310 THE CONTINUITY OF THE RACE 



in this fashion. Among these are milk production in cattle, egg production 

 in fowl, racing ability in horses, leaf shape in tobacco, and many types 

 of disease resistance in crop plants. There is also much inferential evi- 

 dence that many important human qualities are inherited in this way. 

 Breeding experiments are, of course, out of the question in human 

 crosses; but from detailed observational data Davenport concluded that 

 skin color in the Negro-White cross probably involves four pairs of genes, 

 the genes in two of the pairs having a much greater effect than do the 

 genes of the other two pairs. 



