478 JOHN W. GOWEN 



brid has a higher initial production than its parents, or, for that matter, any 

 of the purebred races. The hybrid expends its metabolic reserves less rapidly 

 than either of its parents. Taking all inbred races together, the hybrid utilizes 

 its reserves at slightly less than the average rate. The hybrid is chiefly char- 

 acterized by its large initial production. Examination of the pure lines indi- 

 cates that there are slight differences in the rate of expenditure of the initial 

 reserve, even when the obviously different Inbred 92 is not considered. 



WHAT IS HYBRID VIGOR? 



These results show that the vigor of the hybrid is greatest for lifetime egg 

 production, 203 per cent; is less if length of egg laying period is considered, 

 113 per cent; and is still less with life span as the character, 104 per cent. 

 What is the explanation of egg production's high heterosis? Egg production 

 is a character which is in turn dependent on other component characters. A 

 simple breakdown would be, lifetime egg yield is determined by the length 

 of egg laying period, the decline (slope) representing the loss of ability to 

 produce eggs with age, and maximum egg production at the initial phase of 

 the egg laying cycle. Length of egg laying period has already been shown to 

 have 113 per cent heterosis. The slopes of the decreasing egg yields with ad- 

 vancing ages are Inbred 92 — 0.17, Ames I — 0.06, and their Fi hybrids — 0.05. 

 The hybrids show heterosis in that their egg productions decline less rapidly 

 than their best parent, but the heterosis is only 121 per cent. 



Maximum productions, as judged by the three highest days' average 

 yields of the strains, are Inbred 92, 40.7; Ames I, 52.7; and Fi hybrid, 81.4 

 eggs, or the heterosis is 154 per cent. The highest of the component heterosis 

 values is only about half of that noted for lifetime egg yields. It seems not 

 unlikely that if the division into components could be carried further, it 

 would be found that the heterosis values would approach closer and closer 

 to 100. The results consequently argue for heterosis, as the result of the com- 

 bined action of two or more groups of distinct characters which, when jointly 

 favorable, and as frequently truly multiplicative in action, lead to heterosis. 



Analysis of the variation in egg production between races — the heritable 

 fraction controlled through inbreeding — as contrasted with the variability 

 within races — the fraction due largely to environment — shows that about 

 56 per cent of the lifetime egg production is fixed within the races and 44 per 

 cent is due to gene segregation, environment, etc. 



Consideration of the individual records further support this view. Con- 

 trasting the performances of the individual females within the hybrid groups 

 with those in the different inbred races shows that the hybrid has no females 

 with greater production than those of the inbred races. The hybrids are good 

 because, on the average, all members of the cross are good producers. The 

 hybrids include one female laying 3083 eggs and twenty-seven others laying 

 between 2000 and 3000 eggs. The Ames I has one female laying 2016 eggs; 



