34 PHYSICAL BASIS OF HEREDITY 



normals, because a dominant cliaracter may fail to develop, 

 even though its factors be present. Since normal feet 

 never give syndactyls, the normal type must be recessive. 

 But Davenport's definition of a recessive type as one 

 that never shows in the heterozygous condition is in my 

 opinion based on an arbitrary distinction of what is the 

 cause of dominance and recessiveness. The evidence may, 

 I think, be better interpreted as indicated in the same 

 diagram as that for abnormal abdomen (Fig. 11) in that 

 part marked "dry," in which the syndactyl condition 

 would be represented as recessive (heavy line). In the 

 hybrid the character is usually seen only in a few individ- 

 uals, i.e., it is intermediate, overlapping both parent types. 

 While this case shows that it is often only a convention 

 as to which type is called the dominant and which the 

 recessive, I can see no special reason why in these cases 

 of syndactylism the usual convention may not be followed 

 .which recognizes the small F2 class as the recessive. 



Mendelism rests on the theory of a clean separation 

 of the members of each pair of factors (genes). In 

 every heterozygote the factor for the dominant and that 

 for the recessive are supposed to come into relation to 

 each other and then to separate at the ripening of the 

 germ-cells. If we think of the two genes coming together 

 and afterwards separating, it w^ould seem that a favor- 

 able situation might exist for the two to become mixed, 

 and one "contaminate" the other. If any extensive 

 process of this kind occurred the Mendelian phenomena 

 would be so irregular and erratic that they would have 

 little interest. But even those who are inclined to appeal 

 to contamination as an exceptional phenomenon, grant 

 that clean separation of the genes is the rule. The best 

 critical evidence against contamination is in cases in which 

 for many successive generations breeding has taken place 

 from heterozygous forms only (which creates a favorable 

 situation for contamination to take place w^ere it possible). 

 No influence of contamination has been found in such cases. 



