1878.] 
Colouration of the Organic World. 
47 
have been found of exactly the same tint as the flowers in 
which they lurk. Mr. Wallace, on the authority of Sir 
Charles Dilke, mentions a pink-coloured Mantis which, when 
at rest, closely resembles the pink flower of an orchis, and 
is thus enabled to seize unsuspecting butterflies. But we 
should be wrong in ascribing such similarity of colouration 
to the effects of reflected light, or, indeed, of any merely 
physical influence. They are almost certainly due to physi- 
ological causes, and are instances of what is called 
“ protective colouration.” 
There is another class of phenomena which at first sight 
seems due to the aCtion of light. Many inseCts when they 
first emerge from the pupa are abnormally pale, and do not 
take their full mature colouration until after a longer or 
shorter interval of time. It was in virtue of this property 
that an entomologist, commissioned by the German Govern- 
ment to inspect a field where the dreaded Colorado beetle 
had made its appearance, was enabled to decide that these 
inseCt enemies had only just appeared in the mature form, 
and that on turning up the ground a further stock would be 
found in a rudimentary state, as on aCtual trial was found 
to be the case. But this gradual development of colour has 
not been proved to be the result of light. We have reared 
up caterpillars in perfect darkness, and have found their 
colours on reaching maturity no less brilliant than those of 
their fellows which had been exposed to light in the ordinary 
course of nature. In the case of interference-colours a 
change in the physical condition of the integuments, conse- 
quent of their drying and hardening on exposure to the air, 
is doubtless necessary for their development. The evolution 
of the pigment-colours we are at present investigating, and 
believe that it is simply due to a process of oxidation. 
Some other of the phenomena advanced in support of the 
“ light theory ” of organic colouration may also be, with 
great probability, referred to other causes. Thus some 
ascribe to light the faCt that the upper surface of most 
animals is more intensely coloured than their under side. 
Many fishes have a dark back, and a pale greyish blue or 
greenish belly ; but, as Mr. Wallace points out, this ar- 
rangement seems more protective than due to the aCtion of 
light. An enemy — say a sea-bird — looking down from above 
will have difficulty in distinguishing the dark back of the 
fish amidst the water. On the other hand, an enemy looking 
from below will see the pale belly of the fish against the dull 
bluish colour of the sky as seen on looking up through the 
water, and will scarcely deteCt its presence. Now, were 
