3i6 The Study of Animal Life part iv 



characteristic is so advantageously protective as is usually 

 imagined : thus the coloured upper side in soles is very 

 often covered with a layer of sand. Soles come out most 

 at night, most live at depths at which differences of colour 

 are probably indistinct. In shallower water the advantage 

 is likely to be greater, though the white under-side slightly 

 exposed as the fish rises from the bottom may attract atten- 

 tion disadvantageously. Moreover, if we find in a large 

 number of different animals that the side away from the 

 light is lighter than that which is exposed, and if we can 

 show that this has in many cases no protective advantage 

 whatever — and I believe that a few hours' observation will 

 convince you that both my assumptions are correct — then 

 there is a probability that the absence of light has a direct 

 influence on the absence of pigment. 



But we are not left to vague probabilities ; Mr. J. T. 

 Cunningham has recently made the crucial experiment of 

 illuminating the under sides of young flounders. Out of 

 thirteen, whose under-sides were thus illumined by a mirror 

 for about four months, only three failed to develop black 

 and yellow colour-cells on the skin of the under-sides. It 

 is therefore likely that the normal whiteness of the under- 

 sides is due in some way to the fact that in nature little light 

 can fall on them, for they are generally in contact with the 

 ground. 



{d') Animate St<r7'onndi)igs. — We have given a few 

 instances showing how mechanical or molar pressures, 

 chemical and nutritive influences, and the subtler physical 

 energies of heat and light, affect organisms. There is a 

 fourth set of environmental factors — the direct influence of 

 organism upon organism. In a previous chapter we spoke 

 of the indirect influences different kinds of organisms exert 

 on one another, and these are most important, but there are 

 also results of direct contact. 



Much in the same way as insects produce galls on 

 plants, so sea -spiders {Pycnogonida) affect hydroids, a 

 polype deforms a sponge, a little worm {Myzostomd) makes 

 galls on Crinoids. Prof. Giard has described how certain 

 degenerate Crustaceans parasitic on crabs injuriously affect 



