VARIATION AND HEREDITY 137 



that the germ-cells or gametes produced by 

 cross-bred organisms may in respect of given 

 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 discon- 

 tinuity 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 is 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 complexity 

 of the contrast, by interaction between differ- 

 ent pairs of characters, and by what is called 

 incomplete dominance but there is no mis- 

 taking the phenomena of Mendelian inherit- 

 ance in their typical expression. The cer- 

 tainty of the matter is evident from the 

 success with which the principle has already 

 been used in prediction and in practice. On 



