DARWINISM 
CHAP. 
196 
when they are not brought into action, is a dirty white. 
These animals are excessively sluggish and defenceless, and the 
power of changing their colour to that of their immediate sur¬ 
roundings is no doubt of great service 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 colour, and the power is much developed in the 
Chameleon shrimp (Mysis Chamseleon) which is gray when on 
sand, but brown or green when among brown or green seaweed. 
It has been proved by experiment that when this animal is 
blinded the change does not occnr. 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 pupae, which 
undergo changes of colour when exposed to differently 
coloured surroundings. This subject has been carefully 
investigated by Mr. E. B. Poulton, who has communicated 
the results of his experiments to the Royal Society. 1 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 
<dven in Professor Meldola’s article on “Variable Protective 
o 
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 was attributed to a special 
function which had been developed by natural selection. 
Later on, in a note to his translation of Weissmann’s Theory 
of Descent, Professor Meldola 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 differently 
coloured leaves on which the insect fed. Mr. Poulton’s 
experiments have thrown much light on this question, since he 
has conclusive^ 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 leaves. 
1 Proceedings of the Royal Society, No. 243, 1886 ; Transactions of the Royal 
Society, vol. clxxviii. It pp. 311-441. 
