94 THE DATA OF BIOLOGY. 



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 appropriated and oxygen given off; without 

 them, is the gas from which the carbon is taken, 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 pri- 

 marily constitute 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 substance 

 is undergoing disintegration, it is on the other hand absorbing 

 nutriment; and that it may continue to exist, the one pro- 

 cess must keep pace with, or exceed, the other. If further 

 we ask under what circumstances these combined changes 

 are possible, there is the reply that the medium in which the 

 protozoon is placed, must contain oxygen and food — oxygen 

 in such quantity as to produce some disintegration; food in 

 such quantity as to permit that disintegration to be made 

 good. In other words — the two antagonistic processes taking 

 place internally, imply the presence externally of materials 

 having affinities that can give rise to them. 



Leaving those lowest animal forms 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 slightly specialized. In 

 these we see a correspondence between certain actions in the 

 digestive sac, and the properties of certain surrounding 

 bodies. That a creature of this order may continue to live, 

 it is necessary not only that there be masses of substance in 

 the environment capable of transformation into its own tis- 

 sue, but also that the introduction of tliesc masses into its 



