196 DARWIXISM chap. 



■when they are not brought into action, is a dirty Avhite. 

 These animals are excessively sluggish and defenceless, and the 

 power of changing their colom* to that of their immediate sur- 

 roundings is no doubt of great serA^ce to them. Many of the 

 flatfish are also capable of changing their colour according to 

 the colour of the bottom they rest on ; and frogs have a 

 similar power to a limited extent. Some Crustacea also 

 change coloiu-, and the power is much develoj^ed in the 

 Chameleon shrimp (Mysis Chamfeleon) which is gTay Avhen on 

 sand, but brown or green when among broAvn or green seaweed. 

 It has been proved by experiment that when this animal is 

 blinded the change does not occui\ In all these cases, 

 therefore, we have some form of reflex or sense action by 

 which the change is produced, probably by means of pigment 

 cells beneath the skin as in the chameleon. 



The second class consists of certain larvae, and pup?e, which 

 undergo changes of colour when exposed to difterently 

 coloured surroundings. This subject has been carefulh' 

 investigated by Mr. E. B. Poulton, who has communicated 

 the results of his experiments to the Eoyal Society.^ It had 

 been noticed that some species of larvae which fed on several 

 different plants had colours more or less corresponding to the 

 particular plant the individual fed on. ^Numerous cases are 

 given in Professor Meldola's article on " Variable Protective 

 Colouring" {Proc. Zool. Soc, 1873, p. 153), and while the 

 general green coloration was attributed to the presence of 

 chlorophyll beneath the skin, the particular change in corre- 

 spondence to each food-plant Avas attributed to a special 

 function Avhich had been developed by natural selection. 

 Later on, in a note to his translation of "Weissmann's Theory 

 of Descent, Professor ]\Ieldola seemed disposed to think that 

 the variations of colour of some of the species might be 

 phytophagic — that is, due to the direct action of the difterently 

 coloured leaves on which the insect fed. Mr. Poulton's 

 experiments have thrown much light on this question, since he 

 has conclusively proved that, in the case of the sphinx cater- 

 pillar of Smerinthus ocellatus, the change of colour is not due 

 to the food but to the coloured light reflected from the leaA-es. 



o 



^ Proceedings of the Boyal Society, 'So. 2iZ, 1SS6 ; T raiisactions of the Royal 

 Society, vol. clxxviii. B. pp. 311-441. 



