tain operations, and to trace them upwards to their secondary cause, is a rational and a satisfactory task. But when we would account for the mode of operation, and unfold the nature of the cause itself, we go beyond the reach of our faculties, and all is mystery and confusion. From certain experiments, we are enabled to infer the existence of gravity, and to calculate its laws; but how it operates, and in what manner it exists, we must be satisfied to remain in igno

rance.

In our researches therefore into the laws and the properties of life, let it be our endeavour, first to observe, and then to arrange the facts, and afterwards to draw such inferences as the laws of right reason will permit. Much will indeed remain unexplained, but nothing will be either contradictory or confused. The knowledge which we shall thus attain, though narrow will be certain; and perhaps we may find, that when we have rejected that style of reasoning which I have attempted to expose, we shall also reject the Sceptical notions and the Atheistical views, which such a style will uniformly encourage.

CHAPTER VI.

LIFE-its Definition-In what Respects living Substances differ from dead-The distinct Phanomena of Life, in the Vegetable, the Animal, and the Human Creation.

THE distinguishing character of all living creatures, from the highest to the lowest, being active existence, we may therefore define life to be "Inhe"rent activity." A stone and a tree equally exist, but the former will remain for ever passive and unmoved, excepting by external force; the latter has an inherent activity, which causes motion in its parts, without any external agency. External circumstances may indeed call forth these active powers, from a state of previous suspension ; but no external circumstances can give them existence. The activity of a living substance is an inherent and an independent quality; it may be destroyed, but cannot be imparted either by material impulse or by chemical agency. The ground in which it is sown does not infuse a vegetating activity into the seed, it only brings into

motion those inherent powers which existed, and still continued to exist, though temporarily dormant. The chemical agencies of heat, moisture, &c. do not act upon a living substance, in the same manner as upon inert matter. All changes which chemical agencies produce upon inert matter, consist entirely in the new disposition of its constituent parts: but when by these external influences a seed is made to vegetate, the new disposition of its parts is a consequence of other prineiples, of absorption, secretion, and nutrition. These arise from its inherent activity, and are not dependent upon any chemical powers, strictly so called; for both the mode of action and the product is different from what would be the result of chemical agency alone. An inert body increases in magnitude only by the accretion of fresh matter to the surface. A living body grows by the developement from within, arising from the conversion and assimilation of extraneous substances to those of its own frame. Nor is this simply the accretion of external matter to the vegetable body; but a change is wrought in the moment of assimilation.

The temporary suspension of active powers does not by any means invalidate the existence of life. The existence of a power is one thing, the exercise of it is another. As long as the inherent activity may be recalled or restored, that substance is justly considered as alive. The grain of corn is a living substance, though it shews no more signs of internal activity than a pebble; because that activity, though suspended for a time, is capable of restoration. The

conclusions to be drawn from this consideration are of a very important nature, as will hereafter be shewn.

This inherent activity, though it be the universal character of all living substances, will be found to vary very considerably both in degree and in effect. In the vegetable world it is exercised in little more than in the circulation of fluids, in the appropriation of nutriment, and in the reproduction of its kind. The plant has the power of absorbing a certain moisture from the earth, and of converting it into a nutritive fluid. In the ascent and return of sap, we trace a simple but a very perfect circulation; in the secretion again of juices, and in the reproduction of its kind, we discover phænomena very nearly resembling those of the animal economy. In the power also which it possesses of repairing either structure which has been injured, or parts which have been destroyed, the activity of the plant is peculiarly displayed. It may here be incidentally remarked, that this power of ready restoration, appears to be most vigorous in the lower orders of the creation. The most inert and most defenceless beings, would seem to be endued with the readiest means of repairing those external injuries, to which, from their situations, they are the most exposed. The tree will reproduce its branch, the lobster its claw, while equal losses in the dog, the horse, and the man, are beyond the powers of restoration. It should, lastly, be observed of plants, that their inherent activity

enables them to resist those chemical agencies, to which, after that activity has ceased to exist, they readily yield. While the tree is alive, the atmosphere has little or no effect in rotting the bark, but when it dies, the bark, the fibres, and the roots decay by the external action of the air, in the same manner as the dead body yields to those chemical agencies which the living body was enabled to resist. We have a curious instance, indeed, of this, in the effect of cold on the living and on the dead body. The living will resist the action of congelation, while the dead will be split by the freezing of its juices.

As we ascend in the scale of the creation, the phænomena of active existence increase upon our notice. It has been remarked, that the first process which distinguishes the animal from the vegetable, is digestion, and that this peculiar characteristic is observable even in the lowest degrees of the animal creation. From the process of digestion we are led upwards to the consideration of the nervous system, of irritability, contractility, and other phænomena which accompany animal life. Some, however, of these properties, such as digestion, respiration, &c. though they are peculiar to the higher order of the creation, are, in themselves, nothing more than the extension of those which we observe in vegetable. It is not because the animal has a nervous system, that it is therefore essentially distinguished from the vegetable, but it is because there is a higher princi