250 RAYMOND PEARL 



4. Very low chick and adult mortality. 



5. A higher egg production in practically all adult daughters 

 than would be expected from the gametic constitutions per se 

 of the parent forms, the latter being definitely known from their 

 pedigrees and from their behavior (cf 578), or that of their 

 full sisters in other types of matings. 



I cannot escape the conviction that in some way the first 

 four of these facts are connected with the explanation of the fifth. 

 There the matter must be left for the present. 



This case points to the importance of the physiological study 

 of individuals in genetic work involving crossing. Only the most 

 superficial aspects of this subject have ever been touched. The 

 'increased vegetative vigor' of first crosses is clear in some 

 instances, but very far from being so in others, and nobod ' has 

 ever shown by a clean-cut physiological investigation why or 

 how the phenomenon occurs. Every breeder of experience knows 

 that this is but one of many interesting and fundamentally sig- 

 nificant physiological matters in connection with hybridizing 

 and cross-breeding which need investigation. The animal breeder 

 knows further that there are real objective phenomena, and not 

 mere idle superstitions of the fancier at the basis of those things 

 which the latter calls Clicking' and 'prepotency,' for example. 

 No doubt these things depend on simple genetic laws, but the 

 point is that we do not now know scarcely'' anything definite (i.e., 

 scientifically exact) about the phenomena, to say nothing of their 

 underlying laws. The richness of the field which still remains 

 quite unworked on the purely physiological side of genetics is, 

 I think, only appreciated by the experienced breeder. 



SUMMARY AND DISCUSSION OF RESULTS 



. The facts and their i7iterpretation 



In this paper is presented a detailed analysis and interpreta- 

 tion of a rather extensive series of data regarding the inheritance 

 of fecundity in the domestic fowl. The basic data are derived 

 from trap-nest records extending over a period of years. They 

 include records from (a) pure Barred Plymouth Rocks; (6) Cor- 



