iv] THE SECOND GRADE 87 



is not the fate of all organisms. There is a higher 

 and a lower, for some are more independent, more 

 powerful than others. 



It is now that the importance of size is seen ; for 

 increase of size means increase of independence. 

 Most of the forces of the outside world act only on 

 the surface of the organism ; but its own forces spring 

 from the whole mass of its substance. The energy 

 necessary for action is let loose by the chemical 

 breaking-down of the molecules of protoplasm and 

 by their combination with oxygen. This, in a primi- 

 tive cell, is a function of all the molecules, and so of 

 its total bulk. In the higher animals, where loco- 

 motive power is delegated to the muscles, the relation 

 still holds good; the three dimensions and so the 

 shape of the thigh-muscles of a jerboa and a kangaroo 

 are approximately the same, and so the surface- 

 volume ratio will hold accurately. If the length of 

 the kangaroo is ten times that of the jerboa, then the 

 surface of his thigh-muscles will be a hundred, the 

 bulk a thousand times as great. Of the outside forces 

 (all antagonistic or at best passively resistant to the 

 organism) that of gravity only is proportional to its 

 mass. If it alone were to be considered, size would 

 make no difference to the animal's movements the 

 weight to be moved would increase proportionately 

 with the forces that were to move it, since both are 

 proportional to the mass of the whole. 



