THE COLORATION OF ANIMALS 59 



in the middle of the eighteenth century, recognized clearly, and gave 

 beautiful descriptions of what we now call colour- adaptation. It is 

 true that he gave them only as isolated instances, and was far from 

 recognizing the phenomenon of colour-adaptation in general, or even 

 from inquiring into its causes. From the time of Linne, the endeavour 

 to establish new species overshadowed all the finer observation of 

 life-habits and inter-relations, and, later on, after Blumenbach, 

 Kielmeyer, Cuvier, and others, the eager investigation of the internal 

 structure of animals also tended to divert attention from these 

 cecological relations. In systematic zoology, colour ranked only as 

 a diagnostic character of subordinate value, because it is often not 

 very stable, and indeed is sometimes very variable ; it was therefore 

 found preferable to keep to such relatively stable differences as are to 

 be found in the form, size, and number of parts. 



Charles Darwin was the first to redirect attention to the fact 

 that the colouring of animals is anything but an unimportant matter : 

 that, on the contrary, in many cases it is of use to the animal, e. g. in 

 making it inconspicuous ; a green insect is not readily seen on green 

 leaves, nor a grey-brown one on the bark of a tree. 



It is plain that the origin of such a so-called ' sympathetic ' 

 coloration, harmonizing with the usual environment of the animal, 

 can be easily interpreted in terms of the principle of selection ; and it 

 is equally evident that it cannot be explained by the Lamarckian 

 principle of transformation. Through the accumulation of slight 

 useful variations in colour, it is quite possible for a green or a brown 

 insect to arise from a previous colour, but a grey or a brown insect 

 could not possibly have become a green one simply by getting into 

 the habit of sitting on a green leaf ; and still less can the will of the 

 animal or any kind of activity have brought the change about. 

 Even if the animal had any idea that it would be very useful to it to 

 be coloured green, now that it had got into the habit of sitting 011 

 a leaf, it could not have done anything towards attaining the desirable 

 green colour. Quite recently the possibility of a kind of colour- 

 photography on the skin of the animal has been suggested, but there 

 are many species whose colouring is in contrast to their environment, 

 so that the skin in these cases does not act as a photographic plate, 

 and it would, therefore, have to be explained how it comes to pass 

 that it functions as such in the sympathetically coloured animals. 

 I do not ask for proof of the chemical composition of the stuff which 

 is supposed to be sensitive to light. Whether this be iodide of silver 

 or something quite different, the question remains the same: how 

 comes it that it has only appeared in animals to which a sympathetic 



