302 EVOLUTION, GENETICS, AND EUGENICS 



and Sumatra; G. lafayette, the Ceylon jungle fowl; G. somicrate, the 

 gray jungle fowl of southern India; and G. varius, the jungle fowl of 

 Java. These birds are rather wild and do not hybridize well in cap- 

 tivity. Yet occasional uncontrolled hybrids have been produced which 

 are of interest in throwing some light on the origin of the various do- 

 mestic breeds. Darwin was of the opinion that all the domestic breeds 

 have been derived from G. gallus, for sometimes in India cocks of G. 

 gallus invade flocks of domestic fowls and mate with the hens, produc- 

 ing fully fertile offspring. The same has been claimed, since Darwin 

 wrote, for the other species, and some authors are inclined to believe that 

 domestic fowls of different breeds may have been derived from differ- 

 ent wild species of Gallus. The evidence for the latter view, however, 

 is less abundant than for Darwin's contention. 



SECONDARY EFFECTS OF CROSS-BREEDING AND INBREEDING 



A. CROSS-BREEDING 



Hybrid vigor (heterosis). — It has long been known that the 

 crossing of different races, varieties, or even species of animals or plants 

 result in the production of first-generation hybrids characterized by 

 a greater sturdiness, vitality, and size than either parent-species. This 

 effect has received the name hybrid vigor or heterosis. A good example 

 of this effect is the common mule, which is large and strong, thrives 

 under adverse conditions, and is hardier than either parent. It has 

 the disadvantage, or possibly advantage, of being sterile, a fact which 

 makes it necessary to hybridize two species every time we want another 

 mule. 



Some of the manifestations of hybrid vigor as observed in various 

 crosses are as follows: 



a) Hastening of maturity. — This is particularly advantageous in 

 plants reared in regions where the growing-period is short. Thus 

 hybrid strains of cereals may be valuable because they can be harvested 

 sooner than pure-bred strains. It is also true that hybrid plants, such 

 for example as tomatoes, have a larger as well as an earlier yield. 



b) Increased longevity. — Pearl has shown that hybrid strains of fruit 

 flies have a longer average life-span than pure races. The same is 

 true for a number of hybrid races of plants, as brought out by Gaertner. 



c) Better viability. — The writer has shown that the hybrids pro- 

 duced by crossing the eggs of the fish Fundulus heteroclitus with the 

 sperm of F. majalis were frequently more viable, faster growing, and 

 more vigorous than the pure-bred young of either species; but the 

 hvbrids from the reciprocal cross showed much-reduced viability. 



