THE PRINCIPLE OF VITAL AFFINITY. 173 
became their roots and leaves, and then to arrange themselves there, in those 
peculiar forms by which the numberless species of the vegetable world are cha- 
racterized. 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 retain 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 principles, first of vegetables and then of ani- 
mals, 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 propor- 
tion of carbonic acid in it diminished, and the proportion 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 existence 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, in both divisions of organized 
beings, Nature has provided, not for the permanent existence, but for the deve- 
lopment and decay of successive generations of individuals, and thus for the per- 
petuation of the species. These are the subjects of investigation 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, 2. ¢., modifying, in organ- 
ized 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, although we cannot explain the introduction of living beings into 
the world, any more than we can explain the dissemination of the stars through- 
out space,—although we must always regard the appearance of organized bodies 
on the earth’s surface as the clearest indication which human knowledge presents 
of the subjection of the universe, not only to general laws, but to an arbitrary 
Will, superior to these laws and changing them at pleasure,—yet I think it may be 
said that we have nearly as clear an insight into the designs and arrangements 
of Providence for the maintenance of living beings upon earth, and for the eternal 
reproduction of them there, so long as these laws shail be in force, as we have 
into those by which the movements of the heavenly bodies are directed and 
controlled. 
I. Our first business is to study the facts that have been ascertained in regard 
to the simplest form of chemical change to which the term vital may be applied, 
VOL. XVI. PART II, 2 x 
