136 EVOLUTION 



characters be of the pure parental types, and 

 consequently incapable of transmitting the 

 opposite character; that when such pure 

 similar gametes are united in fertilization, the 

 individuals so formed and their posterity are 

 free from all taint of the cross; that there 

 may be, in short, perfect or almost perfect 

 discontinuity between these germs in respect 

 of one of each pair of opposite characters." 

 This idea of the segregation of the dominant 

 and the recessive characters in two different 

 sets of germ-cells is the essence of Mendelian 

 theory. 



Before passing from this important and 

 fascinating subject, we may emphasize two 

 points. There i^ no dubiety in regard to the 

 clear cases of Mendelian inheritance. Cases 

 that seem to be non-Mendelian may turn 

 out to be Mendelian — disguised by the com- 

 plexity of the contrast, by interaction be- 

 tween different pairs of characters, and by 

 what is called incomplete dominance — but 

 there is no mistaking the phenomena of 

 Mendelian inheritance in their typical ex- 

 pression. The certainty of the matter is 

 evident from the success with which the 

 principle has already been used in prediction 

 and in practice. On the other hand, there 

 are also many heritable characters which 

 blend, and do not conform to the Mendehan 



