48 The Action of Light upon the [January, 
this arrangement of colours reversed the fish would be much 
more readily seen, either from above or from below, and the 
chances of its escaping from its enemies would be much 
reduced. At the same time it must be confessed that this 
explanation is not admissible in all cases of a similar ar- 
rangement of colour. Thus in many Crustaceans unable 
to swim, and therefore not liable to be seen by any enemy 
from below, the under surface is much paler than the back. 
Similarly slugs — whether creeping upon the ground, upon 
the stems or leaves of vegetables — are liable to be espied 
from the back or the sides, but never from beneath ; yet in 
most cases their under surface is decidedly paler than their 
back. These instances, and others which might he adduced, 
certainly seem to agree better with the supposition of a 
darkening influence due to the freer action of light upon the 
upper side than with that of a protective distribution of 
colour. 
But from the whole of the evidence before us, especial 
attention being paid to the case of the deep-sea anemones, 
we are forced to conclude that the colouration of an animal 
species is not, in the mathematical use of the word, a 
function of the amount of solar radiations to which it is 
exposed. That this conclusion does not compel us to deny 
the influence of sunlight upon the hues of all animals, 
under all conceivable circumstances, scarcely needs to be 
stated. 
The faCt that Lepidopterous larvae are in a majority of 
cases, partially at least, of a green colour is not inexplicable. 
They retain in their bodies, in an undecomposed state, the 
chlorophyll of the leaves upon which they have fed. Larvae, 
on the other hand, whether Lepidopterous or Coleopterous, 
which feed not upon leaves, but upon wood, roots, seeds, 
&c., not containing chlorophyll, may naturally be found de- 
ficient in this green colour, without our taking the presence 
or absence of light into account. Hence we need not 
wonder that the caterpillars of the goat-moth and the 
wood-leopard, the larvae of the Longicornes, Buprestidae, 
Dynastidae, &c., are not green : they have not been consuming 
a green pigment. But why have we comparatively so few 
green butterflies and moths, and so many green birds and 
green beetles ? The green colours found in birds and in 
beetles — with the exception of such forms as Cassida, a leaf- 
feeder, are due not to a pigment, but to the interference of 
light, so that their formation must be explained on different 
principles. The paucity of green butterflies may, perhaps, 
be traced to the faCt that chlorophyll is a mixture of two 
