CHAP. VIII.] THE CHEMISTKY OF LIFE. 85 



in this, that the new substance, in being deposited on the 

 crystal, does not undergo any change whatever, except the 

 change from the liquid to the solid state ; while, on the 

 contrary, no new substance can become part of an organism 

 until it has undergone a certain transformation by the 

 agency of the organism itself, which makes it similar in 

 point of chemical constitution to the substance of the 

 organism, and fit to become part of it. This process is Assimila- 

 what is called assimilation. ^^°^' 



The statement, that a living organism is constantly losing 

 substance by waste and acquiring other substance by 

 accretion, though true, is thus shown to be inadequate ; 

 and we must enlarge it by saying, that the organism is 

 constantly losing substance by waste, and acquiring new 

 substance ivhich it transforms into its own substance by 

 assimilation. This double process of assimilation and 

 waste is general among organisms, vegetable as well as 

 animal ; and it has been very generally regarded as the 

 only characteristic which is common to them all : '^ but I 

 shall have farther on to give reasons for believing that 

 there are other characteristics which are equally common 

 to all organisms whatever. 



The substance of an organism consists of chemical com- Organic 

 pounds of high complexity, which are not found in the *'°"^" , , . 

 inorganic world, but are formed by the action of the 

 organism itself on the food that is brought from without. 

 This process of forming the substance of the organism by 

 assimilating the food is always going on ; and the opposite 

 process of waste is always going on, consisting in chemical 

 changes that transform the compounds of which the sub- 

 stance of the organism consists, into compounds of simpler 

 chemical constitution, which are no longer capable of 

 forming part of the organism, but are excreted.^ 



1 Comte (Positive Philosophy, Harriet Martineau's condensed transla- 

 tion, vol. i. p. 362) quotes with approbation De Blainville's definition of life 

 as a " double interior motion, general and continuous, of composition and 

 decomposition ; " evidently meaning that life is a process of assimUation 

 and waste. 



^ Even this statement may need some qualification. No doubt all 

 organisms grow by assimilation, but it does not appear certain that all the 



