like their Divine Author, independent of time and exempt from change.

ESSENTIAL CONDITIONS OF LIFE.

The Forces of Nature are constant; we do not conceive of them as beginning, or changing, or ending; the Laws of Nature appear to us invariable; but the forces and the laws are manifested only in relation to particular conditions. Thus in the case of life, regarded as a manifestation of forces according to laws, we find it to be limited to 'organic' structures composed of certain sorts, and certain combinations of matter1. Much of the matter which composes living bodies is capable of assuming the gaseous form, as Carbon, Hydrogen, Oxygen, Nitrogen. Other parts are capable of appearing in solution, as Phosphate of Lime, Carbonate of Lime, &c. All the substances named exist in nearly all plants and animals; and it appears, though we do not know how, necessary to the exhibition of vital phenomena, that they should be present necessary, I mean, according to this actual plan of creation, the only one we are acquainted with, or can justly conceive of as possible. For though it may be urged, and cannot be doubted, that other elements, other forces, and other laws might be employed for other systems of life; that is a conjecture which may be useful when thinking of the possible inhabitants of other planets, as Mercury, or Jupiter, or Neptune, but is not suited to the Land, Sea, and Air, with which our History of Life is connected. Deprive the atmosphere of its carbonic acid, -plants disappear; let phosphate of lime be absent-not only vertebrate animals vanish, but a large part of both the animal and vegetable races would languish and become unproductive1. Without supposing oxygen or the other elements to be entirely absent, any material change in their relative quantity must greatly affect the relative abundance of different races of living beings whose dependence on these elements is different in degree.

Life is dependent on a continual loss and restoration of parts in its organic fabric. One great part in this process is maintained by the atmosphere, from which all plants and all animals draw supplies of gaseous elements suited to their constitution. Plants absorb the carbonic acid, which exists in the atmosphere to the extent of Toooth part by weight,

1 Phosphate of Lime occurs in so many animals, and in so many plants, in some part or other, as to be regarded by eminent writers as an invariable accompaniment of life.

and yield oxygen in return; while on the other hand, animals inspire this oxygen, and evolve in exchange carbonic acid. Thus appears a real and necessary relation between the atmosphere as it is, and the double system of life which is in operation. If we change the constitution of the atmosphere, all the relations in which it is so important must be changed also, and amongst the most obvious of these are the reciprocally dependent races of plants and animals. It has been conjectured by Brongniart, that the very rich series of vegetable forms, including many ferns, in the old carboniferous deposits, may have been favoured in their amazing growth, not only by high temperature and humid atmosphere, but by a greater proportion of carbonic acid in the air. Dr Daubeny has submitted this to a trial, in vessels properly supplied with a regulated artificial atmosphere, and the result is not unfavourable to the speculation.

Again, animal life depends upon the previous exercise of vegetable life; for ultimately all animals subsist upon plants, as these feed upon the atmosphere. Perhaps nothing is more surprising than the immense diversity of the forms and qualities of the plants, coupled with the almost equal dependence of all vegetative life upon the same atmosphere chemically everywhere almost identical. Upon this vast variety of plants, innumerable hosts of herbivorous animals feed, and they minister to the appetites of the Carnivora. It is conceivable that while the plants remain unchanged, the Herbivora might vary, or might become the prey of different flesheaters; but it is perhaps not conceivable that with such an atmosphere as ours, under such conditions as now obtain, there could be generally any great variation in the relative total amount of vital energy in plants, compared with animals. There seems also a high degree of permanence in the relation of Herbivora compared with Carnivora. Our marine Cetacea might be replaced by Enaliosaurians; our Gasteropoda by Crustacea; our fishes by Cephalopoda; but the researches of geology seem to shew that from the earliest periods, carnivora and herbivora, plants and animals, have been combined into the same general relations of mutual dependence as at present.

Subject to these conditions, life appears in all the habitable spaces of the land, sea and air, filling each with beings capable of enjoying their own existence, and of ministering to the bodily wants and intellectual longings of the one observing and reflecting being to whom God has committed the wonderful gift of thoughts which reach back beyond the origin of his race, and stretch forward to a brighter futurity.

In the elements of land, air, and water, both plants and animals are fitted to live by means of contrivances varied in almost every individual case, but always to be conceived of as elegant adaptations to some conditions of matter - adaptations to gravity, to force of wind, to depth of water, to degrees of light, to periodicity of seasons, and even to local and limited occurrences. Regard the eye which in its perfect state, as in man, is destined to feel the presence or absence of light, to distinguish the colours of the several rays, and to perceive the forms of the luminous surfaces. What is it but a triple photographic lens with six curved surfaces calculated for three different media; calculated for achromaticity and spherical aberration; provided with a variable self-adjusting aperture, and a variable self-adjusting focal length, adapted to something better and more sensitive than a collodionplate-the beautifully expanded and guarded retina, which after the fraction of a second of time is ready to receive a new impression with undiminished energy. Regard the two eyes, the natural stereoscope, by whose beautiful joint action solids take their proper aspect, distance is estimated, and the landscape acquires that instructive composition which our artists delight to imitate.

Again, regard this wonderful organ, modified to