THE BIOLOGY OF DAILY LIFE. 5 



We find, as a matter of observation, that the 

 sustenance of those plants which are suited for 

 the food of animals, is itself derived (not from the 

 mineral world in general), but most chiefly from those 

 mineral particles which had previously been incor- 

 porated into organisms. 



This completes the picture. We see the mutual 

 dependence of the three kingdoms, as regards supply 

 upon each other. It is in fact this very question of 

 supply which most clearly marks the boundaries 

 of those Three Kingdoms, united and yet distinct, 

 which we call animal, vegetal, and mineral kingdoms. 

 This mutual dependence we shall name the "LAW OF 

 INTERCHANGE." But in the following chapters we 

 shall only need to keep in mind that well established 

 and generally acknowledged portion of this Law, 

 which declares that, ANIMALS DRAW DIRECTLY OR INDI- 

 RECTLY THEIR WHOLE SUSTENANCE FROM THE ORGAN- 

 IZED MATTER WHICH PLANTS HAVE ELABORATED. 



While this Law of Interchange may be well said to 

 be generally acknowledged, yet some more or less 

 obvious deductions or corollaries, consequent upon 

 this law, are either not seen as they ought to be, or 

 are practically disregarded.* Let us consider some 

 of these in the following chapters. 



* Any reader may see for himself a remarkable example of this 

 practical disregard of a principle, which at the same time is fully 

 stated in words in this very article on " ALIMENT " in Quain's 

 Dictionary. We have seen it stated (to put the sense of the 

 above quotation shortly) : " We have to look upon the organic 

 matter employed as aliment for animals, not only as yielding the 

 material, but also as supplying the force, &G. This force-supply 

 being derived from the latent solar force locked up in the com- 



