232 RAYMOND PEARL 



Matings of the second cross-bred (Fo) generation 



The Fi birds discussed in the preceding sections were mated 

 in all possible ways inter se and with the parent forms. The 

 results of these matings will be discussed in the present section. 



At the outstart it should be noted that in spite of the fact that 

 as many Fo birds were hatched and reared as the available facili- 

 ties would permit, nevertheless, the number of adult daughters 

 available for fecundity study is small in case of some of the mat- 

 ings. There are several reasons for this. Besides the obvious 

 one such as mortality, depredations of thieves, hawks, crows, 

 rats and the like, there is another important but not so obvious 

 one. This is the failure or great difficulty experienced in getting 

 certain of the Fo cross breds to grow into normal, full-sized, 

 healthy adult birds. After rather wide experience in handling 

 cross-bred chicks, I am convinced that certain gametic combina- 

 tions which are to be expected on Mendelian theory, and can be 

 produced in the expected numbers in the breeding pen , are never- 

 theless physiologically abnormal or unsound. Such birds do 

 not make a normal growth, but in spite of the best care and atten- 

 tion grow up into stunted weaklings, which always show, both in 

 their structure and their physiological economy, the effect of this 

 retarded, abnormal development. I am further convinced that 

 this result is primarily due to the hereditary constitution of 

 the individuals in question. Certain combinations of hereditary 

 factors do not produce physiological sound and vigorous zygotes. 



Of course, there is nothing novel in such a result. It is of a 

 piece, for example, with the parts respecting the relation between 

 hereditary constitution and physiological vigor in maize, which 

 have been so clearly set forth and analyzed by Shull (45, 46, 47) 

 and East (9). Other examples of the same phenomenon might be 

 cited. The whole phenomenon is precisely what would be ex- 

 pected from Johannsen's general conception of inheritance and 

 ontogeny (22). 



This relationship between hereditary constitution and physio- 

 logical constitution or normality takes on particular significance 

 when one is dealing with fecundity. As has been pointed out 



