XIV . INTERMEDIATES 163 



Whether the conclusions will be justified by further 

 investigation, time alone can show. It is, however, 

 clear that the most exact knowledge which' we 

 possess at present is in favour of the assumption 

 that Mendelian segregation occurs also in the much- 

 debated case of the mulatto. 



Davenport's explanation involves the conception 

 of two independent factors^ each sufficient by itself to 

 produce a similar effect, the effect being intensified 

 when both are working together. The idea of a 

 series of similar factors, or multiple factors as they 

 are generally termed, owes its origin to Nilsson-Ehle. 

 When investigating the inheritance of red and white 

 colour in the grain of wheat this observer found that, 

 while i-ed was dominant, reds could throw whites 

 in varying proportions. Careful analysis of many 

 such families showed that they fell into three groups, 

 in which the proportion of reds to whites was 

 respectively 3 : i, 15 : i, and 63 : i. These results 

 can ' be explained on the assumption that there 

 are three independent factors, each of which can 

 produce, independently, the red effect. If we call 

 them A, B, and C we can express the reds which 

 give a 3 : I ratio either as Aabbcc, aaBbcc, or aabbCc; 

 the reds which give a 1 5 : i ratio, as AaBbcc, AabbCc, 

 or aaBbCc; and the reds which give a 63 ; i ratio 

 as AaBbCc. For a white-grained plant only arises 

 from the union of two gametes abc. As the results 

 of a large series of experiments Nilsson-Ehle showed 

 that the conception of multiple factors was in full 

 accordance with the experimental data, and it is 

 now generally accepted by students of genetics. 

 In the case of the wheat grains it cannot be stated 



