340 BIOLOGY. [BOOK v. 



exists. Force is espoused anew to matter, from which moreover 

 it was never separated, except in the brain of metaphysicians. 

 There t is no longer inertia in the universe. Behind all these 

 phenomena is concealed an extended substance, never immobile, 

 and which is itself its own mover. If we still often speak of 

 forces, it is from pure habit, or by pure artifice of language. In 

 final analysis, the idea of force may always be brought back to 

 movements of an active material substance. 



Without speaking even of the planetary movements, which 

 carry everything along, we know that those bodies most solid 

 and immovable in appearance resolve themselves into atoms 

 and molecules, ceaselessly animated with rapid movements. 

 Naturally, these molecular movements also take place in organ- 

 ized bodies. There, they are varied very differently, since 

 these bodies are nourished, that is, are the seat of incessant 

 molecular exchanges, and are decomposed without intermission 

 to be recomposed. 



The molecular movements are, then, effectuated in every living 

 substance, but all living bodies are not endowed with motility. 

 Motility may be defined as the property which certain organized 

 bodies have, either of totally changing their position in space, 

 or of contracting, or of becoming shorter, or of modifying for 

 the moment their form, spontaneously and independently of any 

 mechanical action of exterior origin. 



Definitions, with whatever precision we may endeavour to 

 give them, are always adjusted with difficulty to all the peculiar 

 facts which they embrace. There are certain movements, for 

 example, which we do not very well know how to classify ; we 

 wish to speak of movements still incompletely studied, and 

 known under the name of Brownian movements. The botanist, 

 Robert Brown, was the first to prove that the fine particles of 

 mineral dust, well pounded, moved, apparently spontaneously, 

 when held in suspension in a liquid medium. In fact, all 

 corpuscles having from three to four thousandths of a millimetre 

 in diameter, execute in liquids, under the microscope, a kind of 



