278 THE EVOLUTIO^' THEOEY 



of different families, in regard to which Poulton showed tliat in their 

 early youtli they possess the power of adapting themselves exactly 

 to the colour of their chance surroundings. It is obvious that the 

 protection which the caterpillar would gain from being coloured 

 approxi7}iatelu like its surroundings would be insufficient, for instance 

 because the surroundings may be very diverse, since the species lives 

 upon different, variously coloured plants and plant-parts. Thus a 

 facultative adaptation arose. Selection gave rise to an extraordinarily 

 specialized susceptibility on the part of the different cell elements 

 of the skin to differences of light, and the result of this is that the 

 skin of the caterpillar invariably takes on the colouring which is 

 reflected upon it in the first few^ days of its life from the plants and 

 plant-parts by which it is surrounded. Thus the caterpillars of one 

 of the Geometrida3, AQivpJnchms hetularia, take on the colours of the 

 twig between and upon which they sit, and they can be made black, 

 brown, white, or light green quite independently of their food, accord- 

 ing to the colour of the twigs (or paper) among which they are reared. 



Colour-change in fishes. Amphibians, Reptiles, and Cephalopods, 

 depends upon much more complex adaptations. In their case a reflex- 

 mechanism is present which conducts the light-stimulus affecting 

 the eye to the brain, and there excites certain nerves of the skin ; 

 these in their turn cause the movable cells of the skin which 

 condition the colouring to change and rearrange themselves in 

 the manner necessary to bring about the harmonization of colour. 

 On this depends the colour-change of the famous chameleon, and also 

 the scarcely less striking case of the tree-frog, which is light green 

 when it sits on trees, but dark brown when it is kept in the dark. 

 All these are secondary reactions of the organism in which the 

 external stimulus is, so to speak, made use of to liberate adaptive 

 variations, either permanently or transitorily. In the caterpillars 

 colour-changes are permanent, that is, it is only the young caterpillar 

 which takes on the colour of its surroundings ; later it does not change, 

 even when it is exposed to different light, or intentionally placed upon 

 a food-plant of a different colour. In fishes, frogs, and cuttlefishes, 

 on the contrar}^ the reaction of the colour-cells to light only lasts 

 a little longer than the light-stimulus, and it changes with it. The 

 purposiveness of this difference of reaction is obvious. 



We cannot say to what degree the direct influence of external 

 conditions is eflectively operative on the germ-plasm, or how far, by 

 persistently repeated slight changes, the determinants and the parts 

 of the body determined by them may be made to vary in the course 

 of generations ; that is to say, how large a part this direct influence 



