THEORIES OF DEVELOPMENT.
There have been various applications of the general principle of Development, by means of which an attempt has been made to explain the origin of all things by Natural Laws, so as to exclude the necessity of any Divine interposition, either for the creation of the world, or for the introduction and establishment of Christianity itself. It has been applied, first, to explain the origin of worlds and planetary systems, by showing that, certain specified conditions being presupposed, there are fixed mechanical laws which might sufficiently account for the production of the earth and of the other planets and satellites of our Solar System, without any special interposition of Divine power at the commencement of the existing order of things. It has been applied, secondly, to explain the origin of the various tribes or races of vegetable and animal life, and especially the production of the human race, by showing that the existing types may have sprung, by a process of gradual development, from inferior races previously existing, and that these again may have been produced by the action of chemical agents in certain favorable conditions. It has been applied, thirdly, to explain all the most important phenomena of Human History, and to illustrate the law which is supposed to determine and regulate the progressive course of civilization, so as to account, on natural principles, for the origin and prevalence of the various forms of Religion, and even for the introduction, in its appointed season, of Christianity itself, without having recourse to anything so utterly unphilosophical as the idea of a Divine Revelation, or the supposition of supernatural agency. And it has been applied, fourthly, to explain the order, and to vindicate the use, of those additions both to the doctrines and rites of primitive Christianity, which Protestants have denounced as corruptions, but which Popish and Tractarian writers defend as developments, of the system that was originally deposited, like a prolific germ or seed, in the bosom of the Catholic Church.
It is the more necessary to examine the various forms of this theory, because unquestionably it can appeal to not a few natural analogies, which may serve, on a superficial view, to give it the aspect of verisimilitude. For many of the most signal works of God have been manifestly framed on the principle of gradual growth, and matured by a process of progressive development. We see in the natural world a small seed deposited in the earth, which, under the agency of certain suitable influences, germinates and springs up, producing first a tender shoot, then a stem, and branches, and leaves, and blossoms, and fruit; and every herb or tree, "having seed in itself," makes provision for the repetition of the same process, and the perpetuation and indefinite increase of its kind. The same law is observed in the animal kingdom, where a continuous race is produced from a single pair. And even in the supernatural scheme of Revelation itself, the truth was gradually unfolded in a series of successive dispensations; the First Promise being the germ, which expanded as the Church advanced, until it reached its full development in the Scriptures of the New Testament. These and similar instances may suffice to show that, both in the natural and supernatural Providence of God, He has been pleased to act on the principle of gradual and progressive, as contradistinguished from that of instant and perfect production; and they may seem, at first sight, to afford some natural analogies in favor of the radical idea on which the various modern Theories of Development are based. In such circumstances it would be an unwise and dangerous course either to overlook the palpable facts which Nature and Revelation equally attest, or to deny that they may afford signal manifestations of the manifold wisdom of God. Nor is it necessary for any enlightened advocate of Theism to betake himself to these expedients; he may freely admit the existence of such cases of gradual development, he may even appeal to them as illustrative of the order of Nature, and the design which that order displays; and the only question which he is at all concerned to discuss amounts in substance to this: Whether the method of production which is pursued in the ordinary course of Nature can account for the original commencement of the present system of things?
But the state of the question, and the right application of the argument, may be best illustrated by considering each of the four forms of the theory separately and in succession.
SECTION I.
THEORY OF COSMICAL DEVELOPMENT, OR OF THE PRODUCTION OF WORLDS AND PLANETARY SYSTEMS BY NATURAL LAW.—"THE VESTIGES."
The doctrine of a Nebular Cosmogony was first suggested by some observations of the elder Herschell on those cloud-like appearances which may be discerned in various parts of the heavens by the aid of the telescope, or even, in some cases, by the naked eye. It assumed a more definite form in the hands of La Place, although even by him it was offered, not as an ascertained discovery of science, but simply as a hypothetical explanation of the way in which the production of the planets and their satellites might possibly be accounted for by the operation of the known laws of Nature.
The explanation of the whole theory may be best understood by dividing it into two parts: the first being that which attempts to account for the formation of planets and satellites, on the assumption of the existence of a central sun, and of certain other specified conditions; the second being that which undertakes to account for the formation of the sun itself, on the assumption of the existence of a diffused nebulous matter in space, or, as it has been aptly called, "a universal Fire-Mist."[28]
When the theory is limited to the explanation of the origin of the planets and their satellites, the original condition of our solar system is assumed to have been widely different from what it now is; the sun is supposed to have existed for a time alone, to have revolved upon his axis, and to have been surrounded with an atmosphere expanded by intense heat, and extending far beyond the limits of our system as it now exists. This solar atmosphere revolved, like the sun itself, around its axis; but its heat, constantly radiated into sidereal space, gradually diminished, and the atmosphere being contracted in proportion as it cooled, the rapidity of its rotation was accelerated, until it reached the point at which the central attraction was overcome by the centrifugal force, and then a zone of vapor would be detached or thrown off, which might either retain its form as a nebulous ring, like the ring of Saturn, or first breaking into fragments, from some want of continuity in its structure, and afterwards coalescing into one mass, might be condensed into a planet as the vapor continued to cool. These rings or planets, thus detached from the central atmospheric mass, would continue to revolve, in virtue of the force originally impressed upon them, and their motion would be nearly circular, in the same plane and in the same direction with that of the sun. The first planet, so formed, must have been that at the extreme limit of our solar system; the second the next in point of remoteness from the centre, and so on; each resulting from the operation of the same natural laws, and emerging into distinct existence at that precise point in the gradual cooling and contraction of the atmosphere at which the centrifugal became stronger than the centripetal force. But each planet might also be subjected to the same process of cooling and contracting, and might therefore throw off, under the operation of the same mechanical laws, zones of vapor more or less dense, which might consolidate into moons or satellites, and which should also revolve, like the planets, round their primary. Thus, Uranus has six satellites, and Saturn seven; while the latter has also thrown off two zones so perfectly uniform in their internal structure that they remain unbroken, and constitute a double ring around the planet.
In this first form of the theory, which assumes the existence of the sun and its atmosphere, and the rotation of both round an axis, La Place sought to give a scientific form to the speculations of Sir William Herschell on the condensation of Nebulæ, by proving simply the dynamical possibility of the formation of a planetary system by such means, according to the known laws of matter and motion; but he did not affirm the scientific certainty of his conjecture, and far less the actual production of the solar system in this way. He has been followed by M. Comte, who has attempted to furnish, if not a complete demonstration, at least a plausible mathematical verification, of the hypothesis.[29] Utterly excluding all supernatural agency in the work of creation, he equally excludes from the problem which he attempts to solve, the origin of the sun and its atmosphere; and confining himself to the task of accounting, in the way not of demonstrative certainty, but merely of plausible hypothesis, for the formation of the planets and satellites of our solar system, he conceives the theory of La Place to be susceptible of such a numerical verification as is sufficient to give it a high degree of verisimilitude. Assuming that the periodic time of each planet must be equal to that of the portion of the solar atmosphere of which it was formed at the era when it was thrown off, and combining the theorems of Huygens on the measure of centrifugal forces with Newton's law of gravitation, he establishes a simple equation between the time of the rotation of each zone or section of the solar atmosphere, and the distance of the corresponding planets. On applying this equation to the various bodies of our system, he found that the periodic time of the moon agrees, at least within the tenth of a day, with the duration of the earth's revolution, when her atmosphere is supposed to have extended to the moon; and that the periodic times of the planets maintain a similar correspondence with what must have been the duration of the solar revolution when they were severally thrown off from its atmosphere. It is the less necessary, however, to enter on a detailed exposition of his argument, because he admits that it can afford at the utmost only a probable proof of an hypothesis; and further, because it is expressly limited to the production of the planets and their satellites, while not only is the existence of the solar atmosphere presupposed, but also its existence in a certain state, and with several determinate conditions; while no account whatever is given of the origin either of the sun or its atmosphere, and none of the laws or conditions on which the whole process of development is confessedly dependent.