78 HEREDITY [CH. 



In the account given above of the colour-factors 

 in the sweet-pea it has been shown that at least two 

 separate elements are required to produce colour (in 

 this case red), and a third if blue is to be present in 

 addition. But for the production of the various shades 

 or distribution of colour further factors are known, 

 e.g. for the intensification or dilution of colour, and for 

 making the wing-petals of the same or difierent colour 

 from the standard. Similar phenomena are concerned 

 with colour in animals, of which domestic varieties 

 of the rat provide a simple instance. Rats, other 

 than albinos, are in general either 'self-coloured,' 

 with little or no white (this, if present, is confined 

 to the ventral surface), or 'hooded,' i.e. white with 



flowers (snapdragon) experiment shows that the chromogen itself 

 may be absent. 



As the purple colour in sweet-peas is due to more complete 

 oxidation of a chromogen than red, so in animals colour-physiologists 

 find that the series yellow, brown, black, may represent successive 

 oxidation-stages of the same chromogen by the same ferment. The 

 various colours of mice, for instance, are not therefore to be regarded 

 as necessarily produced by different ferments, but the inherited 

 'colour-factors' determine to what stage the oxidation of the 

 chromogen shall be carried. Some confusion has arisen from 

 the assumption that the ' factors ' postulated by students of heredity 

 are actual specific colour-ferments, while they may be rather 

 determinants which cause the oxidation of the chromogen to 

 proceed to a particular stage, and may be compared with the 

 factors which determine the production of a rose-comb or single- 

 comb in fowls. 



