DYNAMICS IN EVOLUTION. 65 



But, it will be asked, what is surface-tension ? If a drop of 

 water falls through a vacuum, as long as it is falling in space 

 it will present very nearly the form of a sphere. This is due 

 to the fact that the very outermost layers of molecules which 

 lie at insensible distances apart, attract each other with a force 

 that is apparently very much greater than that developed by 

 the action of gravitation. It results from this that the remain- 

 ing molecules contained within this outer molecular film are 

 held as within an elastic bag, the walls of which are of exactly 

 the same strength at every point. This exactly equal strength 

 and elasticity of the outer molecular film at every point on the 

 surface also causes it to fall into a condition of spherical, 

 statical equilibrium. This is true because the molecules at 

 every point on the surface of the drop are of exactly the same 

 size, therefore every one attracts its fellows that are in contact 

 with it, with exactly the same force at every point on that 

 surface. Now, let any change in the dimensions of these 

 molecules occur at any point on the surface of such a fluid 

 sphere, such as may be accomplished by chemical transforma- 

 tions and recombinations, such as oxidation or decomposition, 

 and it must follow that the surface-tension or reciprocal pull 

 of the molecules upon each other, adjacent to the point of 

 such disturbance, must be increased or diminished. The 

 inevitable result of this will be that the form of the drop must 

 instantly change in order that a new condition of statical 

 equilibrium may be attained. The surface of the drop is 

 finite and returns in every direction upon itself since it is 

 approximately spherical. Any deformation of the drop due to 

 surface-tensional disturbances will therefore affect the shape and 

 curvature of some or all of the surface of the drop so that its 

 shape may become very irregular, provided its surface-tension 

 be disturbed at a number of irregularly distributed points 

 simultaneously ; but, since the drop is a finite mass made up 

 of solid particles moving freely among themselves, no matter 

 how much the drop may be deformed or how irregular it may 

 become, its most superficial layer of molecules will always form 

 a closed surface. This fact is important, since, no matter how 

 irregular in shape an Amoeba may become, its outermost stratum 



