Principles of Heredity 129 



of existing characters, not with variation, which he does 

 not discuss. 



Nevertheless Professor Weldon has some acquaintance 

 with the general fact of variability in certain peas, which 

 he mentions (p. 236), but the bearing of this fact on the 

 difficulty he enuntiates escapes him. 



Results of crossing in regard to seed characters : 

 normal and exceptional. 



The conditions being the same, the question of the 

 characters of the cross-bred zygotes which we will call 

 Alt's depends primarily 011 the specific nature of the 

 varieties which are crossed to produce them. It is un- 

 necessary to point out that if all All's are to look alike, 

 both the varieties A and B must be pure not in the 

 common sense of descended, as far as can be traced, 

 through individuals identical with themselves, but pure in 

 the Mendelian sense, that is to say that each must be at that 

 moment producing only homogeneous gametes bearing the 

 same characters ^1 and B respectively. Purity of pedigree 

 in the breeder's sense is a distinct matter altogether. The 

 length of time or if preferred the number of generations 

 through which a character of a variety has remained pure, 

 alters the probability of its dominance, i.e. its appearance 

 when a gamete bearing it meets another bearing an antago- 

 nistic character, no more, so far as we are yet aware, than 

 the length of time a stable element has been isolated alters 

 the properties of the chemical compound which may be 

 prepared from it. 



Now when individuals (bearing contrary characters), 

 pure in the sense indicated, are crossed together, the 

 question arises, What will be the appearance of the first 



B. 9 



