_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.) CV. _The Assumption of a Final Cause in the structure of each part of animals and plants is as inevitable as the assumption of an Efficient Cause for every event. The maxim that in organized bodies nothing is_ in vain, _is as necessarily true as the maxim that nothing happens_ by chance. (IX. 6.) CVI. _The Idea of living beings as subject to_ disease _includes a recognition of a Final Cause in organization; for disease is a state in which the vital forces do not attain their_ proper ends. (IX. 7.) CVII. _The Palætiological Sciences depend upon the Idea of Cause: but the leading conception which they involve is that of_ historical cause, _not mechanical cause._ (X. 1.) {25} CVIII. _Each Palætiological Science, when complete, must possess three members: the_ Phenomenology, _the_ Ætiology, _and the_ Theory. (X. 2.) CIX. _There are, in the Palætiological Sciences, two antagonist doctrines:_ Catastrophes _and_ Uniformity. _The doctrine of a_ uniform course of nature _is tenable only when we extend the nation of Uniformity so far that it shall include Catastrophes._ (X. 3.) CX. _The Catastrophist constructs Theories, the Uniformitarian demolishes them. The former adduces evidence of an Origin, the latter explains the evidence away. The Catastrophist's dogmatism is undermined by the Uniformitarian's skeptical hypotheses. But when these hypotheses are asserted dogmatically they cease to be consistent with the doctrine of Uniformity._ (X. 3.) CXI.