Chap. iJ OF NUTRITION. 85 



laws of non-living chemistry, the combination does not seem to 

 be accomplished in definite proportions. It is a sort of alloy. 



In plants the power of assimilation seems more energetic. It 

 is in the mineral medium, in efiect, that the plant must seek 

 its aliments direct ; consequently we see the green parts of 

 plants assimilate at once the carbon of aerian carbonic acid, and 

 incorporate it immediately with complex organic substances, 

 ternary and quaternary. The same synthetical power is exercised 

 in certain circumstances on the azote of the air, and, normally, 

 on the azote of the ammoniac salts drawn by the roots of plants 

 from the soil. 



In the animal, the true phenomena of assimilation are generally 

 exercised at the expense of albuminoidal substances already 

 elaborated. It is an important and remarkable fact that the 

 organic assimilated substances have never, previously, the same 

 composition as those which form the assimilative anatomical 

 elements. The musculine, the elasticine, and the like, peculiar to 

 every species of cell, of fibre, and so on, are, in effect, met with, 

 nowhere apart from the elements which they constitute and 

 reconstitute incessantly ; they are formed in the animal organism 

 at the expense of the living liquids, by isomeric catalyses. ' 



The anatomical elements can assimilate a great number of 

 substances, but they have necessarily their own special affinities, 

 entirely analogous to those of the bodies of mineral chemistry. 

 Thence result a choice and a selection, which, for a long time, 

 appeared intelligent, though here intelligence no more enters 

 than in the afiinity of chlore for hydrogen, of anhydrous sul- 

 phuric acid for water, and so on. These chemical combinations 

 formed in the substance of the anatomical elements have excessive 

 instability, and they are the more unstable the more life rises to 

 a superior degree, the more it is animalised. Thus chemical 

 instability is much greater in the anatomical animal elements 

 than in the vegetal elements.^ In these last we cannot dissever 



1 Ch. Robin, AnatomU Microscopique des Aliments AnaUmiques. 8vo. 

 Palis, 1868. ' Ch. Robin, loo. cU. p. 65. 



