C.
The basis of all Natural Systems of Classification is the Idea of Natural Affinity. The Principle which this Idea involves is this:—Natural arrangements, obtained from different sets of characters, must coincide with each other. (viii. 4.)
CI.
In order to obtain a Science of Biology, we must analyse the Idea of Life. It has been proved by the biological speculations of past time, that Organic Life cannot rightly be solved into Mechanical or Chemical Forces, or the operation of a Vital Fluid, or of a Soul. (ix. 2.)
CII.
Life is a System of Vital Forces; and the conception of such Forces involves a peculiar Fundamental Idea. (ix. 3.) 24
CIII.
Mechanical, chemical, and vital Forces form an ascending progression, each including the preceding. Chemical Affinity includes in its nature Mechanical Force, and may often be practically resolved into Mechanical Force. (Thus the ingredients of gunpowder, liberated from their chemical union, exert great mechanical Force: a galvanic battery acting by chemical process does the like.) Vital Forces include in their nature both chemical Affinities and mechanical Forces: for Vital Powers produce both chemical changes, (as digestion,) and motions which imply considerable mechanical force, (as the motion of the sap and of the blood.) (ix. 4.)
CIV.
In voluntary motions, Sensations produce Actions, and the connexion is made by means of Ideas: in reflected motions, the connexion neither seems to be nor is made by means of Ideas: in instinctive motions, the connexion is such as requires Ideas, but we cannot believe the Ideas to exist. (ix. 5.)