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322 as there are creatures which, although fixed to one habitat, not capable of progression that is, are endowed with all the senses, although it may be in some modified, or less active form than in the higher animals. For "the nervous system has been detected in every division of the animal kingdom, and almost in every class, and it is everywhere connected with sensation and motion."

Note 4, p. 188. Be sensible through a medium, &c.] The medium, that is, made diaphanous and motive by colour or sound, acts, by a succession of undulations, upon the eye or the ear, and finally, through the humours of the former and the air in the latter, upon the sentient part of these organs; so that there is an evident analogy between these undulations and the impulses which maintain locomotion until lost in the state of rest. This succession of impulses may well apply to the changes which, without change of locality, are slowly and silently going on in bodies, and be compared to colouring matter which permeates and gradually combines with each molecule of wax up to saturation; but every substance cannot, of course, be thus affected—a stone, for instance, owing to the condensation of its particles, cannot be made receptive of colour.

Note 5, p. 189. The air is mobile in the highest degree, &c.] The nature and properties of the atmosphere were imperfectly known, as has been said, in the age of Aristotle. It was deemed necessary to sensation that