82 BIOLOGY. [BOOK i. 



other vital properties. Let but the nutritive liquids impregnated 

 with oxygen cease to reach the nervous cells, to bathe them, 

 to excite them, to renew them, immediately motility, sensibility, 

 and thought vanish ; the animal re-descends, for a time, or for 

 ever, to the level of unconscious organised beings. 



From this physico-chemical point of view we can now, without 

 much difficulty, form an idea of the totality of the molecular 

 movements which form the essential basis of life. Every living 

 being is constituted, in a general manner, of colloidal substances 

 more or less fluid, more or less solid, holding in solution salts, 

 gases, and so on. A portion of these salts and gases has been 

 introduced from without, and is ready to combine itself with the 

 unstable colloidal substances ; some are the result of combina- 

 tions already effected ; but this process of combination and 

 separation cannot stop ; for the atoms of atmospheric oxygen 

 mingle themselves ceaselessly with the organic molecules, separate 

 them, disaggregate them by virtue of their powerful affinities 

 for certain elements which form part of their complex molecules. 

 After a time more or less short, the oxygen, by a slow oxydation, 

 would have thus destroyed the living substance, if food had not 

 likewise been introduced from without into the texture of the 

 living being. These renovating substances, after having often 

 undergone preparatory chemical changes, after having become 

 nutriments, that is to say, after having acquired a chemical com- 

 position and a physical state which assimilate them to the living 

 substance, identify themselves with it. One by one their mole- 

 cules take the place of those which have been destroyed. The 

 living being, thus incessantly restored, lasts, continues to live, 

 and would live indefinitely, if this molecular movement never 

 slackened. 



But we now know, through the magnificent generalisations of 

 modern chemistry and physics, that in the world there are. only 

 atoms in some degree animated, that these atoms transmit to each 

 other mutually the movement which impels them, or which they 

 engender, and that this movement, without ever being annihi- 



