138 The Study of Animal Life PART n 



in all that it touches; and this we call the living matter. 

 As this living matter breaks down into simpler substances, 

 or as it causes surrounding substances to break down, 

 energy is set free for use in movement. We may make 

 a diagram of this process. The steps go up and down; the 

 top one we call protoplasm (Fig. A). This shows only one 

 line of ascent and one of descent. There may be many, all 

 going on at the same time, as is shown in Fig. B. But 

 these are much too simple ; they show continuous ascending 

 and descending stairs, but each step does not really result 

 directly from the one below, but must result from two or 

 more stairs meeting ; and at each meeting there must be 

 substances formed which are useless, and begin to break 

 down, or are cast out of the system at once. 



8. Growth. The power of growth, of adding to itself 

 substance of the same nature as itself, is the real mystery 

 of living matter. A crystal grows out of its solution, the 

 star or pyramid is built up with perfect regularity, but the 

 process is much simpler than the growth of living matter. 

 The substance of which it will be formed is already there, 

 but the protoplasm has to make its own substance as it 

 grows. This is the true difference between the two pro- 

 cesses, and not, as is usually stated, that a crystal grows by 

 depositing matter on its surface, while a cell grows by putting 

 matter within itself. For when two cells fuse, which often 

 occurs, growth is really as much by aggregation as in the 

 case of a crystal, and such manner of growth is made 

 possible simply because the two cells are masses of matter 

 of equal complexity. But when less complex matter is given 

 to a cell it cannot add that matter to itself until it has been 

 transformed into substance as complex as itself. This 

 change can only be effected within the little laboratory of 

 the cell itself. The fact that the growth of a crystal may 

 be endless, while that of a cell is limited, which is usually 

 cited as the distinctive difference, is a consequence of the 

 necessity of the protoplasm for forming its own substance 

 within its own substance. For when a cell grows in size 

 the ratio of its surface to its volume constantly decreases; 

 and therefore, since new material can only be absorbed 



