478 GENERAL CONSIDERATIONS [Ch. XIX 



The conclusion to which de Varigny arrived was that the 

 nanism provoked in the small vessels is caused by a diminution 

 of movement and consequently of exercise, for in the restricted 

 medium the movements are fewer and slighter, since the food is 

 near at hand. Also, the snails will exercise more in a body of 

 water with a broad, exposed surface than in a narrower one, 

 because they crawl so much on the surface films. 



This interpretation is sustained by all the results excepting 

 those obtained from a varying number of individuals in masses 

 of water of the same volume and form. The fact that the 

 individuals grow larger the fewer they are, may be accounted 

 for, thinks de Vaeigny, on psychological grounds ; for, in the 

 crowded room, movements will be restricted from very much 

 the same cause that makes one move less rapidly in a street 

 containing impediments than in one which is quite clear. 



It may well be doubted whether the bottom of this matter 

 has yet been reached. Probably nanism is produced by several 

 causes : such as insufficient supply of mineral constituents in 

 the water, especially of calcium salts, products of excretion in 

 the water, and exercise. There is, however, much reason for 

 believing that Hogg's conclusion is one which, with our fuller 

 knowledge, we can hardly improve upon — that, in I'espect to 

 the size attained as in other qualities, the snail has the power 

 of "adapting itself to the necessities of its existence." 



§ 3. General Considerations relating to the Action 

 UPON Growth of External Agents 



1. Modification of Rate of Growth. — Protoplasm is composed 

 of three kinds of substances: the living plasma, the formed sub- 

 stance, and the watery chylema. Accordingly, growth involves 

 three processes : that of the formation of new plasms, assimila- 

 tion; that of the manufacture of formed substance, secretion; and 

 that of the absorption of water, imbibition. Anything which 

 affects these three processes will affect the rate of growth. 



The action of external agents upon the rate of growth is of 

 two general kinds. There is, first, the general effect ; and, 

 secondly, the specific effect. The general effect is more or less 

 similar in all protoplasmic masses (organisms) subjected to the 

 agent ; it is the result of some immediate and necessary modifi.- 



