86 BIOLOGY. [Book ii. 



the chemical combinations but by the aid of energetic chemical 

 agents. Moreover, it is much less easy to interrupt the vital 

 movement in the plant than in the animal. But in them 

 both, chemical instability, in various degrees, is the very 

 condition of life. Every combination too stable is the equivalent 

 of death. 



Nutritive assimilation has naturally, as a condition, a corre- 

 sponding disassimilation. In order that new substances may 

 incorporate themselves with an anatomical element it is abso- 

 lutely necessary that other substances yield their place to them. 

 In effect, incessantly a portion of the substances which formed 

 part of the anatomical element ceases to resemble the fundamental 

 substances, and severs itself from them. The substances thus 

 have not, by reason of the severance, ceased to be complex 

 albuminoidal substances, but generally they have become more 

 oxydised, and have passed into the state of crystallisable matters, 

 — they have taken a step to return to the mineral world. 



As to the mineral substances expulsed from the anatomical 

 element, certain of them merely pass through without undergoing 

 any change. This is the case with sundry salts, azote, water, 

 and so on. Other mineral compounds, however, are formed 

 therein by direct combination, just &.s they would have been 

 formed in a retort. In this fashion are produced in animals, 

 the alkaline carbonates, the lactates, the ammoniaco-magnesian 

 phosphates, the phosphates of lime, the urates, carbonic acid, and 

 so on. 



The nutritive exchange is not effected in all the tissues with 

 the same energy. In general it is in the cell, properly so called, 

 or in tissues foi-med by cellular aggregations, that this double 

 current attains its maximum of power. Nutrition can often be 

 effected without the succour of special circulatory apparatus. 

 The exchange of nutritive materials frequently is achieved in 

 this case from step to step with sufficient rapidity. Things take 

 place thus in certain inferior organisms simply polycellular, in 

 th& crystalline of the eye of mammifers, and so on. 



