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LIFE ON THE EARTH.
THE subject which, by your command, I have the gratification of bringing before the notice of the University of Cambridge, is not offered as new, though, in consequence of being at the present time subjected to that scrutiny which always arises on the production of new evidence, it wears a somewhat novel aspect. For certainly the history of life is a theme which can never have been absent from the mind of a contemplative naturalist. It never can have been absent, because in all the classifications, in all the systems by which we vainly task ourselves to represent the divine idea of nature, we have invariably looked for a beginning, a progress, and a possible end. Standing by the stream of life, we have surveyed the variations in its course, and appealed to history and experience, for the data which might guide us to a right view of its incessant fluctuations, and its recurring uniformities. We have thus found all nature, organic and inorganic, to be harmoniously combined in mutual dependence; the worlds of
matter and of life linked together by peculiar associations, which endure through long time amid varying phenomena, all suggestive of appointed succession and definite purpose. Perceiving that every one thing exists as part of a system, that all effects are parts of a series, and that the whole is compacted together and kept in perpetual movement by some determinate and general principles, we are constrained to believe that so perfect a plan must be permanent and subject to no material change. Nor is this inference much disturbed by the fact that in every stage we perceive vicissitudes from the greater to the less, and from the less to the greater : from the simple to the complex, and from the complex to the simple. For the changes which thus manifest themselves appear, by sufficient experience, to be often repeated in cycles of measurable duration, and governed by laws which appear of perpetual efficacy. Thus amidst all the diversity of nature nothing appears accidental, nothing indefinite, nothing unforeseen, and it is this consideration which makes the study of creation hopeful. We may never fathom the mystery of the origin of life or the inner constitution of matter, but every step we take in accordance with sound reason and reverent feeling brings us nearer to a better understanding of the problem, and is a step in the right direction. Let us, then, bring into view the facts discovered in relation to the Origin and Succession of Life on the Earth, not with the expectation that we can altogether penetrate the scheme of creation, but to get a further insight into it, and a clearer perception of the appointed laws of nature, by the use of those senses with which the Almighty has endowed us. Let us hope that while we study the external world, and, perhaps in vain, strive to master its wonderful history, we shall at least enlarge and correct our ideas, and truly perform the part assigned to us in the large field of creation which we are enabled and invited to contemplate.
Nature, in a large sense, is the expression of a DIVINE IDEA, the harmonious whole of this world of matter and life. Man, included in this whole, is endowed with the sacred and wonderful power of standing in some degree apart, so as to observe the course, investigate the laws, and measure and direct the inexhaustible powers which surround him and penetrate him. The knowledge thus slowly gathered is contained in two great HUMAN IDEAS, the idea of force, as producing phenomena, and of time as determining the succession and duration of these. The ideas of constant force and perpetual time are suggested to us in various ways by the repeated occurrences of nature; uniform measures
of force and time are obtained by observation of the earth's constant force of attraction toward its centre, and unchanging velocity of rotation on its axis. By comparing phenomena with uniform measures of force, and uniform measures of time, we obtain, or strive to obtain, for each natural effect, a correct numerical valuation in terms of the force employed and the time consumed in the production of the effect.
There are, or appear to be, different kinds of force, producing different kinds of effects—as magnetism and gravitation ;—but they are often comparable, and capable of valuation under the one comprehensive idea of relative magnitude. Thus all natural effects known to us are measured or conceived to be measurable, by units of force, operating through units of space in units of time. And nature appears to us to be the sum of these effects combined into a 'System,' harmonious, mutually dependent, and preserved entire amidst an endless succession of limited vicissitudes. In expressing our conceptions of this well-adjusted ‘System of Nature' we employ the term 'Laws;' and the more comprehensive these are, - the higher the abstractions which they represent—the less do we conceive them to be variable ; so that the most general laws which we reach or strive to reach, are conceived to be,
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 matter?. 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
1 In modern language matter is said to be known to us only by effects cognizable by our senses; these effects are due to forces; matter is the seat of these forces; or, if we will, it is a collection of centres of force.