Since we cannot go back to the first origin of living bodies, our only resource in investigating the true nature of the powers which animate them consists in examining their structure, and tracing the union of their elements, Our knowledge of these points is too imperfect for us to draw all the necessary inferences. The minute branches of vessels and nerves, and the intimate structure of the organs in general, elude our imperfect means of research: our analysis of fluids is also very incomplete, and there are, probably, several of which we have no means at all of subjecting to examination. Yet, though our knowledge of organization be not sufficient to enable us to explain all the facts presented to our observation by living bodies, we may, by means of it, re cognise them, even in an inactive state, and trace their remains after death. No inanimate matter has that fibrous and cellular texture, nor that multiplicity of volatile elements which form the charac ters of living bodies, whether alive or dead. Thus, while inorganic solids are only composed of many-sided particles, attracting each other by their surfaces, and receding only for the purpose of se parating; while they are resolved into a very limited number of elementary sub stances, and are formed merely by the combination of these elements, and the aggregation of these particles ; while they grow only by the juxtaposition of new particles, which are deposited exteriorly to those already existing, and are de stroyed only by the mechanical separa tion of their parts, or the decomposing agency of chemical means ; organized bodies, made up of fibres and laminae, whose intervals are filled by fluids, are resolved almost entirely into volatile ele ments, grow on bodies similar to them selves, and separate from these only when they are sufficiently developed to act by their own powers ; constantly assimilate foreign matters to themselves, and, inter posing these between their own particles, grow by the operation of an internal pow er, and perish at last by this interior prin ciple ; indeed, by the very effect of their lite.
An origin by generation, a growth by nutrition, and a termination by death, are the general characters common to all or ganised bodies ; and if several of such bodies possess these functions only, and such as immediately depend on them, and have only the organs required for their performance, there are many others exercising particular functions which re quire appropriate organs, and also modi fy the general functions and their organs.
Of all the less general powers, which presuppose organization, but which do , not seem to be necessary results of struc ture, those of sensation and voluntary motion are the most remarkable, and ex ert the greatest influence over the other functions. We are conscious of the exist ence of these powers in ourselves, and we attribute them, by an analogical mode of reasoning, to many other beings, which we therefore name animated beings or animals. They seem to be necessarily connected together ; for the idea of vo luntary motion contains in itself that of sensation ; since volition. cannot be con ceived without desire, and without a feel ing of pleasure or pain. The goodness, which we observe in all the works of na ture, will not allow us to believe that she has formed beings with the power of' sen sation, that is, with a susceptibility of pleasure and pain, without enabling them at the same time to approach to the one and fly from the other, at least to a cer tain degree. And if, among the too real
misfortunes which afflict our species, one of the most affecting is the sight of a man of sensibility deprived by superior force of the power of resisting oppression ; the poetic fictions, most apt to excite our pity, are those which represent sensible beings inclosed in immovable bodies ; and the tears of Clorinda, flowing with her blood from the trunk of a cypress, ought to arrest the blows of the most savage man.
Independently of the chain, which unites these two powers, and of the double apparatus of organs which they require, they produce also several modi fications in the faculties common to all organized bodies ; and these modifica tions, joined to the two peculiar powers, constitute more particularly the essential nature of animals. Thus, in respect to nutrition, vegetables being attached to the earth, absorb nutritive fluids directly by their roots ; these almost infinitely sub. divided, penetrate the smallest intervals of the soil, and, if we may use the ex pression, travel to a distance in quest of nourishment for the plant to which they belong ; their action is quiet and constant, being liable to interruption only when drought deprives them of the necessary juices. Animals, on the contrary, fixed to no spot, but frequently changing their abode, required the power of transporting with them the provision of fluids neces sary for their nutrition ; they have there fore an interior cavity to receive their food ; and on its inner surface there are the openings of' absorbing vessels, which, to use the energetic language of Boer have, are real internal roots. The size of this cavity, and of its orifices, allowed in several animals the introduction of solid substances. These required instruments for their division, and liquors for their so lution ; in a word, nutrition was no longer performed by•the immediate absorption of matters in the state in which the earth or atmosphere furnished them ; it was necessarily preceded by various prepara tory operations, which, taken altogether, constitute digestion.
Thus digestion is a function of a secon• dary class, peculiar to animals. Its ex istence, as well as that of the alimentary cavity in which it takes place, is render ed necessary by the power which animals have of voluntary motion ; but it is not the only consequence of that power.
Vegetables, having few faculties, are simple in their organization ; being com posed almost entirely of parallel or slightly diverging fibres. Moreover, their fixed position admitted of the general motion of their nutritive fluid being kept up by sim ple external agents ; thus it ascends by means of suction in their spongy or ca pillary texture, and also through the in fluence of evaporation, from the surface ; it is rapid in a direct ratio to this evapo ration, and may even become retrograde when that process ceases, or when it is changed into absorption by the moisture of the atmosphere.