212 HEREDITY ANT) SEX 



these first hybrids, further decline does not take 

 place. 



An alternative view, that is also Mendelian, has been 

 offered by Bruce and by Keeble and Pellew. Vigor, it 

 is maintained, is in proportion to the number of domi- 

 nant factors, and in proportion to the number of these 

 factors present whether in a hybrid or in a homozygous 

 (duplex) condition. 



On this view the hybrid is vigorous, not because it is 

 hybridous, so to speak, but because in its formation a 

 larger number of dominant factors (than were pres- 

 ent in either parent) have been brought together. 



A third view is also compatible with the evidence, 

 namely, that there may exist factors that are them- 

 selves directly concerned with fertility. There is one 

 such case at least that has been thoroughly analyzed 

 by Pearl. 



Pearl studied for five years the problem of fertility 

 in two races of fowls, viz. barred Plymouth rocks and 

 Cornish Indian games. The main features of his 

 results are shown in the diagram (Fig. 102). He finds 

 that the winter output of eggs, which is correlated 

 with the total production, is connected with two factors. 

 One factor, designated by L h is a non-sex-linked char- 

 acter. If it is present, an average of less than 30 eggs 

 is produced in the winter season. There is another 

 factor, L 2 , that is present in the barred rocks, but not 

 in the Indian game. If present alone, the winter out- 

 put is again about 30 eggs on an average. If, how- 

 ever, both Li and L 2 are present, the winter output 

 is more than 30 and may be as great as 90, or in rare 

 cases 100-120 eggs. 



