158 Life as a Physical Phenomenon. 



As I have already intimated, the cells draw from the 

 surrounding fluids the materials necessary for their growth, 

 and it is a remarkable fact that every kind of cell has the 

 power, or faculty, of selecting and appropriating jast such 

 materials as arc needful for its own peculiar structure. 

 This faculty is, however, confined within certain limits. 

 Thus the cells of plants will absorb carbonic acid, water, 

 and ammonia, and unite these elements in the form of 

 cellulose, hut to the animal these materials would fnrnisb 

 only starvation diet. But these same compounds, carbo- 

 nic acid water and ammonia, when once united in the 

 plant, become food for the animal. 



Having traced the history of the cell and its relations to 

 living structures, thus far, it requires no stretch of imagin- 

 ation to perceive clearly that all living forms are funda- 

 mentally alike, and that the cell or more exactly the 

 protoplasm, from which the cell is built, is the real basis of 

 all life. It is also easy to show that the movements of 

 living beings are the result of changes of form in these 

 structural units. Thus muscular contractility, which is 

 regarded as one of the crowning marks of distinction be- 

 tween animals and all other classes of beings, results, sim- 

 plv from the flattening or shortening of multitudes of 

 individual cells, and this effect may be produced by the 

 action of electricity as well as by the force which passes 

 along the line of a nerve, and this phenomenon of con- 

 tractility is, by no means, confined to animals, but is found 

 also in plants, and it might be shown that the identical 

 principle exists also in minerals. But aside from this last 

 statement, it is evident from what we have already sen, 

 that the dillerence in faculty between the lowest plant or 

 the lowest animal and the highest of its class is but one of 

 degree, and not of kind. There would seem to be the 

 widest contrast between the simple being we have figured 

 as the unit of the vegetable world, and the giant tree that 



