THE ADAPTATION OF ORGANISMS 93 



summarise under the heads of heat, light, and elec- 

 tricity ; and, lastly, vital influences, the influences, 

 that is to say, exerted by neighbours, parasites, and 

 that most active of all vital agents, man. We may 

 consider our organism, in short, as a central unit 

 surrounded by an environment everything not 

 the plant in part aiding, in part retarding the 

 organism in its healthy development. The case is, 

 however, not so simple as it looks, for not only may 

 these various influences be all active at the same 

 moment, but they act on and modify each other, 

 and the modified influence may have an entirely 

 different effect on the organism from that which 

 it would have had if it had been unaltered by other 

 conditions. 



It will be useful at this point to quote a few examples 

 from the profusion of literature on the subject of 

 the influence of the environment on the organism. 



It has been shown by several experimenters 

 that, when bred in confined spaces, the offspring of 

 certain shrimps, &c., develop into dwarf forms, and 

 that a definite relation exists between the size of 

 molluscs and that of the vessel in which they are Meehani- 



grown. The influence of changes in the pressure ?i 



T -, ,. i P ., . influences- 



lateral and vertical ot the environment, more 



especially on the form of aquatic plants, on the shape 

 of corals, shells, sponges, trees, &c., has been the 

 subject of research by many investigators, while 

 the effect of pressures and tensions on cell form and 

 planes of division has also occupied the attention 

 of both botanists and zoologists. 



Looking next at chemical influences, tadpoles and 

 young fish, when well supplied with oxygen, develop chemical 

 more rapidly than under normal conditions ; drought influences, 

 induces encystation and latent life in many lower 



