448 PACKARD— ORIGIN OF MARKINGS OF ORGANISMS. [Dec. 2, 



Much light, it seems to us, has been thrown on the actual mode of 

 origin of stripes and bars in general by Keeble and Gamble^ in their 

 valuable essay on the color physiology of the higher Crustacea. 

 They hold that the adult color pattern of Hippolyte varians '* is 

 determined by the environment and not inheritance." "The 

 adult pattern arises in spite of difficulties which are thrown in its 

 way. Gaps are bridged over, stripes are evolved, stripes are made 

 to bars, and bars blend to a monochrome," How a bar is formed 

 by a bar of shadow is thus stated : 



'* We know (see Section IX) that the pigments of the chromato- 

 phores expand into the branches under certain conditions of illumi- 

 nation {eg., on dark background) and contract into the centres in 

 diffuse light. Imagine a practically transparent Hippolyte (such, 

 for example, as the earliest adolescent faint brown-lined forms) 

 resting persistently, as it does rest, in such a situation that a bar of 

 shadow falls across it, whilst over all the rest of its body light falls. 

 In the region of the bar of shadow the chromatophores will expand. 

 In the rest of the body they will be contracted to mere dots. 

 Grant that where the conditions are favorable to the activity of 

 the chromatophores growth will be greater than where its contents 

 are aggregated in the manufacturing centre — a supposition which is 

 no more unreasonable than that which supposes that functional 

 activity favors growth — then in the region of shadow new 

 chromatophores are formed, either by budding from or in relation 

 to an existing centre. These in turn give rise to new centres in a 

 like manner. The bar of shadow is now reflected on the surface of 

 the animal by a bar formed of chromatophore branches. Hippo- 

 lyte has grown into its surroundings." 



'' Further, if we accept a recent explanation of absorption color 

 photography, we can see how the color of this bar of shadow comes 

 to resemble that of the object that casts it. For Wiener has shown 

 (1895) that a combination of substances may exist so sensitive to 

 light as to be decomposed thereby, and give rise to a pigment of 

 the same color as that of the incident light. If we postulate such 

 a substance or combination in the chromatophores of Hippolyte, 

 then the bar which we have just seen produced will be of the same 

 color as that of the object which throws the shadow. We may then 

 picture the mode whereby Hippolyte varimis becomes infinite in 

 variety." 



^ Phil. Trans. Roy. Soc, London, Vol. 196, 1904, pp. 327-8. 



