S4 BIOLOGY. [Book i. 



cerebral or psychical activities. Lewes says : " Life is a series of 

 definite and successive changes of structur e and of composition, 

 which act upon an individual without destroying his identity." 

 In speaking of structui'e, this definition excludes the activities of 

 purely mineral chemistry, which the fitrst does not, but it also 

 forgets the cerebral activities, and besides it does not embrace 

 the vital acts that take place in the plasmatic liquids,' such as 

 the blood, the lymph, which, though destitute of structure, are 

 endowed with life, as we shall presently see. 



The definition of H. Spencer, "The continual agreement 

 between interior and exterior relations," has the fault of being 

 too abstract, and of soaring so high above facts, that it ceases to 

 recall them. Besides, just by reason of its vague generality, it 

 might also be applied to certain continuous chemical phenomena. 



It would be better to descend nearer to the earth, and to limit 

 ourselves to giving a short summary of the principal vital facts 

 which have been observed. Doubtless life depends upon a two- 

 fold movement of decomposition and renovation, simultaneous and 

 continuous ; but this movement produces itself in the midst of 

 substances having a physical state, and most frequently a morpho- 

 logical state quite peculiar to them. Finally, this movement 

 brings into play diverse functions in relation with this morpho- 

 logical state of the living tissues, habitually composed of cells 

 and fibres endowed with special properties. 



Let us say then that '* life is a twofold m oveme nt of simul- 

 taneous and continual composition and decomposition, in the 

 midst of plasmatic substances, or of figurate anatomical elements, 

 which, under the influence of this in-dwelling movement, perform 

 their functions in conformity to their structure." 



