400 D. J. SCOURFIELD OX MENDELISM AND MICROSCOPY. 



For example, " round yellow " peas were crossed with " wrinkled 

 green" peas, and the result in the second generation was as 

 follows: 315 round yellow, 101 wrinkled yellow, 108 round 

 -iron, and 32 wrinkled green, or approximately in the ratio of 

 9 RY + 3 WY + 3 RG + 1 WG, which ratio is evidently 

 due to a combination of 3 R + 1 W and 3 Y + 1 G. The 

 number of different types actually present was, however, not four, 

 but nine, for it was found by the evidence of subsequent 

 generations that, on the average, out of every 9 RY there 

 were — 



4 containing W and G in addition to R and Y 



2 „ W only ,, „ „ 



9 G 



1 ,, R and Y only — i.e. pure in both characters ; 

 out of every 3 WY there were — 



2 containing G in addition to W Y 



1 ,, WY only — i.e. pure in both characters ; 

 out of every 3 RG there were — 



2 containing W in addition to RG 



1 „ RG only — i.e. pure in both characters ; 



while every WG was found to be pure. 



The above combinations and their ratios could be predicted from 

 the formula (1 R + 2 RW + 1 W) x (1 Y + 2 YG + 1 G). 



It is'of the greatest importance to notice that of the four classes 

 mentioned above as having different outward characteristics, two 

 represent combinations of characters not present in the original 

 parents — namely, " wrinkled yellow " and " round green " ; and as 

 it has also been shown that in each of these classes there was a 

 definite proportion of pure forms, the result is that, even in the 

 second generation, Mendel obtained two new races which were 

 constant without any further selection. 



When three pairs of characters were united in one cross the 

 result was also found to agree with what might have been 

 predicted from a combination of the ratio 3 : 1 for each pair. 

 In this case eight classes differing in outward appearance were 

 obtained, and approximately in the ratio of 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. 

 Mendel did not undertake the herculean task that would be 

 involved in carefully following up the development of hybrids 

 combining in themselves more than three pairs of characters, but, 

 in the course of his experiments, he did actually meet with the 



