I ought probably, in this place, to pay some attention to the theory of life advanced with much ingenuity by Dr. Darwin. But not feeling disposed unnecessarily to expatiate, I shall avoid a detail on this subject, it being sufficient to remark that his sensorial power appears too physical to solve alone the phenomena of life; it is attributing a power to matter, which I believe, however modified, or refined, it can never assume.
Having thus taken a cursory view of the most prominent opinions which have fallen within my observation, and endeavoured to shew them, rather as the scintillations of imagination, than the effulgent light of reason, suited to guide us through this mazy labyrinth, of metaphysiological investigation. I shall now proceed with what I presume at least the more unexceptionable explanation, and better adapted to the wisdom that regulates all nature.
From the most remote periods of antiquity, philosophers have not been inattentive to the peculiar differences that discriminate animate from inanimate matter, and under some modifications the distinction has been attributed to a principle called life, which not sufficiently understood in its nature, is only to be known by its phenomena, or symptoms.
Writers of high rank in the literary world, have, in their ardour to define its operations, called life a forced state, in consequence probably of observing, that when all external agents are withdrawn, its effects cease to be evinced in a plenitude of action. But were it becoming in me to cavil about modes of expression, I would only call the manifest symptoms of life forced as dependent on external agencies for their continuance. Life being rather the quality that distinguishes dead from living matter, and which may consist in an aptitude to action, and can remain for some time after its active effects cease to be obvious.
This aptitude will continue in some animals longer than others, probably owing to the peculiar manner in which they are influenced by stimuli; look at the large class of hybernating animals; though they are in their retreat to all appearance dead, none of the evident symptoms of life shewing existence, yet we may conclude that its influence still pervades their systems and preserves their bodies, composed of a variety of elements disposed by their properties to run into discomposition, from the disorganizing effects of chemical action.
This disposition of animal bodies to action, it may be observed, admits of increase or diminution. When the same substances produce more action, the aptitude may be supposed accumulated, or its energies increased, when less action, we may suppose it diminished; and when no action at all, under any circumstances we may conclude it destroyed, and here the capability for action ceasing altogether, discovers the difference between dead and living matter. Hence remarking its various vicissitudes with respect to energy, and its regeneration when not too much impaired, I am disposed to refer its origin to some source capable of supply, and not to an inherent or insulated quality.
Seeking for the medium through which this vital influence immediately operates, I am induced to turn to the brain as the point where all the powers of the animal appear more completely concentrated, and its continuations the (nervous elongations) as the active agents of life, existing more or less through the whole body. This proposition, I shall indeavour to support by direct and collateral arguments, adduced with as much perspicuity and brevity as possible.
Much may be argued from the importance of the brain in the economy of the system and the rank it occupies in intellectual operations. And though the mode of connexion between mind and matter, and the living principle and it, be not demonstrable to the senses, and will probably forever remain among the arcana of nature, yet we continually witness their effects and may conceive them a quality impressed under particular circumstances on the nervous system: possibly something in the way that bodies are endowed with the power of affinity or principle of gravity. And although I leave it to the researches of the metaphysician to explain how mental phenomena are produced through the agency of matter, and how the sentient principle acts again through the same mean. Still we may trace their proximate cause to the nervous medulla and brain, as the common centre of communication between all parts, and as the direct medium through which external substances act, and which again produce a re-action.
Injuries or inflammations of the brain are attended immediately with derangement of organs, or the most destructive consequences; whereas injuries of other parts, essential to the powers of life, and therefore called vital, appear rather by indirect means to impair the bodily functions; necessary to the proper performance of which, there is a very delicate organization of the whole, existing in close dependence on the circulation, or (the tout ensemble) of organic life.