THE MECHANISM OF LIFE 539 



the more solid a body is, the less its molecules are in 

 motion. Motion is least in the hardest bodies, and greatest in the 

 thinnest liquids ; in the latter the intensity of the motion is almost 

 equal to that in gases, which, as is well known, is so great that the 

 molecules repel one another. Between the two limits of very thin 

 liquids and very hard bodies the solidity of the structure varies, 

 becoming greater the harder the body is. There is, in fact, a cer- 

 tain molecular structure in every simple solution. If, e.g., a crystal 

 of pure salt be put into a vessel containing distilled water, after 

 some time it dissolves, and the molecules of the salt become 

 scattered by diffusion uniformly throughout the liquid, so that in 

 every volume of the latter, even the most minute, the same per- 

 centage of salt molecules is contained. In other words, an attrac- 

 tion between the molecules of the salt and those of the liquid takes 

 place, and about every one of the former is grouped a certain 

 number of the latter. The only difference as regards this grouping 

 or structure between the mobile liquid and the solid body is that, 

 during the active molecular motion in the liquid, molecules are 

 continually being drawn away from their groups and replaced by 

 others, so that the structure is continually being destroyed and re- 

 formed ; while in the solid body, where the motion of the molecules 

 is slight, the structure can exist for a long time undisturbed. This 

 continual reformation of structure in the liquid is, however, of 

 fundamental importance to living substance, for only where there 

 is a possibility of continual outgo and income of molecules can a 

 metabolism exist, and without this living substance is inconceivable. 

 But this continual change of molecules does not hinder the con- 

 tinual appearance of differentiations of form in certain places re- 

 sulting from molecular and atomic groupings in the living substance. 

 Just as a stream of water or a gas flame can maintain a very 

 definite form, although at no two successive moments do the same 

 molecules produce that form, so living substance, in spite of its 

 liquid nature, can show certain continual differentiations of form, 

 which exist so long as the causes for the definite grouping of the 

 molecules and atoms remain the same. 



This consideration is of great importance, for it enables us to under- 

 stand the general phenomena of the construction of form in living 

 substance. The apparent paradox that living substance, although 

 its components are undergoing continual change, can possess in 

 many cases a constant and often extraordinarily complex form, is 

 at once explained. Let us imagine a cell that possesses various 

 kinds of differentiations, for example, the flagellate infusorian 

 Poteriodendron^hich besides its nucleus is provided with a flagellum 

 and a contractile myoid-fibre (Fig. 263). In each of its individual 

 differentiations the particles are arranged in a specific manner, in 

 the nucleus differing from that upon the surface of the protoplasm, 

 in the flagellum differing from that in the myoid-fibre. Never- 



