140 Discussion of Colour-Evidence [ch. 



factor. In three cases among plants (Sweet Pea, Stock, 

 Orchids) however we already know that the production of 

 colour {sc. sap-colour) requires the fortuitous concourse of 

 two complementary factors which have independent distribu- 

 tions in gametogenesis, and individuals lacking either of 

 these factors are completely devoid of colour. 



In the light of this discovery we naturally ask whether 

 it is not probable that the sap-colours of plants in general 

 may not in reality be produced by pairs of complementary 

 factors. It is tempting also to speculate on the possibility 

 that the colours of animals may have a similar nature. At 

 present however the objection holds that in no species of 

 animal have two pure albinos been found to produce coloured 

 offspring when mated together, and the F^ ratio from the 

 cross albino x coloured is always 3 coloured : i albino, never 

 9 : 7. But it may be suggested with great plausibility that 

 this simply indicates that every individual, coloured or 

 albino, contains one of the two factors, and the question 

 whether colour is a single — or a double — factor character 

 remains undecided. If a variation were to occur by which the 

 supposed common factor was omitted from the composition 

 of an albino, albinos bearing respectively each of the two 

 factors could be raised, and nothing would then preclude the 

 production of coloured individuals by crossing the two sorts 

 of albinos together. 



At first sight some of the facts related in regard to fowls 

 seem to supply evidence of this kind. White Silky x a 

 recessive white strain produces F^ fully coloured. But 

 neither parent is an albino in any strict sense, for both have 

 eyes fully pigmented. As a matter of fact also the Silky 

 breed, though quite white in plumage when adult, often — 

 perhaps always — has some buff colour in its down. The 

 resemblance is therefore far from being complete. 



Another case which suggests a similar interpretation is 

 that of eye-colour in the mouse, for there black eyes result 

 in F^ from crossing certain pink-eyed mice having coloured 

 coats with certain albinos. But here again one of the 

 parents is obviously not albino, and as we now know from 

 Miss Durham's observations, the eye of the coloured parent 

 though ostensibly pink, really contains a small but definite 

 amount of pigment. 



