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- 



