76 THE PHILOSOPHY OF BIOLOGY 



for mechanical work is not done by it, nor is heat 

 dissipated — or at least these processes occur to an 

 insignificant extent. 



are synthesised to — 



(proteid 

 fat 

 carbohydrate 



Carbon dioxide \ Metabolism 

 Water - of the green 



Nitrate J plant 



Chemical energy 

 at high potential. 



Chemical energy at \\'ott "^ c^vste'D^ 

 low potential 



The " working substance " of our organic cycle has 

 therefore returned to its original state. 



We have considered the process of metabolism 

 in two categories of organisms, the typical animal 

 and the green plant, and we have combined these so 

 as to obtain a picture of a reversible cycle of physico- 

 chemical processes. When w^e speak of the " organism " 

 in the most general sense, we mean that it exhibits 

 these two modes of metabolism. This is, of course, 

 not the case in any actual organism which we can 

 investigate, or at least the typical modes of be- 

 haviour which characterise animal and plant life 

 are not seen in any one individual. But we find that 

 there is no absolute distinction between the two 

 kingdoms. The plant may exhibit a mode of nutrition 

 closely resembling that of the animal (as in the 

 insectivorous plants), and it is possible that photo- 

 synthetic process, in the general sense, may be present 

 in the metabolism of some animals. Certain lower 

 plants, the zoospores of algae, exhibit movements 

 identical in character with those of lower animals. 

 At the base of both kingdoms are organisms, the 



