skilful, and however well versed in the theory and practice of his art, cannot be sure of the working qualities of his new machine or instrument until he has subjected it to an actual trial. Operations in engineering, such as the construction of roads and bridges, of harbours, breakwaters, and sea-walls-or in drainage and irrigation, require to be tested by actual use before their success is certain. Inventions in the departments of chemistry and optics must also be verified by experience, and cannot be trusted to mere scientific anticipation. The aptitude of a room for hearing sounds cannot be determined, à priori, by the theory of acoustics, and must be made the subject of experiment before it can be known with certainty. In all such cases as these, the experimenta fructifera of Bacon must be applied, in order to determine how far the general predictions of theory have sufficed for guiding the operations of the practical man. ୮ In comparing the prophetic powers of physical and political science, we must bear in mind that no science can properly be said to predict anything.' The general affirmations of a science apply, indeed, equally to the future and to the past; but this is true of political as well as physical science, 'so far as human nature, the subject-matter of politics, is unchangeable.' Scientific prediction, properly so called, is made in physics by the application of the principles of a scientific art, not of a mere science or theory, to the facts of an actual case. Now we may observe that, when the question relates to some mechanical contrivance of man's hands-some operation upon external nature-the uncertainty of physical prediction is, if not as great as that of political prediction, at least very considerable. When the sciences of mechanics, hydraulics, and acoustics come to be applied to constructive works-to fortifications, dykes, moles, docks, harbours, breakwaters, lighthouses; to the supply and transport of water by aqueducts, pipes, drains and sewers; to railways, roads, bridges, viaducts, canals, paving, to the size and shape of rooms, to ships, artillery, carriages, and machinery of all sorts-in short, to the various departments of engineering and architecture, the utmost uncertainty often prevails as to the future practical results, and the widest diversity of opinion exists among competent pro fessional men with respect to the application of the principles of their art. It likewise often happens, that works constructed under the best practical advice fail of their purpose, just as laws made by the counsel of the ablest statesmen often fail in producing their anticipated effects. In both cases, the failure is owing to an imperfect determination of the future. $ 4 Another important class of physical sciences is that which describes the mineral structure of the earth, and the living beings upon its surface. These are geology and mineralogy, with the sciences relative to the vegetable and animal kingdoms, and to the physical nature of man. The crust of the earth, and its component minerals, are not affected by any constantly-recurrent periodic changes, but they appear to be the result of a successive aggregation of parts, and they continue to be modified by atmospheric influences, by the action of superficial water, and by internal volcanic agency. "Geology, therefore, is partly historical,' and narrates successive changes in the earth's surface; and partly it describes the existing strata, their superposition, their character and component parts, and the animal and vegetable remains embedded in them. So far as the existing state of the superficial portion of the earth remains unchanged, the descriptions of geological science will continue to apply to the future. Minerals, and their respective properties (including crystallography), are subject to no variation; and the results of scientific observation, when embodied in a proper classification and nomenclature, will therefore be as true of minerals hereafter as they are at the present moment. Geography, in like manner, is merely a descriptive science, and does not predict the future. Such predictions as appear to belong to geography relate, in fact, not to the science itself, but to our knowledge of it, and to the progress of civilized man over the surface of the earth. Such, for instance, is the celebrated passage in the tragedy of Seneca,(") which has been applied to the discovery of America. (6) 'Venient annis sæcula seris,' &c. Med. 575-9. See Bacon's Essay on Prophecies; and A. de Humboldt, Examen Critique de l'Histoire de la Géographie du nouveau Continent, tom. i. p. 162-80. " § 5 Animated nature, on account of the phenomena of reproduction, growth, and death, is subject to a series of periodic changes. Though it does not move in fixed cycles, each individual member of the great aggregate lives only for a period, and the species is renewed by the perpetual generation of new individuals.(7) Each individual is mortal, but the race never dies. In the vegetable kingdom, each individual resembles every prior and co-existing individual of the same species in its physical properties. Being destitute of sensation, and neither conscious of pain and pleasure, nor endowed with any intelligence, plants are necessarily a mere succession of identical terms in a long series. In the animal kingdom, each individual resembles every prior and co-existing individual of the same species, not only in its physical, but in its sentient and mental properties. Animals, unlike plants, are all provided with organs of sensation, more or less perfect; they are all conscious of pain and pleasure; they have all an instinctive guide of their actions; and the highest orders of mammalia possess a limited share of reason. But (with the exception of the inconsiderable alterations produced by the domesticating influence of man) the successive animals of the same species are unchanged. ($) Their faculties do not enable them to accumulate knowledge, or to profit by the experience of their predecessors. The more intelligent mammalia are docile when man is their teacher, and obedient when man is their ruler; but they can neither teach nor govern one another. Hence their nature is stationary, not progressive; they are bound, by the laws of their existence, to an unvarying condition, secure, indeed, against degeneracy, but incapable of improvement; and (7) This idea is expressed with equal truth and dignity of language by Virgil with reference to bees, whose life, according to him, lasts only for seven years: 'Ergo ipsas quamvis angusti terminus ævi Excipiat-neque enim plus septima ducitur æstas— (8) On the unchangeability of animal species see above, ch. ii. §§ 4, 5. one generation of animals is as like another, as one generation of plants is like another. The lion and the horse of the present day as much resemble the lion and horse of the ages of Aristotle and Pliny, as the oak or the pine of the present day resembles the oak or the pine of the same epochs. Each generation of animals differs from its predecessors, in being composed of different individuals, but resembles its predecessors in being composed of individuals whose attributes cannot be distinguished from those of any other generation. Like the planets, an animal species is always in motion, but it moves perpetually in the same orbit-' mobilitas immota manet.' It is another and the same. Its character is unchanging, though its component parts are not identical. Hence, the classifications and descriptions of Cuvier and other scientific naturalists exhaust the entire subject-matter of animal life: animals have no history for the past, and no prospect of progress for the future; and when their anatomy and physiology have been determined by accurate observation, the description will be as applicable to future, as it is to the previous and present generations of each species. $ 6 Similar propositions apply to man, so far as he is an animal. When his anatomical structure has once been described, when the observations and researches of physiologists have ascertained the normal action of his bodily organs, and when pathology has investigated their morbid deviations from the healthy type, the scientific truths thus established have no special reference to the past, and they will retain their applicability so long as the physical nature of man is perpetuated without change through successive generations. Harvey's discovery of the circulation of the blood was not limited to the time before Charles the Second; it is as true of the present as of any other generation, and no one doubts that it will continue true hereafter. All the ascertained facts of human physiology-all accurate descriptions of the processes of sensation, motion, digestion, secretion, &c., are independent of time; and, inasmuch as they relate to a subject, which, though not identical, is perpetually renewed without variation, which is, as it were, reprinted from the same type, re-coined from the same die, and re-cast from the same mould, they will be as true hereafter as they are at the present time. (9) The internal functions of the human body are, during life, for the most part withdrawn from our observation, and they can only be investigated by indirect means. Even, however, when they are known, both in their healthy and morbid state, the number of possible concurring influences upon their action is so great as to render prediction obscure and uncertain. Hence, medicine and surgery are, to a great extent, merely tentative arts. The nature of the malady is often obscure, and the diagnosis uncertain; the mode of treatment is still more doubtful, and the most skilful physician will often abstain from giving a confident opinion, either upon the future course of the disease or the efficacy of his remedies. All that concerns the expectation of life is subject to great uncertainty; though, by including large numbers of persons, an average probability for every successive period of life may be calculated. Medical science has made a successful inroad into the future, and subjected a large portion of it to the control of human skill and human knowledge; but it must be confessed that much of the territory still remains inaccessible to the prescience of the physician. The power of anticipating the course of a disease in animals, and applying to it a proper curative treatment, is still more restricted than in man, inasmuch as animals are unable to describe their sensations, and to afford that assistance in exploring the nature of a morbid affection which the physician derives from a human patient. (1) Animals, however, are less liable to morbid derangement than man in the natural and undomesticated state, their life is generally terminated either by the voracity of some other animal, or by atmospheric changes; if they escape one of these two causes of mortality, they appear to die, not by disease, but by natural decay.(") Vegetable life is still less liable to disease (9) Upon the extent of prediction in physiology, see Comte, Phil. Pos. tom. iii. p. 458. (11) Above, ch. ii. § 4. (10) See above, ch. vi. § 5. |