248 Experimental Transformation of Speeies 



independent determiners quite untenable for all unit char- 

 acters. We know now as a matter of fact that each " de- 

 terminer " produces more than one effect. The students had 

 simply described the most noticeable characteristic, over- 

 looking many others. Among the sweet peas, for example, 

 the petal called the standard is erect in purple flowers and 

 drooping in white flowers. In the garden pea round seeds 

 are associated with tendrils on the leaves, and colored seed 

 coats with purple flowers. On the other hand, many traits 

 of plants and animals that seem to be simple characters have 

 been found to depend for their manifestation upon two or 

 more independent factors. 



'Determiners and Faetors 



Mendel's assumption of determiners present in the germ 

 cells was in complete accord with all of his findings, and, as 

 we shall see, with certain fundamental facts in the structure 

 and behavior of the germ cells. It was his good fortune that 

 he selected for his experiments the characters of the garden 

 pea with which he actually worked, since there are other sets 

 of contrasting characters that he might have chosen and 

 that would have led him hopelessly astray. The relatively 

 simple cases did yield the significant mathematical formulas 

 that unlocked the more complex situations. 



William Bateson and R. C. Punnett in England found 

 two strains of sweet peas with white flowers which were in- 

 distinguishable from any of their outward characters. The 

 microscope showed one of them to have a longer pollen grain 

 than the other. After crossing these two varieties, all of the 

 succeeding generation blossomed out in gay purple flowers. 

 This unexpected result was looked upon as a case of re- 

 version or throw-back, since the wild Sicilian sweet peas 

 from which the cultivated varieties had been derived have 

 such purple flowers. Further breeding resulted in a distri- 

 bution of the segregated forms into two groups, in the pro- 

 portion of nine colored to seven white. If we assume that 



