TRANSITION OF INERT INTO ORGANIC SUBSTANCES.

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The Application of Physiology. — After the foregoing details of the chief points in the economy of living nature, it will not, in conclusion, be inappropriate to exhibit some of the great truths of Physiology; to trace their connection with other subjects of human inquiry; and to point out some of the uses to which a knowledge of this department of science is practically applicable.

Physiology, taken in its largest acceptation, holds a most prominent place in the circle of human knowledge. We have seen it trace the development of the perfect man through the grass of the fields back to the common mineral elements of the crust of the earth. It also enlists in its service Anatomy, Chemistry, and not a few departments of general Physics; and it connects itself with Agriculture, Political Economy, and the science of Legislation and Government, hardly less than with Medicine, Surgery, and the preservation of health.

The most striking truth in physiology is, that organic existences, including alike the highest races of mankind and the meanest vegetable organisms, are, in their material composition, derived from the mineral matter of the earth. The properties of the simple elementary substances entering into the vegetable and the animal kingdom have been ascertained with exactness; in their mineral condition, they have been fully investigated; and their chemical properties and combinations in inorganic nature are well understood. And such knowledge leads to a second great principle in physiology— namely, that there is nothing in the properties of these several elements of the organic world by which any tendency is given to them, beyond the rest of mineral matter, to combine together to produce any form of organic life, however simple, and however transitory. When, then, in connection with this undeniable truth, it is considered that, for long periods of time, our planet, the earth, must have been, from physical circumstances, totally incapable of supporting any form of organic existence, the conclusion follows, that the appearance of organic existences on the earth implies an exercise of Infinite Power, by which mineral matter was endowed with the new property of passing into the first species of animal and vegetable life. It is vain to say that such statements as these lie without the pale of inductive science. Man's natural curiosity loudly asks whence came organic species, whenever he considers the undeniable truth that the surface of the earth must have lain for ages destitute of such existences; and the answer which, by the original constitution of his mind, he is compelled to give, is, that such a change on the mere matter of the crust of the earth, as its transition into living forms, could not have occurred without the interposition of Omnipotent Power.

This great truth, then, is not the less a natural inference of the human faculties, because it does not strictly fall within the limits of physiology; it owes its origin to the operation of the great principles of human belief, implanted in the mind by its primitive constitution, and on the knowledge supplied by the cultivation of physiology. The next great step in the progress of man's knowledge of organic nature lies strikingly within the limits of physiological science. It is the conclusion that each organic species had its origin in a separate creative power. It is vain for any one pretending to the character of a philosopher to maintain that the idea of a transmutation of species is far more simple and far more in accordance with Infinite Wisdom. Every one endowed with a philosophic spirit must at once reject this idea, simply because there is not a shadow of foundation in its behalf to be met with in the whole of nature. It is impossible to pronounce with certainty, from physiological evidence, whether the several races of plants and animals at present living, or discovered by geological evidence to have formerly existed, had their origin from one pair, or from many pairs; but the

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strongest evidence does exist against the supposition that one species of a lower grade can pass successively into other species of a higher grade. On the contrary, nature has guarded each species from change with the most sedulous care. By artificial means, and within certain limits, man can make various changes in the species, both of plants and animals; and accidental circumstances, without man's interference, produce like variations. But it is fully ascertained that, as soon as those circumstances, whether designed or accidental, which have caused a variation have ceased to operate, then the species returns to its original state.

As soon as physiology, by drawing upon the philosophy of mind, has overcome the difficulty attendant on the first appearance of organic nature on the surface of the earth, it traces out by observation all that belongs to the economy of organic existence. By such observations physiology has made known the conditions necessary for the development and maintenance of organic existences as well as for their reproduction; and as far as physiology has yet discovered, so long as these conditions can be maintained in respect to each species, no tendency is shown to its becoming extinct. At the same time, a question may arise whether organic species have the power of unlimited existence, as species, so long as the ordinary known conditions necessary for the maintenance of the individuals continue unimpaired, or whether a species be, like the individual, capable only of a limited existence; so that, as in the case of the individuals of which it is composed, its prolific life at last begins to fail. The experience of mankind on the earth is probably not yet sufficiently extended to afford any sufficient data for debating this question. But the striking analogies between individual life and the life of a species in other respects, must prevent us from positively affirming that species can only die by a failure in the ordinary conditions under which the individuals of that species are seen at present to live and thrive. In the case of the individuals of every species, no matter how abundantly the conditions of their life are supplied, there comes at last a period when their susceptibility of availing themselves of those conditions declines, so that decay and death are the inevitable consequences. In the species of plants and animals best known to physiologists, no tendency has ever yet been remarked to degenerate, except that which owed its origin to a failure in the conditions necessary for the existence of the individuals of that species. The dodo is an example of a species which has become extinct within the records of history; but a single case hardly affords a sufficient ground even for conjecture; and it is, perhaps, right at present for physiologists to content themselves with the belief that the dodo perished from fortuitous causes interfering with the external conditions necessary to enable the individuals of that species to live and thrive on the earth's surface.

Here, however, in stating the great truths of physiology, the importance of the law of death in the organic world, as taking that part of nature entirely out of the category in which mineral nature exists, must not be omitted. In physiological nature, then, the law of death may be thus stated-no perfection of organism, no completeness in the supply of the conditions of existence, can prevent any living individual whatever from at last failing to derive the means of maintenance from those conditions, and from falling into a state of decay and dissolution. Such a law is exclusively known in physiological nature, there being nothing the least analogous to it in the case of inert matter.

A more practical truth of physiology is, that each species multiplies in proportion as the circumstances under which it is placed are favourable to the maintenance of the individuals of that species. This law appears to admit of no exception. In short, phy

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siological principles are quite sufficient to settle the questions whch have arisen as respects the law of population. No country can support more inhabitants than it can supply with the means of maintenance. It is not necessary that the soil of that country should produce enough of corn and cattle to feed all its inhabitants; but then it must produce something else by means of which food can be obtained from other countries. If the inhabitants are skillful workmen, they may convert raw material, derived from other countries, into manufactured goods; and for the value of their workmanship they may receive enough of corn and cattle to satisfy their wants. There may be mines of mineral wealth in demand among agricultural nations; and in exchange for this wealth they may obtain a sufficiency of corn and cattle. Still the great law remains unaffected that the number of people which a country can maintain cannot exceed that for which it possesses the means of providing food.

Physiology enters upon another practical question of vast importance—namely, whether the soil of a country can be renewed independently of the application of existing organic matter. Every crop which is taken off a field carries with it a certain amount of soil; not, indeed, equal to its actual weight. because a great part of the substance of each crop is derived from the air, and from the rains. Hence a soil necessarily becomes exhausted by repeated crops. It is renewed by the application of manure; but as manure, in common circumstances, is obtained from organic matter, it is plain that the organic matter of a country must be continually declining by being again reduced to mineral matter; unless it be proved that under some circumstances at least soil can be renewed from the mineral kingdom. The annual waste of organic matter in every country is enormous-that is to say, a large quantity of organic matter is continually passing back into the mineral state, under such processes as putrefaction, combustion, and the respiration of animals. Plants, no doubt, are continually converting inorganic matter, such as the carbon of carbonic acid and the hydrogen of water, into their own substance. But the organic substances required for food contain not only hydrogen and carbon, but also nitrogen; and therefore, unless it be proved that ammonia, which is the chief source of the nitrogen of plants, be constantly produced in the mineral kingdom, it must be confessed that there is a continued irreparable destruction of organic matter upon the earth's surface. Here there is a controversy among chemical authorities-some contending that ammonia is continually formed in the mineral kingdom; others that the ammonia which appears in a soil is derived solely from the decomposition of organic matter. On the determination of this question our speculations rest as to the future history of the organic kingdoms on the surface of our planet. If there be a continual destruction of organic matter without any corresponding renewal from the mineral kingdom, then a time will come when plants and animals must perish for want of the means of subsistence at present supplied to them by the soil. It is no part of our purpose to enter upon this controversy; but the evidence at present seems to be in favour of the unlimited power of mineral nature to produce ammonia, and therefore to supply that important constituent of the food of plants which otherwise must be derived from the destruction of organic matter.

Another speculative question bearing on the fortunes of the animal kingdom is sometimes debated in works of physiology. We have already remarked, that the carbonic acid, which is continually thrown into the atmosphere by the respiration of animals, is as constantly decomposed and removed by plants for their own support. It is a common view that our atmosphere must, at a very early period, have contained all the carbon, in the form of carbonic acid, which now exists in the organic

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kingdoms, and in the soil of the earth. If such were the case, the atmosphere, however fit to support the life of vegetable organisms, must, it is said, have been totally unfit to maintain the life of animals. The supposition then is, that through the vast preponderance of the vegetable kingdom, for many ages, on the surface of the earth, the carbonic acid was gradually reduced in proportion down to its present small measure; and that the carbon so abstracted from the carbonic acid is that which now forms so large a proportion of the bodies both of plants and animals, and so large a proportion of the soil of the earth. And now that the animal kingdom has begun to preponderate, and a greater proportion of carbonic acid is produced by the respiration of animals than is decomposed by the food of plants, this change will go on increasing, until at last the atmosphere will become again unfit for the support of animals, owing to the great accumulation of carbonic acid. The determination of this question involves several considerations. It is true that the forests which covered the earth in ancient times are fast disappearing; but it is also true that these forests are replaced by cultivated crops. Shall we then say that if all the arable parts of the earth become covered with crops, those crops will not destroy as much carbonic acid as the ancient forests? And if this be the case, then the carbonic acid will not undergo any material increase. One thing is certain, that the .animal kingdom, as respects its constituent carbon, can only increase at the expense of the vegetable kingdom; so that, while there must remain the same quantity of carbon at the earth's surface, a larger proportion will certainly be contained in the animal kingdom than in the vegetable, owing to the destruction of the ancient forests. But if the whole quantity of carbon contained jointly in the crops, and in the animal kingdom, and in the soil, remains equivalent to the quantity now in those three conditions, no change can take place in the quantity of the carbonic acid in the atmosphere. Again, it is perhaps impossible that the animal kingdom can increase so fast as to deteriorate the air much, when it is considered, that the only part of the animal kingdom that can be regarded as on the increase, is man himself, and the animals subservient to him.

An easy answer to the difficulty which has been here raised is, that by computation, from very probable data (“ Edinburgh New Philosophical Journal,” July, 1845), the conversion of the whole carbon of the soil, and of living plants and animals, into carbonic acid, would not more than double the small proportion of that gas existing at present in the atmosphere.

The connection of pestilential diseases with deficiency of the means of subsistence has too little engaged the attention of legislators. It is true there are certain diseases of an epidemic character, such as small-pox, measles, scarlet fever, which prevail even among the best fed orders of society. It is undeniable, however, that even these epidemics are far more fatal when joined with an insufficiency of food. In modern times, it is hardly possible to conceive the ravages which, in the earlier ages, epidemics inflicted upon the human race. At those periods, agriculture had made but very slender progress; and what surprises us is not so much that the nations of Europe suffered from such diseases as that they did not suffer even more. Were the same circumstances which so often prevailed in those countries again renewed in the present crowded state of many of the countries of Europe, the devastation would be far greater than even we find to be recorded of those times. Great sanitary improvements have taken place in all the countries of Europe, defective as that kind of legislation still is. But when the rapid increase of our great towns, without any previous means being secured for their proper drainage and ventilation is considered, physiology cannot too loudly pro

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claim, not only that virulent epidemic diseases may arise under such circumstances, but extend their ravages even beyond the limits of those localities in which imperfect regulations prevail.

As respects the general maintenance of health, physiology supplies many important precepts; although nature in this respect has hardly left man to be governed by physiology. Hunger forces man to the highest activity for the preservation of his life; and under this appetite, aided by common sense, a body of popular dietetic rules has arisen, the habitual observation of which, more from imitation than from reflection, serves to preserve individuals in health. It is only by seeking a variety of food that man is sure to obtain all the chemical constituents required for the maintenance of his bodily frame. We have already shown that each of the simple elements, of which the human body is composed, is continually passing out by various excretory channels; and that, unless replaced, nutrition becomes deficient, and the function of that part which fails to receive its just supply is necessarily impaired. It doubtless often happens that the digestive powers are too feeble to extract the substances required from one kind of food, while they may be sufficient to obtain them from another. The desire of a variety of food, then, is plainly a species of instinct implanted in man for the purpose of securing the perfect nutrition of the animal frame.

Physiology is the handmaid of medicine; and in its largest sense, it even includes pathology. The relation between physiology proper and pathology has no parallel in other departments of knowledge. In physical science, as there is no death, so there is no disease. The mere derangement of machinery invented by man is very different from the state of disease in physiological nature. But not to waste time on a subject scarcely relevant to our present purpose, it is at least manifest that in the derangement of machines there is not, as in the case of disease, a power inherent in them to rectify and to restore themselves to their former state of efficiency. Such a power, however, is what characterizes pathology in particular. It is sometimes said that in meteorology, storms and tempests, as contrasted with calm weather, are the diseases of the atmosphere. But even in this department there is no close analogy between the two cases. A mere disturbance of the equilibrium of the atmosphere, on which every sudden change of the weather depends, bears but a very remote analogy to the pathological states to which living nature, and in particular the human nature, is subject. But when we come to chemical science-to that science which treats of the combinations of bodies, and of their actions and reactions upon each other, then we perceive at once how totally different physical nature (and under chemistry the whole of physical nature falls) is from organic nature, in respect to that class of phenomena which constitute the special department of physiology termed pathology. In chemical nature there is no individuality, unless, rejecting the idea of the infinite divisibility of matter, we pronounce each atom of a chemical substance to possess an individuality. And this view at present supplies us with the best and most correct notion of the grand distinctions existing between physical and physiological nature. The individuals, then, of which chemistry treats are mere atoms of simple bodies-every massive simple body is merely a group of individual atoms-each of these atoms is, in a very definite sense, an independent individual; it possesses all the properties which belong to the mass or aggregate in which it is seen to exist; by being separated from that mass it loses nothing; in the present system of things it is imperishable; it knows no decay, it knows no fatigue, it knows no exhaustion of its properties, it knows no dissolution or death. From its first creation to the time when the Eternal shall pronounce the fiat of its extinction, it