Ill] OF MOLAR AND MOLECULAR FORCES 53 



The difference between the two classes ^i phenomena is somewhat 

 akin to the difference between the forces which determine the 

 form of a rain-drop and those which, by the flowing of the waters 

 and the sculpturing of the sohd earth, have brought about the 

 complex configuration of a river ; molecular forces are paramount 

 in the conformation of the one, and molar forces are dominant 

 in the other. 



At the same time it is perfectly true that all changes of form, 

 inasmuch as they necessarily involve changes of actual and relative 

 magnitude, may, in a sense, be properly looked upon as phenomena 

 of growth ; and it is also true, since the movement of matter must 

 always involve an element of time*, that in all cases the rate of 

 growth is a phenomenon to be considered. Even though the 

 molecular forces which play their part in modifying the form of 

 an organism exert an action which is, theoretically, all but 

 instantaneous, that action is apt to be dragged out to an appreciable 

 interval of time by reason of viscosity or some other form of 

 resistance in the material. From the physical or physiological 

 point of view the rate of action even in such cases may be well 

 worth studying ; for example, a study of the rate of cell-division 

 in a segmenting egg may teach us something about the work done, 

 and about the various energies concerned. But in such cases the 

 action is, as a rule, so homogeneous, and the form finally attained 

 is so definite and so httle dependent on the time taken to effect 

 it, that the specific rate of change, or rate of growth, does not 

 enter into the moriihological problem. 



To sum up, we may lay down the following general statements. 

 The form of organisms is a phenomenon to be referred in part 

 to the direct action of molecular forces, in part to a more complex 

 and slower process, indirectly resulting from chemical, osmotic 

 and other forces, by which material is introduced into the organism 

 and transferred from one part of it to another. It is this latter 

 complex phenomenon which we usually speak of as "growth." 



* Cf. Aristotle, Phys. vi, 5, 235 a 11. eVet yap airaaa Kivrjcm iv xP^''Vy '''''^• 

 Bacon emphasised, in like manner, the fact that "all motion or natural action 

 is performed in time : some more quickly, some more slowly, but all in periods 

 determined and fixed in the nature of things. Even those actions which seem 

 to be performed suddenly, and (as we say) in the twinkling of an eye, are found 

 to admit of degree in respect of duration." Nov. Org. XLVI. 



