THE WONDER OF LIFE 543 



uncoloured form. It is carried to the fatty bodies and 

 accumulates there in numberless granulations, re-assuming 

 the violet colour, which shines through the brownish 

 integment and makes the grub effectively harmonious 

 with the colour of the stamens amid which it works. 



In the case of the Tree-Sloth, the green colour of the 

 hair is due to Algae which might grow elsewhere, and as 

 there are other instances of plants of low degree growing 

 on living hairs, the Sloth's case is no particular puzzle. 

 In Baron Albert von Sack's Voyage to Surinam (1810) 

 there is a surprisingly early reference to this peculiar case 

 of protective resemblance. 



' The colour and even the shape of the hair are much in 

 appearance like withered moss, and serve to hide the animal 

 in the trees, but particularly when it gets that orange- 

 coloured spot between the shoulders, and lies close to the 

 tree ; it looks then exactly like a piece of branch where 

 the rest has been broken off, by which the hunters are often 

 deceived '. 



Account must also be taken of cases where the particular 

 colour depends physiologically on that of the surroundings. 

 Some caterpillars have a sensitive period during which 

 their colour or that of the pupae is influenced, within certain 

 limits, by the surrounding colour. This has been proved 

 experimentally by Professor Poulton, Schroder, and others. 

 How the colour of the reflected light affects the pigmen- 

 tation of the animal is obscure, but the fact is certain in a 

 few cases, and it is to be associated with the fact, also 

 proved in a few cases, that the mortality among pupae is 

 in part dependent on the degree of their inconspicuousness 

 against certain backgrounds. It is stated that the colour 



