CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 75 



on in a plant pre-suppose a surrounding medium containing 

 at least carbonic acid and water, together with a due supply 

 of light and a certain temperature. "Within the leaves 

 carbon is being assimilated and oxygen given off ; without 

 them, is the gas from which the carbon is abstracted, and the 

 imponderable agents that aid the abstraction. Be the nature 

 of the process what it may, it is clear that there are external 

 elements prone to undergo special re-arrangements under 

 special conditions. It is clear that the plant in sunshine 

 presents these conditions and so effects these re-arrange- 

 ments. And thus it is clear that the changes which consti- 

 tute the plant's life, are in correspondence with co-existences 

 in its environment. 



If, again, we ask respecting the lowest protozoon, how 

 it lives ; the answer is, that while on the one hand its sub- 

 stance is ever undergoing oxidation, it is on the other hand 

 ever absorbing nutriment ; and that it may continue to exist, 

 the assimilation must keep pace with, or exceed, the oxidation. 

 If further we ask under what circumstances these combined 

 changes are possible ; there is the obvious reply, that the 

 medium in which the protozoon is placed, must contain oxy- 

 gen and food — ox} r gen in such quantity as to produce some 

 disintegration ; food in such quantity as to permit that dis- 

 integration to be made good. In other words — the two 

 antagonistic processes taking place internally, imply the pre- 

 sence externally of materials having affinities that can give 

 rise to these processes. 



Leaving those lowest animal forms revealed by the mi- 

 croscope, which simply take in through their surfaces the 

 nutriment and oxygenated fluids coming in contact with 

 them, we pass to those somewhat higher forms which have 

 their tissues partially specialized into assimilative and re- 

 spiratory. In these we see a correspondence between certain 

 actions in the digestive sac, and the properties of certain sur- 

 rounding bodies. That a creature of this order may continue 

 to live, it is necessary not only that there be masses of sub- 



