THE MINGLING OF RACES §^^ 



an example from poultry, two races of white birds may be 

 crossed and produce a bird with the full pigmentation of 

 the jungle fowl. This is because one of the races is white, 

 through the absence of factor b, the other through the 

 absence of factor a, and the hybrid brings together the two 

 factors essential to full coloration. 



One of the commonest expressions of this law is seen in 

 the union of races both of which are unable phenotypically 

 to express their full developmental potentiahties, due to the 

 absence from each of some developmental factor. When this 

 factor is different in the two races their union may result in 

 the hybrid possessing more development-stimulating factors 

 than either of the parental races possessed. Accordingly, 

 the hybrid may show an exceptional capacity for growth. 

 This result is known as hybrid vigor, or heterosis. We 

 naturally look for evidence of heterosis in the first generation 

 hybrid between two races. 



In later generations some of the individuals possess both 

 of the developmental factors in question. Others possess 

 neither and others will possess one or the other so that we 

 should expect in a later hybrid population to find a great 

 variabihty in capacity for growth. 



The case of hybrid vigor is well illustrated in maize. 

 When any variety of maize is inbred it tends to produce 

 dwarfed offspring. Indeed, the ears developed on such 

 self-fertiHzed plants produce a small proportion of viable 

 seeds. If, on the other hand, any female flower is poUinated 

 with any other plant the large ears are produced and these, 

 when planted, develop into vigorous offspring. If two 

 depauperate products of inbreeding are mated in corn the 

 offspring show this hybrid vigor. 



Coming now to the traits of human races the first question 

 that we have to consider is whether any of them are inherit- 

 able. It would seem to be foolish to raise this question, but we 

 do so because such inheritance has been denied. 



On the physical side we know of not a few traits that are 

 inherited in accordance with simple Mendelian laws. Thus 

 brown eye color is dominant over the absence of brown 

 pigmentation in the iris, as exhibited by blue eyes. The first 

 generation shows a dominance of the brown eye color and in 



