226 
COLOUR IN NATURE 
CHAP. 
in Amphibians. The power of colour-change is 
familiar in the case of the common frog. A very- 
little experimentation will show that in it the 
general colour is dull and dark against a back¬ 
ground of earth or peat, and bright yellow-green 
among fresh herbage. In spite, however, of this 
power of colour-change, and of individual colour 
variation, there is much constancy of marking. In 
our common British frog there are two longi¬ 
tudinal stripes running down the sides of the body 
which seem to be absolutely constant, and which 
appear in the larva at the commencement of the 
metamorphosis. All those who have kept tadpoles 
in confinement must have noticed this fact, and 
learnt to regard it as a sign that their pets will 
shortly require a complete change of environment— 
from water to land. Markings of this kind occur 
constantly both in Amphibians and Reptiles, and are 
of much importance. Their origin has been investi¬ 
gated in one case only, in a snake, and it has been 
found that, as might be expected, they are closely 
related to internal structures. Further investigations 
on the same lines would be of much interest. That 
they are constant throughout large groups and 
dependent upon “ laws of growth ” has long been 
maintained upon theoretical grounds by Professor 
Eimer and his school. 
As to the details of the mechanism of colour, the 
slight development of the epidermis, and the power 
of colour-change, show at once that the elements 
important in coloration must occur in the true skin 
or dermis. The epidermis does contain a small 
amount of pigment granules, but these are unim- 
