Principles of Heredity 127 



and experiments relating to this extraordinarily interesting- 

 subject. As de Vries writes in his fine work Die Muta- 

 tionstheorie (i. p. 580), " a study of the seed-differences of 

 inconstant, or as they are called, ' still ' unfixed varieties, is 

 a perfect treasure-house of new discoveries." 



Let us consider briefly the possible significance of these 

 facts in the light of Mendelian teaching. First, then, it is 

 clear that as regards most of such cases the hypothesis is 

 not excluded that these recurring sports may be due to the 

 fortuitous concurrence of certain scarcer hypallelomorphs, 

 which may either have been free in the original parent 

 varieties from which the modern standard forms were 

 raised, or may have been freed in the crossing to which the 

 latter owe their origin (see p. 28). This possibility raises 

 the question whether, if we could make '''pure cultures " of 

 the gametes, any variations of this nature would ever occur. 

 This may be regarded as an unwarrantable speculation, but 

 it is not wholly unamenable to the test of experiments. 



But variability, in the sense of division of gonads into 

 heterogeneous gametes, may surely be due to causes other 

 than crossing. This we cannot doubt. Cross-fertilization 

 of the zygote producing those gametes is one of the causes 

 of such heterogeneity among them. We cannot suppose it 

 to be the sole cause of this phenomenon. 



When Mendel asserts the purity of the germ-cells of 

 cross-breds he cannot be understood to mean that they are 

 more pure than those of the original parental races. These 

 must have varied in the past. The wrinkled seed arose 

 from the round, the green from the yellow (or vice versa, 

 if preferred), and probably numerous intermediate forms 

 from both. 



The variations, or as I provisionally conceive it, that 

 differentiant division among the gametes of which variation 



