Dr Alison on the Principle of Vital Affinity » 143 



rounding them, to enter into their substance, by their roots 

 and leaves, or by the organs which soon became their roots 

 and leaves, and then to arrange themselves there, in those 

 peculiar forms by which the numberless species of the vege- 

 table world are characterized. I apprehend we must also 

 regard it as an ultimate fact, that they were endowed with 

 the power of so modifying the chemical relations of the ele- 

 ments composing those absorbed matters, as to select and re- 

 tain certain of these elements, and allow others to pass away 

 from them, to decompose the carbonic acid, fix the carbon, 

 and invest it with those peculiar affinities for the water, the 

 hydrogen of the water, and a few other elements, contained 

 in the surrounding media, by which all the proximate princi- 

 ples, first of vegetables and then of animals, and therefore 

 the whole substance of organized beings, are formed. 



But it is important to have a precise exposition, although 

 not an explanation, of the power thus exercised by the first 

 plants ; and it is still more important and satisfactory to be 

 able to shew how, by the exercise of these and analogous vital 

 powers, the atmosphere must have been gradually changed, 

 the proportion of carbonic acid in it diminished, and the pro- 

 portion of oxygen increased ; how it became fitted, and is kept 

 fitted, for the residence, first of cold-blooded and then of warm- 

 blooded animals ; how most of the other conditions of exist- 

 ence of these animals have been, and still are, continually 

 prepared for them by these living actions of vegetables ; how 

 all the variety of the textures of all organized bodies, from 

 the origin of vegetables to the death and decomposition of 

 animals, are continually formed and maintained ; and how, 

 both divisions of organized beings. Nature has provided, not 

 for the permanent existence, but for the development and de- 

 cay of successive generations of individuals, and thus for the 

 perpetuation of the species. These are the subjects of inves- 

 tigation in the chemical department of physiology ; and if it 

 can be shewn, that, by a few simple laws, regulating what 

 we call vital attractions and affinities, i, e., modifying, in or- 

 ganized bodies, the attractions and affinities to which matter 

 is everywhere liable, provision is made for all this succession 

 and continual renewal of the phenomena of life ; then, al- 



