THE LIVING SUBSTANCE II 



brain. It has been suggested that the transmission of 

 a nerve impulse may be connected with a wave of gelation 

 sweeping along the nerve, an almost instantly reversible 

 change in the density of the material. If this is true, 

 there is some analogy with the transmission of sound 

 in air, the sound "waves" representing temporary 

 conditions of density. Darwin, when experimenting 

 with that remarkable insectivorous plant, the sundew 

 (Drosera), found that if one side of the leaf received a 

 stimulus (e.g., caught a fly), the sensitive hairs on the 

 other side moved after a time. During the interval, 

 he noted that a wave of cloudiness passed across the 

 leaf, apparently a condition of temporary or reversible 

 gelation. This activity was prevented by such an- 

 aesthetics as ether or chloroform, and if their action 

 on the nerve tissue of animals is analogous, we can 

 understand how their effects are produced. 



10. The colloidal particles of the living substance, Energy of 

 each consisting of many molecules, bear electric charges, 

 They are immersed in or surrounded by water, con- 

 taining dissolved materials which themselves bear 

 positive or negative charges of electricity. The whole 

 forms a system in which attraction and repulsion, and 

 therefore gelation or liquefaction, depend upon electric 

 states. When particles of a colloid are brought to- 

 gether, or when they are driven apart, as the result of 

 electric forces, new states are produced, leading to 

 fresh changes. Thus, without going into further de- 

 tails, we gain some idea of the physical phenomena 

 implied when we say that living protoplasm is a dy- 

 namic system of atoms and molecules. We also see 

 how the life processes are dependent upon the presence 

 of water and of non-living matters in solution ; in other 

 words, the protoplasm molecules, though the exclusive 



