144 Pattern showing in Albino [CH. 



marked mouse and a self-coloured blue may be represented 

 as due to the re-combinations of the two pairs of factors. 



1. S, self colour. s, its absence, viz. Dutch-marked. 



2. >j dense colour. d, its absence, viz. dilute colour. 



Black is common to both parents, and so need not be represented. 

 Dutch-marked black x Blue 



sD Sd 



f\ Black self colour 



SsDd 



&>... Black self colour Black Dutch Blue self colour Blue Dutch 

 9 SD 3s> 3 Sd i sd 



The recognition of these subtraction-stages becomes 

 important when we attempt to estimate the minimum 

 number of factors necessary to produce the results we per- 

 ceive. Symbolically the various subtraction-stages may be 

 represented as depending on the removal of distinct factors, 

 but physiologically they may be caused by special quantita- 

 tive subtractions from one of the factors causing the produc- 

 tion of colour, and consequently an economy of hypothesis 

 may be made. 



In exceptional cases the pattern which an albino form is 

 carrying, if the expression be permitted, can be actually 

 recognized by inspection though no real colour is developed. 

 For example Lock noticed the following case in edible peas 

 (Pisum sativum). Certain varieties of peas have brown 

 anastomosing lines on the coats of the seeds. These peas 

 are called maples in England (pois perdrix of French seeds- 

 men). The maple skin occurs only in the seeds of strains 

 which have coloured flowers. Such plants crossed with an 

 ordinary white-flowered type having a plain seed-coat gave 

 this result : 



Parents Flowers coloured x Flowers white 



coats maple coats plain 



f\ Flowers coloured 



coats maple 

 J% Flowers coloured Flowers white 



9 coats maple 3 coats plain 4 coats plain: some showing 



traces of mapling 



Among the white-flowered group in F z were some plants 

 which bore seeds showing traces of the maple marking, not 

 as brown lines, for no actual pigment seemed to exist, but as 

 a damasked pattern showing where the mapling would have 



