4 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



so by merely laying on the fresh molecules outside 

 those already formed ; protoplasm, on the other hand, 

 makes the fresh food, which may or may not, indeed 

 need not, have the same composition as itself, an 

 essential part and parcel of itself. 



In the next place we observe, that while a crystal 

 under the conditions just now mentioned will grow 

 so long as it is supplied with matter of similar 

 chemical constitution, living matter only grows 

 when assimilation goes on at a quicker rate than 

 destruction or waste. Save for the difficulties of ex- 

 perimenting, there is no reason why all the sulphate 

 of copper in the world should not (a) be brought into 

 one huge crystal, and (0) so remain. It is not so with 

 living matter ; for every organism there appears to be 

 a limit of growth, and when that is reached, all the 

 succeeding matter assimilated goes for a different pur- 

 pose. The organism, ceasing to grow, begins to repro- 

 duce its kind, and, in the very simplest cases, produces 

 an individual exactly similar to itself. This act of 

 reproduction appears to be, next to sustentation, the 

 primary work of every organism, and when that is 

 completed, we often observe that the parent organism 

 begins to lose its activity ; it becomes the prey of other 

 living organisms ; or, undergoing gradual decay, the 

 complex mass of albuminous matter, which we call 

 protoplasm, and associate with life, falls away into 

 constituent molecules of a less high degree of chemical 

 complexity. 



Assimilation, growth, reproduction, death, are, 

 as here explained, four phases in the history of living 

 matter which at once and sharply distinguish it from 

 crystalline or other dead material. 



Nor is this all ; if we set one crystal against 

 another of similar composition, or if we try to rouse 

 or stimulate a crystal, we get no response. With living 

 matter the case is very different ; roused either by 



