II] OF SURFACE AND VOLUME 57 



there are other cases in which the ratio of surface to mass may 

 change the whole condition of the system. Iron rusts when exposed 

 to moist air, but it rusts ever so much faster, and is soon eaten away, 

 if the iron be first reduced to a heap of small filings ; this is a mere 

 difference of degree. But the spherical surface of the rain-drop 

 and the spherical surface of the ocean (though both happen to be 

 ahke in mathematical form) are two totally different phenomena, 

 the one due to surface-energy, and the other to that form of mass- 

 energy which we ascribe to gravity. The contrast is still more 

 clearly seen in the case of waves: for the little ripple, whose form 

 and manner of propagation are governed by surface-tension, is 

 found to travel with a velocity which is inversely as the square 

 root of its length; while the ordinary big waves, controlled by 

 gravitation, have a velocity directly proportional to the square root 

 of their wave-length. In hke manner we shall find that the form 

 of all very small organisms is independent of gravity, and largely 

 if not mainly due to the force of surface-tension: either as the 

 direct result of the continued action of surface-tension on the 

 semi-fluid body, or else as the result of its action at a prior stage 

 of development, in bringing about a form which subsequent chemical 

 changes have rendered rigid and lasting. In either case, we shall 

 find a great tendency in small organisms to assume either the 

 spherical form or other simple forms related to ordinary inanimate 

 surface-tension phenomena, which forms do not recur in the 

 external morphology of large animals. 



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

 of that principle of simihtude which we have already discussed in 

 regard to several of its manifestations. We are coming to a con- 

 clusion 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 Fonn, 

 because in the most familiar illustrations of organic form, as in our 

 own bodies for example, these two factors are inseparably asso- 

 ciated, 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 produced, by its gradual 

 and unequal increments, the successive stages of development and 



