12 The Problems 



RATIOS 



Jl Ab class may produce either all Ab's, 

 [2 or both Ab's and ab's. 



class may produce either all AJ3's, 

 2 or both All's and Ab's, 



2 or both AB's and .Z?'s, 



4 or all four possible classes again, namely, 



^ A's, Ab's, aB's, and ab's, 



and the average number of members of each class will 

 approach the ratio 1 : 3 : 3 : 9 as indicated above. 



The details of these experiments and of others like 

 them made with three pairs of differentiating characters are 

 all set out in Mendel's memoir. 



Professor de Vries has worked at the same problem in 

 some dozen species belonging to several genera, using pairs 

 of varieties characterised by a great number of characters : 

 for instance, colour of flowers, stems, or fruits, hairiness, 

 length of style, and so forth. He states that in all these 

 cases Mendel's principles are followed. 



The numbers with which Mendel worked, though large, 

 were not large enough to give really smooth results * ; but 

 with a few rather marked exceptions the observations are 

 remarkably consistent, and the approximation to the num- 

 bers demanded by the law is greatest in those cases where 

 the largest numbers were used. When we consider, besides, 

 that Tschermak and Correns announce definite confirmation 

 in the case of Pisum, and de Vries adds the evidence of his 

 long series of observations on other species and orders, 

 there can be no doubt that Mendel's law is a substantial 



* Professor Weldon (p. 232) takes great exception to this state- 

 ment, which he considerately attributes to " some writers." After 

 examining the conclusions he obtained by algebraical study of Mendel's 

 figures I am disposed to think my statement not very far out. 



