22 The Problems 



part of the heritage. The theory farther demands and 

 by the analogy of what we know otherwise not only of 

 animals and plants, but of physical or chemical laws, 

 perhaps this is the most serious assumption of all that 

 the structure of the gametes shall admit of their being 

 capable of transmitting any character in any intensity 

 varying from zero to totality with equal ease ; and that 

 gametes of each intensity are all equally likely to occur, 

 given a pedigree of appropriate arithmetical composition. 



Such an assumption appears so improbable that even 

 in cases where the facts seem as yet to point to this 

 conclusion with exceptional clearness, as in the case of 

 human stature, I cannot but feel there is still room for 

 reserve of judgment. 



However this may be, the Law of Ancestral Heredity, 

 and all modifications of it yet proposed, fall^ short in the 

 respect specified above, that it does not directly attempt 

 to give any account of the distribution of the heritage among 

 the gametes of any one individual. 



Mendel's conception differs fundamentally from that 

 involved in the Law of Ancestral Heredity. The relation 

 of his hypothesis to the foregoing may be most easily 

 shown if we consider it first in application to the pheno- 

 mena resulting from the cross-breeding of two pure 

 varieties. 



Let us again consider the case of two varieties each dis- 

 playing the same character, but in the respective intensities 

 A and a. Each gamete of the A variety bears A, and 

 each gamete of the a variety bears a. When they unite in 

 fertilisation they form the zygote Aa. What will be its 

 characters ? The Mendelian teaching would reply that 

 this can only be known by direct experiment with the two 

 forms A and a, and that the characters A and a perceived 



