xx THE INFLUENCE OF ENVIRONMENT 397 



tageously. It may be asked whether the absence of 

 pigment is not in part the direct physiological result of 

 the relative absence of the light-stimulus in each indi- 

 vidual case. 



The beginning of an answer is to be found in Mr. J. T. 

 Cunningham's crucial experiment of illuminating the 

 under-sides of young flounders. Out of thirteen whose 

 under-sides were thus illuminated 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 depends in part on the conditions of individual 

 nurture, to the fact that in natural conditions little light 

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

 the ground. 



(d) Animate Surroundings. 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 



~ o ' o 



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 (Pycnogonidae) affect hydroids, a 

 crab deforms a coral, a little " worm " (Myzostoma) makes 

 galls on Crinoids. Certain degenerate Crustaceans para- 

 sitic on crabs destroy the reproductive organs of their 

 hosts, and some internal parasites produce slight modifi- 

 cations of structure. Interesting also are the shelters 

 or domatia of some plants, within which insects and mites 

 find homes. 



Looking backwards, we recognise that environment 

 may influence the organism in varying degrees. There 

 may be direct results, rapid parries after thrusts, or the 

 results may be indirect. Some animals are more suscep- 

 tible and more plastic than others. Young organisms, 

 such as caterpillars and tadpoles, are more completely 

 in the grasp of their environment than are the adults. 



