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appear, that the brain or sensorium is more particularly the organ which is subject to that species of sufferance called fatigue. From these facts we may perceive the necessity of sleep, which consists in a temporary suspension of sensation, volition, and thought; and is a resource of nature, whereby the powers of life recover themselves after satiety and fatigue, which are provided as guards to warn us when nature is in danger of being strained, either by repletion, or over-exertion; and it is evident that such barriers were absolutely necessary, in order to set bounds to operations which are only occasionally requisite, and which would otherwise depend on the caprice of the will. The exercise of sensation and voluntary motion, in a moderate degree, is conformable to the intention of nature, and therefore salutary; and it is only when they are excessive, that they tend to wear out the powers of life, and more espe cially if these are not duly recruited by sleep. Man is formed for action, and without it neither mind nor body can attain its vigour. But the excess may lead to untimely decay, as the deficiency to a torpid vegetable kind of existence. Hence, in reasoning on these things, we must observe the mean. For to such a creature as man it would be no ultimate advantage that he should improve and cherish his unthinking muscular power, at the expense and loss or deterioration of his sensitive and intelligent capacities. We see, therefore, how completely it is in his power to degrade himself, by cultivating the grosser principles
of his frame, or to raise himself to his proper rank and dignity by attention to the nobler capacities of his mind. He can scarcely be perfect in intellect, and perfect in animal power at the same time: and either of these parts of his constitution will suffer, it appears, by exclusive devotion to the other.
I have considered the facts and observations on muscular motion, not only as highly interesting in themselves, but as leading us to distinguish that property of animal life called irritability, which seems to depend on the vis insita of the muscular fibre, from the simpler irritability or contractility of the vegetable structure, below it, in the chain, as well as from that more exquisite and elaborate appendage to animal life called nervous power, above it. Some persons have, indeed, doubted whether any animal, however simple, can have the least consciousness of existence, or the least degree of sensation, without some kind of nervous structure more or less perfect. This, however, would be to assume more than we are warranted in doing, from the latest discoveries in comparative anatomy. As far as our present knowledge reaches, distinct laws seem to belong to each of these states. And at each link of the ascending chain, by general confession, there is an impassable barrier to human research, an incomprehensible and active power working in each, like the law of gravitation in dead matter. We cannot therefore pretend to distinguish these states-viz. vegetation, irritability, and nervous power, any farther than by their effects.
I am disposed to think, notwithstanding, that even this slight view will enable many to correct some erroneous notions, and popular ones too, on this subject.
As an example, I may mention the common impression that the writhings, distortions, and convulsive motions, we sometimes see in animal bodies, are accompanied with pain or feeling. This is proved undoubtedly not to be always the case. It seems, however, a natural association in the mind, and probably a wise appointment to excite our humanity. Hence that philosophy must appear cold and insensible which would attempt to reason away our natural sympathies for suffering, as if they had no foundation in fact. Convulsive motions do sometimes indicate pain. But the most violent pain is often endured without them: and, inversely, the most violent convulsions are often unaccompanied with sensation.
It appears evident that those animals, which possess this irritability in the greatest perfection, are endowed with the least nervous power; and the more perfect the nervous structure, the more it seems to impede the full exercise of the vital irritability. This will be more obvious when we take into our consideration the facts I am about to mention: and here I may notice by the way, that although in the next section I follow some of the views of Herder in his Outlines of the Philosophy of Man, yet I cannot altogether subscribe to some of his conclusions. For after
quoting the opinion of Reimar, who has written oné of the most enlightened works on the subject, 'that the instincts of animals are to be explained from the mechanism, senses and feelings, with which they are endued, and that we must admit particular determinate natural powers, and natural innate capacities, which are susceptible of no farther explication,'* Herder announces that he does not acquiesce in the latter sentiment, but substitutes an opinion, vaguely enough expressed, that "the composition of the whole machine from certain given powers, senses, feelings, and conceptions; in short, the organization of the creature itself constitutes the most sure direction, the most perfect determination, that nature could impress upon her work."
If we examine this position, it leaves us as much in the dark, with respect to the agency of a ruling superintending power, as that of Reimar; and, by implying that the organization of the animal is self sufficient, gives that countenance to the sceptic which I believe Herder did not intend. It seems to imply that we may come to something capable of explanation; and yet makes nothing clearer after all the circumlocutory terms of the proposition. If Herder had not made the objection to the former, his own proposition might have appeared more admissible in itself. But the attempt to make animal organization dependent on its own powers and resources, and to explain by
* See Outlines of Philosophy, Vol. 1. Book 3. Chap. 2.
technical phrases what is inexplicable, however scientifically and cautiously expressed, is too much after the manner of Lucretius. And yet if we take the word nature in the last line, to mean what Herder wishes it to be understood to mean, "Almighty power, goodness, and wisdom,”* the effect may be in part neutralized. It is certain that he did not intend to exclude such an ultimate cause; though the expressions he has used lean to that view of the question.
On the connected chain of functions in Vegetable, Muscular, and Nervous systems.
After the preceding facts and observations on the instinctive motions of the vegetable and muscular fibre, as they are exhibited in the living phenomena of plants and animals, I shall proceed to notice, in a connected series, the leading offices and distinguishing characters of the most prominent links in the natural chain of Being, from the simple functions of plants to the complicated power and intelligence of man.
In plants we may observe the two natural instincts, if such they may be called, of nutrition and propagation; and the results of these are works of divine
* See the Preface to Herder's Outlines.