132 ANIMAL COLOEATION. 



way weed-colour with white spots. Even a Planarian worm 

 which lives in the weed is similarly yellow-coloured, and alsO' 

 a mollusc, Scyllcea pelagicar Mr. Wallace * quotes the above 

 passage to exemplify protective resemblance among marine 

 animals presumably brought about by the action of natural 

 selection. It would be highly desirable to ascertain whether 

 the colour of the animals is not simply due to the pigment of 

 the algae, just as the colour of caterpillars is sometimes due 

 to chlorophyll absorbed directly from their food. Here again 

 we should have to suppose that the pigment passes unaltered 

 through the bodies of two animals, for the carnivorous forms 

 could only get it from those that fed upon the algse. This 

 explanation obviously does not include the " white patches," 

 which look much more like natural selection. 



Considering the resistant nature of many pigmentary sub- 

 stances, vegetable as well as animal, it is at least probable that 

 a large number of cases of colour resemblance, often set down 

 to the action of natural selection, may be due, as in the case of 

 Eunice, to the simple excretion by the skin of these pigments 

 which have been taken in as food. Until more is known about 

 the chemical composition of animal pigments it would be rash 

 to adopt an elaborate explanation when the more simple one 

 would possibly be sufficient. 



That the green colour of many caterpillars is directly due 

 to their food has been shown by Mr. Poulton in some of his 

 highly important contributions to the colour question. This 

 green colour may be partly due to the food contained in the 

 alimentary canal, but it is also in many cases caused by the 

 green colour of the blood and of the epidermal layer. But this 

 green pigment is chlorophyll in a slightly altered condition. 



There remain, however , numerous cases which cannot in 

 * "Darwinism," p. 208. 



