12 • INTRODUCTORY [ch. 



the conformation of the organism itself, whose permanence or 

 equihbrium is explained by the interaction or balance of forces, 

 as described in statics. 



If we look at the living cell of an Amoeba or a Spirogyra, we 

 see a something which exhibits certain active movements, and 

 a certain fluctuating, or more or less lasting, form; and its form 

 at a given moment, just like its motions, is to be investigated by 

 the help of physical methods, and explained by the invocation of 

 the mathematical conception of force. 



Now the state, including the shape or form, of a portion of 

 matter, is the resultant of a number of forces, which represent or 

 symbolise the manifestations of various kinds of energy; and it 

 is obvious, accordingly, that a great part of physical science must 

 be understood or taken for granted as the necessary preliminary 

 to the discussion on which we are engaged. But we may at 

 least try to indicate, very briefly, the nature of the principal 

 forces and the principal properties of matter with which our 

 subject obliges us to deal. Let us imagine, for instance, the case 

 of a so-called "simple" organism, such as Amoeba; and if our 

 short list of its physical properties and conditions be helpful 

 to our further discussion, we need not consider how far it 

 be complete or adequate from the wider physical point of 

 view*. 



This portion of matter, then, is kept together by the inter- 

 molecular force of cohesion ; in the movements of its particles 

 relatively to one another, and in its own movements relative to 

 adjacent matter, it meets with the opposing force of friction. 

 It is acted on by gravity, and this force tends (though slightly, 

 owing to the Amoeba's small mass, and to the small difference 

 between its density and that of the surrounding fluid), to flatten 

 it down upon the sohd substance on which it may be creeping. 

 Our Amoeba tends, in the next place, to be deformed by any 

 pressure from outside, even though sUght, which may be apphed 

 to it, and this circumstance shews it to consist of matter in a 

 fluid, or at least semi-fluid, state : which state is further indicated 

 when we observe streaming or current motions in its interior. 



* With the special and important properties of colloidal matter we are, for 

 the time being, not concerned. 



