 settle this point. Of much significance is the fact that, whether simple or 

 complex, fecundity is shown by these experiments in cross breeding to be a 

 character resting on a definite gametic basis. 



In conclusion, I think it may fairly be said that the investigations here 

 reported show in the first place that different degrees of fecundity are 

 inherited in the domestic fowl, and in the second place, that in all respects 

 wherein it has been possible, considering the inherent difficulties of the 

 material and the character dealt with, to make the test, the method of this 

 inheritance is in entire accord with Johannsen's concept of genotypes. 



