It is through natural agents also that the means of regulation are secured in the present economy of the globe. Through heat, the distribution and arrangement of the vegetable tribes are accomplished; through their mutual relations with the atmospheric air, plants and animals are interbalanced, and neither permitted to obtain a superiority. The condensation of carbon from the air and its inclusion in the strata constitute the chief epoch in the organic life of the earth giving a possibility for the appearance of the hot-blooded and more intellectual animal tribes. That event was due to the influence of the rays of the sun.
Passing from inorganic to organic forms, our author remarks that their permanence is altogether dependent 'on the invariability of the material conditions under which they live. Any variation therein, no matter how insignificant it might be, would be forthwith followed by a corresponding variation in the form.' At this point we are brought to the far-famed 'development theory,' which, since the publication of the 'Vestiges of Creation,' has been the scientific battle field of the naturalists of the world. Professor Draper is, of course, a firm adherent of this theory. He continues:
'The present invariability of the world of organization is the direct consequence of the physical equilibrium, and so it will continue as long as the mean temperature, the annual supply of light, the composition of the air, the distribution of water, oceanic and atmospheric currents, and other such agencies, remain unaltered; but if any one of these, or of a hundred other incidents that might be mentioned, should suffer modification, in an instant the fanciful doctrine of the immutability of species would be brought to its true value. The organic world appears to be in repose, because natural influences have reached an equilibrium. A marble may remain forever motionless upon a level table; but let the surface be a little inclined, and the marble will quickly run off. What should we say of him who, contemplating it in its state of rest, asserted that it was impossible for it ever to move?
'When, therefore, we notice such orderly successions, we must not at once assign them to a direct intervention, the issue of wise predeterminations of a voluntary agent; we must first satisfy ourselves how far they are dependent upon mundane or material conditions, occurring in a definite and necessary series, ever bearing in mind the important principle that an orderly sequence of inorganic events necessarily involves an orderly and corresponding progression of organic life.
'To this doctrine of the control of physical agencies over organic forms I acknowledge no exceptions, not even in the case of man. The varied aspects he presents in different countries are the necessary consequences of those influences.'
Whether we advocate the doctrine of the origination of the human race from a single pair, or from different races at different centres, we are, in Dr. Draper's judgment, alike driven to the conclusion of the transitory nature of typical forms, to their transmutations and extinctions. In the former case, we can only account for diverse races, having different shades of complexion, different varieties of skull, etc., by the admission of the paramount control of physical agents, such as climate and other purely material circumstances; in the latter, we can only account for the varieties visible among the different races themselves on similar grounds.
Variations in the aspect of man are best seen when an examination is made of nations arranged in a northerly and southerly direction, the differences of climate being much greater in this direction than from east to west. These variations do not affect complexion, development of the brain, and, therefore, intellectual power, only. But differences of manners and customs, that is, differences in the modes of civilization, must coexist with diversities of climate. An ethnical element is therefore necessarily of a dependent nature; its durability arises from its perfect correspondence with the conditions by which it is surrounded. Whatever can affect that correspondence will touch its life.
With such considerations the author passes from individuals to groups of men or nations:
'There is a progress for races of men as well marked as the progress of one man. There are thoughts and actions appertaining to specific periods in the one case as in the other. Without difficulty we affirm of a given act that it appertains to a given period. We recognize the noisy sports of boyhood, the business application of maturity, the feeble garrulity of old age. We express our surprise when we witness actions unsuitable to the epoch of life. As it is in this respect in the individual, so it is in the nation. The march of individual existence shadows forth the march of race existence, being, indeed, its representative on a little scale. Groups of men, or nations, are distributed by the same accidents, or complete the same cycle as the individual. Some scarcely pass beyond infancy; some are destroyed on a sudden; some die of mere old age. In this confusion of events, it might seem altogether hopeless to disentangle the law which is guiding them all, and demonstrate it clearly. Of such groups each may exhibit, at the same moment, an advance to a different stage, just as we see in the same family the young, the middle aged, and the old. * * * In each nation, moreover, the contemporaneously different classes, the educated and illiterate, the idle and industrious, the rich and poor, the intelligent and superstitious, represent different contemporaneous stages of advancement. One may have made a great progress, another scarcely have advanced at all. How shall we ascertain the real state of the case? Which of these classes shall we regard as the truest and most perfect type?'
In order to deal with this problem, and to demonstrate the general nature of a movement having such diverse components, we must, continues Professor Draper, select, from a family or a nation, or a family of many nations, such members or classes or states as most closely represent respectively its type or have advanced most completely in their career. In a state the leading or intellectual class is always the true representative. It has passed gradually through the lower stages, and has made the greatest advance.
We are next called to notice that individual life is maintained only by the production and destruction of organic particles, death being necessarily the condition of life; and that a similar process occurs in the existence of a nation, in which the individual represents the organic molecule, whose production, continuance, and death in the person, answers to the production, continuance, and death of a person in the state. In the same manner that individuals change through the action of physical agencies and submit to impressions, so likewise do aggregates of men constituting nations. 'A national type pursues its way physically and intellectually through changes and developments answering to those of the individual, and being represented by infancy, childhood, youth, manhood, old age, and death, respectively.'
This orderly process may, however, be disturbed by emigration, by blood admixture, or by other exterior or interior occurrences, which would involve a corresponding change in the national characteristics and duration; perhaps result in the rapid and total disappearance of the community.