166 SOME RECENT DEVELOPMENTS 



away, and the products of its wasting are much the 

 same as the excretory matters formed by a living 

 animal. The difference is that the living animal 

 or plant has the power of repair, renewal, or 

 regeneration of its living substance, while the dead 

 body has no such power. This renewal is effected 

 at the expense of the food. Food is essential, or 

 we starve and die; and it has been well said that 

 hunger is the essential and diagnostic character of 

 living things. This power of building up living 

 from non-living matter is peculiar to protoplasm, of 

 which we have seen that the cell-bodies, whether 

 of animals or plants, consist. The process is 

 partly a chemical one, a building up of simpler into 

 more complex bodies ; but in part it is something 

 more, something peculiar, the true nature of which 

 we do not understand. The final touch, the con- 

 version of the dead food into living brain, muscle, 

 etc., is effected by a process which we name 

 assimilation, but of which the modus operandi is at 

 present absolutely unknown. 



This conception of protoplasm or the living 

 matter of animals and plants, as undergoing in- 

 cessant change or metabolism as it is termed, is 

 one of much importance. Living protoplasm has 

 been compared to a fountain in which the form 

 remains constant though each component particle 

 of water is in constant movement. In protoplasm 

 as in the fountain, we distinguish two main pro- 

 cesses, an uphill or anabolic process, as the water 

 rises to the crest of the wave, or the food is being 

 built up into the living tissues ; and a downhill or 



