H. Onslow -j-j") 



position or nuinlH»r of rrrUiin chciiiiail ^'rnu|>H'. If thr clian^rr m 

 quantiUitive ami tho pi^tiRMit Ih (iiri'uHo<i, th(> iimount " in H<»liiti«>n" umy 

 be incn»ai»e<l, or the pigment may IxroiiH' a^^rr^at^Ml in .small graimlcH 

 or inassoH. The orange-red pigments of the l*ieri<lae an- all y«H<»w in 

 mpitM)Us solution, the orange colour being probably due t^) inen;i,s«<l 

 oonoentnUion". The jwile yellow butterfly (\diaH hyale^ is said l«» 

 conUvin mostly diffused pigment, wherejus the more <leeply coloun'<l 

 C7. edusa contains gnundar nuwses of pigment between the walls of each 

 scale. 



(3) A similar deeixMiing «)f colour might be j)rodueed by an increase 

 in the number of scales, i.e. several overlapping layers of sc;iles woidd 

 give a deejKT shade than a single layer. 



(4) Finally, the position of the pigment within the scale itself 

 might affect the colour, which would depend im the proximity of the 

 pigment to the upper surface, e.g. it might be chieHy in the upj)er or 

 lower membrane of the scale, or it might be between the two*. 



Clearly a correct decision of these cpiestions is of the utmost im- 

 portance, for if the pigment varies (piantitatively and not <jualit;itively, 

 the factor or f;ictors which control the amount must be capable (»f acting 

 in a quantitative manner upon the pigment-producing mechanism. 

 Now this mechanism, in many if not in all ciises, consists of an enzyme 

 (usually oxidising) which acts upon a colourless chromogen. It is 

 difficult to see how the addition or subtraction of a factor — a change 



* In the yellow flower pigments for example the pale yellow flavone Apigeniu 



O 

 HO 



HO CO 

 becomes the much more deeply coloured pigment Luteolin, 



_ O^ ^ .OH 



HO '" 



HO CO 

 i.e. an extra hydroxyl group appears in the side ring. 



« F. G. Hopkins, Phil. Trans. Vol. 186, B, p. 678, 1895. 

 » W. Geest, Zs. tciss. Imektenbiol. Berlin, Vol. iv. p. 162, 190r». 



* A case of dilution of colour owing to localisation of pigment occurs in the hairs of 

 black and blue mice and rabbits. H. Onslow, Proc. Roy. Soc. B, Vol. 89, p. oG, 1915. 

 Joom. of Gen. viii 16 



