Having thus briefly traced the changes that precede maturity, we may ask what is it that prevents the processes of growth from advancing at the same rate as they have hitherto done ? Why, so long as they are undisturbed by dis ease or unnatural circumstances, should they not advance ad infinitum, or at least why should they not raise man to the strength and dimensions which poets have fabled in their Titans ? The same food, the same atmosphere, the same light and heat, the same electric agencies, by which the organs have been main tained or excited, are still around them and exerting their influence. Why, then, should they never transcend a certain point? Why should the stature, however much it may vary between a Boruwlaski and an O'Brien, yet never rise above a certain measure ? Why does the strength never exceed the powers of a Milo or a Desaguliers, or the intellect surpass the limits of Aristotle, Shakspeare, or Newton? These are interesting but impossible problems. If we say that a certain quantum of vital power is allotted to the growth of man, and that while a portion is expended in raising him to matu rity, the residue must be husbanded for con ducting him through the remaining portion of his duration, else he might suddenly stop short in his career without passing those stages that prepare him for the cessation of his existence ; —what do we gain by such an explanation ? Nothing ; for the term vital power which we employ is but a hypothetical cause, or if more closely examined, is scarcely even this ; it is but an abstract term applicable to a number of actions that do not occur in the inorganic world. The vital power of a body is but the collective manifestation of its vital actions, and to say therefore that only a certain quantum of vital power is inherent in it, is but to express in other words the simple fact that those actions are circumscribed. Discarding this explana tion, shall we sav that the fact must be referred to some deficiency in the media of the being's existence; that, although the aliment, the air, the light and caloric are competent to the pro duction of a certain degree of growth, they cannot extend it, and that, were their conditions different, the animal development would be more perfect. It is easy perhaps to suppose this, but we do not see how it can be proved, nor indeed that existing analogies favour it. On the surface of our globe there is every variety in the temperature, in the humidity, and in the electric conditions of the atmosphere, and every diversity in the articles of food em ployed ; in more limited spheres there are the greatest diversities in these several respects produced artificially by the various occupations of mankind ; and although we find, both among races and individuals, great varieties of deve lopment, which may occasionally be traced to some relation with the media in which they live, these varieties are by no means in propor tion to the differences of the media, and in the majority of cases the former are independent of the latter. In the temperate zone, with a due proportion of animal and vegetable diet, man appears to attain his most perfect deve lopment, and with however great skill he adapts these circumstances, he never surpasses a certain point, and from what we know of his physiology no great alteration in any one of the external stimuli of his existence could be tole rated. A different proportion of the oxygen, nitrogen, and carbon in the atmosphere, we know full well to be noxious ; a larger or smaller quantity of aqueous vapour suspended in it will occasion many well-known maladies ; the same may be said of alterations in the ba lance of the electricity that surrounds us. Great extremes of heat and cold may be borne for awhile, but it is obvious that they are not so well adapted to a healthy state of the system, and therefore to its growth, as intermediate de grees ; and consequently it is not easy to con ceive any degree either above or below these limits consistent even with existence. Fami liar enough also are we with the effects of full and sparing, of simple and mixed dietetics, and with the fact that between certain well known bounds lie thd salutary quantities and qualities. From all which it appears suffici ently evident, that we cannot conceive any difference in the amount or properties of the known stimuli of life, that would be more favourable to the growth of man, than those which are to be found in the range of the known variations, whether natural or artificial. From the beginning there must have been established a direct relation between the organization of the body and the outward elements; the latter are nothing but stimulants adapted to co-exist ing susceptibilities, or to put it more closely, man is not made by, but for or with, the sur rounding agents ; his lungs are fashioned in cor respondence to the atmosphere which he breathes, his digestive organs to the food that is spread so plenteously before him, and his nervous system to the subtle imponderable agents that play about him ; consequently as his organs only act in concert with, and do not result from the media of his existence, a development be yond that which he is known to acquire must proceed quite as much from the former as from the latter ; and the supposition, the value of which we have been endeavouring to estimate, thus falls to the ground. If man could become
a larger, more powerful, or more sagacious animal than he now is, he must not only live in different media, but must possess a different constitution ; in other words, the. characters that distinguish him as a species must be altered. The question, then, that offered itself remains to our apprehension unsolved by either of the hypotheses. The limitation of man's development is like the definite period of his duration, and a hundred other circumstances connected with his existence, an ultimate fact ; no event that we are able to discover intervenes' between its production and the will of the Deity.
Maturity, though varying with every indi vidual, may be said to take place somewhere between the ages of twenty-five and thirty. It is a general opinion that it is a stationary con dition ; that when such changes have taken place in the frame, as render the human being capable of undertaking the various duties and occupations to which adults alone are adequate, there are no further alterations till the period of declining age ; that, in short, growth has entirely ceased. But this idea is not strictly correct, for there is in all probability no period when the system is absolutely stationary ; it must either be advancing to or receding from the state of perfection. This is of course more obvious when we know that augmentation of bulk is only a part of that process which per fects the organization. (See NUTRITION.) It is true that at the adult age the determinate height and figure, the settled features, the marked mental and moral character, naturally give rise to the idea that a fixed point has been attained ; but a little inquiry soon teaches us that the individual is still the subject of some progressive changes. It is the stature only that is stationary, for this depends on the skeleton, which ceases to lengthen before the period we speak of. But the capability of powerful and prolonged muscular exertions increases for some years ; there must consequently be a change in the muscular tissue. The intellectual faculties have not attained their although we do not hesitate to consider them mature; we must therefore infer that there is a correspond ing organic development of some kind in the cerebral substance. Maturity then would, ac cording to this view, require to be dated at a period much later than that which is usually assigned to it. It is enough, however, without referring further, to know that although at the adult period the organs of animal life are so developed, that we cannot consider them im perfect instruments, they are even afterwards the subjects of a perfectionnement. What is commonly meant then by maturity, is in strict ness that period of human existence, during which the processes of growth and decline are passing into each other by such slow degrees as to be imperceptible.
In this important era of the life of man, more important even than the season of adole scence, we must leave him in the full posses sion of all the faculties and energies which his Maker has allotted him, fulfilling his destiny of good and evil, encountering and triumphing over peril, toil, and pain, scaling the rough steep of ambition, threading the dark • intricate paths of gain, labouring for the happiness or misery of his fellow-creatures, supported all the while by the consciousness of a strength that seems never to fail him, of resources never to be exhausted ; we must allow a few years to roll by, and then return to him, when weary, wayworn, and broken with the storms of life, lie has discovered that there are limits to his powers of action and endurance ; that of the objects which he proposed as the ends of his labours, while a few have been accomplished, the majority are either vain or unattainable ; and that a race fresh in vigour, and high in hope, the images of his former self, are over taking and thrusting him away from the scenes of his exertions. What are the revolutions that have transpired in his system ? The formative organs of all the tissues of the body are in reality the tissues themselves ; whether it be a muscle, or a gland, or the coat of a vessel, the parts which essentially produce its growth are nothing more or less than its own constituent molecules, the mutual attrac tions of which in deposition and absorption constitute assimilation ; for there is no proof that vessels are used for any other purpose than that of conveying the nutrient fluids to and from the places, where the ultimate mole cules arrange themselves in the form of tissue. The altered qualities, then, which are presented by the tissues, in whatever organs, in the de cline of life, must depend immediately upon alterations in their own molecular motions and affinities. The nature of these alterations will of course correspond to the nature of each tissue ; and unless we mistake, they will all be found to agree in one character, viz. a sim pler composition, a lower kind of organization than they formerly possessed. But the discussion of this point will be more conveniently deferred till we shall have briefly recited the principal changes in the more important parts of the body.