1 64 MENDELISM chap. 



that the depth of the colour is a clear indication 

 of the number of red - producing factors present. 

 Though a plant with all three factors may be 

 of a rather deeper red than one in which but a 

 single factor occurs, the differences in tint are 

 not sufficiently well marked to allow of a colour 

 classification of the reds in terms of their genetic 

 constitution. 



Nilsson-Ehle's work, however, placed the con- 

 ception of multiple factors on a firm basis, and 

 since then it has been applied in explanation of 

 other cases where a long series of intermediates 

 results from a cross. Especially is this the case 

 where quantitative characters such as size and 

 weight are involved. A cross between a large 

 and a dwarf form may sometimes, as in the Cupid 

 sweet-pea, result in complete dominance of the 

 larger form, followed by a simple 3 : i ratio of 

 large to dwarfs in Fg. But it frequently happens 

 that Fj is intermediate, and that in F^ there is a 

 continuous series of forms ranging between the 

 original small and large parents. In illustration 

 we may take a case from poultry, where a cross 

 was made between a Sebright bantam and a Gold- 

 pencilled Hamburgh, the latter breed being normally 

 almost twice as heavy as the former. The F^ birds 

 were intermediate in size (cf. Fig. 46), though they 

 approximated more closely in this respect to the 

 Hamburgh. The F^ generation raised from the F^ 

 birds showed a wide range of variation, at the 

 extremes of which were birds heavier than the 

 Hamburgh, and others lighter than the Sebright. 

 The results obtained are explicable on the view 



