48 A. E. HILTON ON THE NATURE OF LIVING ORGANISMS. 



which form surface films radiating from numerous catalytic 

 centres, the films which meet combine into firmer membranes ; 

 and these, by the continued action of the plasm in depositing 

 fresh material, become consolidated into cell -walls, connective 

 tissues, or the harder bodies which support or protect the various 

 parts of the organism. It is always, however, in the activities of 

 the plasm, not in the strength of the scaffolding it erects, that 

 the vitality of the organism consists. That is a point of the first 

 importance. Not the cell, but the active plasm it contains, is the 

 unit of life. 



A point which calls for notice before we sum up is the double 

 advantage of the cell-life of the plasm. The membranes of 

 precipitation which form the cell-walls are semi-permeable ; that 

 is to say, they allow some substances to pass through them 

 easily, others not so readily, some only with difficulty, and others 

 not at all. Curiously enough, larger molecules of some sub- 

 stances pass through the cell-walls more easily than smaller 

 molecules of other substances ; that is because the permeability 

 depends, not upon the walls being porous or jDerforated, but upon 

 the degree of solubility, in the membranes, of the substances 

 passing through them. The passage may be in either direction ; 

 or an exchange may be going on, some elements passing out of 

 the cells while others are passing into them. The process, a 

 complicated one, is explainable partly by chemical reactions and 

 partly by the more mechanical law of osmosis — the law by which 

 interchanges between fluids of different densities, when divided 

 by membranes, are governed. Usually, the denser medium gains 

 .at the expense of the other ; and the significance of this in 

 relation to life phenomena is apparent, in view of the colloidal 

 consistency of plasm, its membrane-forming tendency, and 

 watery surroundings. The cells, in fact, serve a double purpose. 

 The permeability of their walls permits the food solutions to 

 circulate through them for the general purpose of sustenance and 

 growth ; while the circumstance that the walls are partially 

 impervious enables certain cells, or groups of cells, to develop 

 characteristics differing from the rest. From small differences 

 thus arising, and the operation of the laws of evolution through 



