256 ZOOLOGICAL PHILOSOPHY 



ducing compounds ; in other cases (animals) of modifying these 

 compounds and altering their character by the addition of new 

 principles to a remarkable extent. 



I must then again impress the fact that living bodies form for them- 

 selves, by the activity of their organs, the substance of their bodies 

 and the various secretions of their organs ; and that they neither 

 find this substance ready formed in nature, nor the secretions, which 

 come purely from them alone. 



It is by means of food, which animals and plants are obliged to use 

 for the preservation of their hfe, that the organs of these hving bodies 

 work their effects. These effects consist in a modification of the food 

 resulting in the formation of special substances, which would never 

 have existed without this cause, and in building up by perpetual 

 alterations and renewals of these substances, the entire body which 

 they constitute, as also its products. 



Whereas all animal and vegetable substances are composed of prin- 

 ciples in very complex combinations, and many of which have been 

 forced into these combinations, man has no power to do the like ; 

 all that he can do is to decompose, alter, or destroy them, or to 

 convert them into various special combinations, always less and less 

 complex. It is only the movements of life that can produce these 

 substances. 



Thus plants, which have no intestinal canal nor any other organ for 

 digestion, and which consequently use for food only fluid substances 

 or substances whose molecules are not aggregated (such as water, 

 atmospheric air, caloric, hght, and the gases that they absorb), yet 

 form out of such material, by means of their organic activity, all the 

 juices that are proper to them, and all the substances of which their 

 body is composed ; that is, they form for themselves the mucilages, 

 gums, resins, sugar, essential salts, fixed and volatile oils, feculae, 

 gluten, extractive and woody matter ; all of them substances arising 

 direct from immediate combinations, and none of which can ever be 

 formed by art. 



Plants certainly cannot take from the soil by means of their roots 

 the substances which I have just named : they are not there, and those 

 which are there are in a more or less advanced condition of degra- 

 dation or decomposition ; lastly, if there were any in a state of com- 

 plete integrity, plants would not be able to make use of them without 

 having previously decomposed them. 



Plants then have formed directly the substances to which I refer ; 

 but when they are outside plants, these substances can only be useful 

 as manure ; that is to say, only after being altered in nature, broken 

 up, and having undergone the necessary degradation to fit them for 



