62 HEREDITY 



present, the comb is walnut. The one factor is, as it 

 were, struggling to make a rose comb, the other to 

 make a pea, and the result is the walnut form. The 

 walnut type is not a hybrid between the two. It is 

 both pea and rose, and as such may breed true. Our 

 ratio of 9:3:3:1 is produced by the independent 

 inheritance of two pairs of characters, only in this 

 case the two dominants produce a special combined 

 result. So far as the subsequent results are concerned 

 it is immaterial whether both dominants be introduced 

 through one parent, as in the walnut x single cross, 

 or whether one dominant be introduced through each 

 parent, as in the rose x pea cross. 



Another interesting case is that of the appearance 

 of the purple colour in crossed " Emily Henderson " 

 sweet-peas, which was mentioned in the Introduction. 

 The facts are that " Emily Henderson " is a pure white 

 variety, the plants being apparently all of one type, 

 except that some have round pollen-grains, others oval. 

 When two of these white-flowered plants are crossed, 

 the progeny are sometimes white-flowered, but sometimes 

 also purple. The appearance of this purple-flowered 

 sweet-pea is a distinct case of reversion; for it is 

 known that all our cultivated forms have been derived 

 from the wild sweet-pea found in Sicily, which has 

 purple flowers. 



As before mentioned these plants, when allowed to 

 self-fertilise, produce coloured and white-flowered pro- 

 geny in the ratio of 9:7. The explanation of this 

 remarkable case, given by Professor Bateson, is that 

 two factors are necessary for the production of colour. 

 Neither of these factors alone produces any colour ; the 

 two must act together. We might compare the case 

 to that of the action of light on sensitised paper. The 

 one type possesses the basis for colour production, but 

 lacks the developing agent. The other has the factor 

 for developing the basis, but has no basis to develop. 

 Hence when the two types are crossed, colour is pro- 

 duced. In the second generation, nine individuals 

 out of every sixteen will have both dominants and will 



