144 ACQUIRED CHARACTERS sec. 



their colours in accordance with their surroundings, and 

 thereby become less conspicuous. Besides the chameleon, I 

 will only mention the Sepias among the Cephalopods. This 

 is a much more delicate action of light than that which merely 

 changes the colour from light to dark, as occurs in so many 

 cases — a sudden action instead of the gradual effect by which, 

 for instance, darkness of tint is produced after some time in 

 man ; and physiologically different, for in the former case it 

 is a question of change in the shape of the pigment -cells 

 under the influence of the nervous system, in the latter of 

 deposition of pigment. 



Such sudden alteration of colour under the influence of 

 light is strikingly shown in certain fishes, e.g. trout, grayling, 

 minnows. In these the colour of individuals kept in the dark 

 becomes lighter on sudden exposure to light in consequence 

 of contraction of the pigment-cells. 



We attach too exclusive an importance to '•' adaptation " — 

 as I attempted to point out by examples previously given — 

 when we think to explain by selection every similarity 

 between the colouring of an animal and that of the ground on 

 which it lives. For, as we have seen, animals may become 

 similar in colour to their surroundings, actually adapted in 

 colour, quite by chance ; for instance, in consequence of the 

 direct, necessary action of light, i.e. of the surrounding colours, 

 and therefore without selection. Many really wonderful 

 cases of adaptation, apparently due to selection, probably 

 come under this category. 



It is also to be noticed that even rapid changes of colour 

 may also be due to a chemical action of light, and not invari- 

 ably to nervous influences. It has been observed, for example, 

 that the pupie of butterflies during their development take on 

 the colours of their surroundings. Pupae have assumed the 

 red colour of a cloth enveloping them, a colour to which they 

 could scarcely be exposed under natural conditions, and to 



