5o8 EVOLUTION, GENETICS, AND EUGENICS 



that hybridization is the rule in all organism? that reproduce sexually, 

 for no completely homozygous individuals exist in such groups, and 

 therefore there will always be a certain amount of factor segregation 

 in gamete formation and of recombination in the process of zygote 

 formation. Also it must be admitted that there are aU grades of 

 heterozygosity within a species and between one species and another. 

 Moreover, sexual reproduction as an adaptation operates chiefly 

 through bringing together a variety of combinations of characters 

 possessed by strains genetically diverse; it is, in fact, a hybridizing 

 mechanism. It seems probable therefore that hybridization as a factor 

 in evolution operates up to the limit of the adaptive possibilities inherent 

 in its mechanism. We may therefore conclude that hybridization is 

 and has been an important evolutionary factor, though we have at 

 present little information as to its precise mode of operation as an 

 agent in species formation. 



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. 



h) 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 Fundidus heteroditus 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. 



