92 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



more or less protected against their enemies by the resemblance 

 thus caused between the colour of their skin and that of their 

 surroundings. Similar observations were made by Agassiz, 

 Ayres, and Storer, on the salmon of the United States, while 

 European naturalists were the first to experiment on Amphibia 

 which exhibit a similar power of assuming a protective hue. 

 Finally, Heincke of Kiel has quite lately published a very 

 careful description of the protective changes of colour ^^ in Gobitis 

 Buthensparri, which exhibits the most conspicuous variations of 

 colour that have as yet been described. 



Before entering oil the discussion of those experiments by 

 Lister and Pouchet on the chromatic function, by which we 

 were first enabled to understand the observations above men- 

 tioned, it will be advisable to describe the structure of the skin 



Fia. 25.— Section of a frog's skin, ep, epidermis, including five pigment-cells ; c, cutis 

 "with black star-sliaped, deep-seated cells ; a and &, a thick single layer of yellow pig- 

 ment cells close under tlie epidermis. 



and the mode of distribution of the pigments in it. ■ One ex- 

 ample — the skin of the frog — will suffice for all cases. The skin 

 (fig. 25) consists of two distinct portions, the epidermis and 

 the cutis. The former (fig. 25, ep) is entirely composed of cells, 

 and the innermost layer contains cylindrical cells; the cutis is 

 chiefly fibrous and encloses nerves, large cavities for glands and 

 cell elements. These last are commonly filled with pigment, 

 and the remarkable changes of colour in the frog's skin depend 

 entirely on the distribution of these highly ramified pigment- 

 cells and their power of shrinking under certain kinds of irrita- 

 tion. The pigment in these contractile cells — known as the 

 chromatophores — is different in difierent individuals and in dif- 

 ferent parts of the body, yellow, brown, black, sometimes even 

 red or green. Besides, the colour of the chromatophores varies 



