34 ON MAGNITUDE [ch. 



Now this is a very important matter, and is a notable illustration 

 of that principle of similitude which we have already discussed 

 in regard to several of its manifestations. We are coming easily 

 to a conclusion which will affect the whole course of our argument 

 throughout this book, namely that there is an essential difference 

 in kind between the phenomena of form in the larger and the 

 smaller organisms. I have called this book a study of Growth 

 and Form, because in the most familiar illustrations of organic 

 form, as in our own bodies for example, these two factors are 

 inseparably associated, and because we are here justified in thinking 

 of form as the direct resultant and consequence of growth: of 

 growth, whose varying rate in one direction or another has pro- 

 duced, by its gradual and unequal increments, the successive 

 stages of development and the final configuration of the whole 

 material structure. But it is by no means true that form and 

 growth are in this direct and simple fashion correlative or comple- 

 mentary in the case of minute portions of living matter. For in 

 the smaller organisms, and in the individual cells of the larger, 

 we have reached an order of magnitude in which the intermolecular 

 forces strive under favourable conditions with, and at length 

 altogether outweigh, the force of gravity, and also those other 

 forces leading to movements of convection which are the prevaihng 

 factors in the larger material aggregate. 



However we shall require to deal more fully with this matter 

 in our discussion of the rate of growth, and we may leave it mean- 

 while, in order to deal with other matters more or less directly 

 concerned with the magnitude of the cell. 



The hving cell is a very complex field of energy, and of energy 

 of many kinds, surface-energy included. Now the whole surface- 

 energy of the cell is by no means restricted to its outer surface; 

 for the cell is a very heterogeneous structure, and all its proto- 

 plasmic alveoh and other visible (as well as in\asible) hetero- 

 geneities make up a great system of internal surfaces, at every 

 part of which one "phase" comes in contact with another "phase," 

 and surface- energy is accordingly manifested. But still, the 

 external surface is a definite portion of the system, with a definite 

 "phase" of its own, and however httle we may know of the distri- 

 bution of the total energy of the system, it is at least plain that 



