The observations of Alfred Russell Wallace in the Malay Archipelago led towards the same conclusions as those indicated by Darwin, to whom Mr. Wallace dedicated his book,[BD] giving the results of research extending from 1854 to 1862, and in confirmation of Darwin's theory, though differing on important points, Wallace published in 1870 his Contribution to the Theory of Natural Selection. Mr. Darwin's views when first promulgated received decided though somewhat guarded support from Sir Charles Lyell, whose geological studies marked him out as a highly qualified witness on the subject, and were strongly favored by Dr. Hooker, author of Introduction to the Australian Flora, while they encountered not a little criticism, and were met with announcements of formal reservations, on the part of distinguished naturalists.
A fuller statement as to the history of his own thought was given by Mr. Darwin in the fifth edition of his work, and also in a letter to Hæckel, author of the bolder venture as to the History of the Creation, and the Evolution of Man. This letter to Hæckel may be given here as having considerable value in its bearing on the formation of the theory of evolution. After referring to his early researches as to lower forms of life, Mr. Darwin proceeds thus—"Having reflected much on the foregoing facts, it seemed to me probable that allied species were descended from a common ancestor. But during several years I could not conceive how each form could have been modified so as to become admirably adapted to its place in nature. I began therefore to study domesticated animals and cultivated plants, and after a time perceived that man's power of selecting and breeding from certain individuals was the most powerful of all means in the production of new races. Having attended to the habits of animals and their relations to the surrounding conditions, I was able to realize the severe struggle for existence to which all organisms are subjected; and my geological observations had allowed me to appreciate to a certain extent the duration of past geological periods. With my mind thus prepared, I fortunately happened to read Malthus's Essay on Population; and the idea of natural selection through the struggle for existence at once occurred to me. Of all the subordinate points in the theory, the last which I understood was the cause of the tendency in the descendants from a common progenitor to diverge in character."[BE] This short passage in personal history may considerably aid others in their study of the theory.
This letter may with advantage be supplemented by one or two brief extracts from The Origin of Species. In one of his most recent editions, our author says,—"It may be metaphorically said, that natural selection is daily; and hourly scrutinizing throughout the world the slightest variations, rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life." Such a metaphorical representation attributing to nature different processes, such as scrutinizing, rejecting, preserving, and working, considerably aids us by pointing to the intellectual conditions involved in the acceptance of the theory of evolution. To this it may suffice if there be added the closing passage in our author's work on species, indicating his view of the relation of all to the supernatural. It is in these words—"Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving,[BF] namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved."[BG] On this passage, Schmidt has remarked—"In this concession, Darwin has certainly been untrue to himself; and it satisfies neither those who believe in the continuous work of creation by a personal God, nor the partisans of natural evolution." In this criticism we have a good example of the manner in which an impression of conflict is fostered. The rejoinder to Schmidt is obvious. Those who value religion seek no concessions, but desire to banish from scientific and philosophic writing all thought tending in this direction. If, however, needless complications are to be avoided, we must have exact statements of the relative positions. The sentence just quoted involves a misrepresentation equally of religion and science. Belief in the supernatural does not imply belief in a continuous work of creation, and therefore does not carry any thing inherently antagonistic to the conception of evolution under natural law. On the other hand, belief in evolution of species under the action of natural law does not decide the question, concerning the origin of life, just as a science of nature can decide nothing concerning the supernatural.
Professor Schmidt is, however, so far from clearly and consistently recognizing the exact limit of the theory, that within the compass of a single page he first affirms the limitation, and then denies it. First vindicating Darwin from the attack of Max Müller as to beginning and end of living organism, he says, "The origin of life has in fact nothing to do with actual Darwinism, or natural selection, unless the principle of selection be extended to the inorganic world of matter,"[BH] an entanglement which Schmidt is willing to avoid, and which Darwin carefully shuns, distinctly stating that "Science in her present state does not countenance the belief that living creatures are now ever produced from inorganic matter."[BI] But Schmidt has no sooner placed himself behind this line of defence, than he attacks Darwin for acknowledging origin of life by creation, maintaining that "it is directly incompatible with the doctrine of descent."[BJ] The theorist must take either one side or the other. Either he must maintain that the theory of descent has "nothing to do with the origin of life," and in that case there is no conflict with religious thought; or that the theory of descent is incompatible with creation, and in that case there is conflict with religious thought, and at the same time inconsistency with science, in respect of its own definition as an explanation of nature, and in respect of its own results which do not involve "spontaneous generation," but do include the position that not a single particle of matter can be originated or destroyed by any power recognized in operation. No one will "dispute the claims of the investigation of nature to its logical inferences," but "where the material substratum is deficient," most reasonable men will demand that distinct acknowledgment be made of the fact, and that the boundaries of science be defined accordingly. If, however, any one be inclined to maintain that "where the material substratum is deficient," all inquiry must terminate, and human thought must refuse to go further, or to attempt to rise higher, this certainly is not science, but an illogical attempt to make the science of nature commensurate with the boundaries of thought,—an arbitrary declaration that "the causal series" within the material universe is the sum total of causality. Such an affirmation can not warrantably attach itself to a theory of descent, as it can not be tested by observational methods, but altogether by reference to the laws of thought determining the value of rationalizing processes. The theory of "natural evolution," implies evolution from something; it postulates a beginning from which it takes procedure, and it exhausts itself in observations concerning such deviations as occur in the annals of natural history. If, therefore, any of "the partisans of natural evolution," take up a position involving denial of a rational acknowledgment of the supernatural, they isolate themselves in so doing, leaving the theory free from responsibility as to their attitude, and taking upon themselves the logical necessity for vindicating their position on grounds with which the theory of evolution itself has no concern.
Having thus vindicated the theory of natural evolution from all share in the denial of creation, and having entered a protest upon purely scientific grounds against the attempt to translate a scientific theory concerning a limited order of facts within the universe into a metaphysical theory concerning the origin of the universe, we are in a position to concentrate upon the theory itself as an attempt to provide a scientific explanation of the history of living organism. And this is here done with the view of ascertaining, in a general and necessarily restricted manner, the value of the evidence presented for its acceptance, thereby ascertaining its relations to the essential characteristics of religious thought.
The first and simplest part of the task is to indicate the favorable impression it has made on the minds of men,—the conquest it has already won for itself,—as a theory carrying within it a large amount of truth, whatever may be its final form, after the very intricate and difficult questions involved have been carefully examined. The theory has carried general approval for the position that "allied species are descended from a common ancestor," or, stating the same view in the manner suggested by experiment, that it is possible to obtain in the history of a single race of animals, considerable deviations in structure, and to give these deviations fixedness or permanence by continuance of selected features along the line of hereditary descent. This has been amply illustrated by the examples of the various orders of pigeons, and of dogs; both classes of animals having been largely experimented upon, and the distinct varieties being easily recognized and popularly known. Comparatively little hesitation exists as to acknowledging that the different orders of pigeons have had a common ancestry; and that the same may be said of the different races of dogs. These examples afford the governing conception of the evolution theory, presenting the type of evidence which has led on to the wider generalization. Under the discussions which the theory has originated facts previously familiar have been contemplated in a different light, as bearing upon a general plan or order of procedure apparent in the history of organized existence. The consequent gain for the theory is altogether favorable to the restricted doctrine that allied species have had a common descent, or that a single type of organism may under the law of evolution lead to the appearance of different orders or races of animals.
The clear advance thus made in our conceptions of the history of the universe will appear by simple statement of negative results following from the admission of the modified form of doctrine just given. These involve the rejection of views previously held, not by religious men in religious interests, but by men generally, as the natural consequence of the want of scientific investigations fitted to guide the public mind. So far as a general conclusion has gained assent, men show no reluctance to accept the clear logical inferences following from the investigation of nature.
Among these results is rejection of fixedness of species as implying impossibility of deviation from a single normal type of structure. The possibility of adaptive changes being granted, the absolute fixedness of species in the rigid sense formerly acknowledged is abandoned. How great the modification of view must be, is much more difficult to decide, and hardly admits of exact statement. There is certainly no denial of distinction of species, nor can such denial ever find acceptance, whatever be the advance of theory, for the distinctions are so broad as to render this impossible. But the whole work of classification of the different orders of animal life, exceedingly difficult in any case, has been rendered much more perplexing in consequence of the accumulation of evidence favoring the doctrine of evolution. What can properly be regarded as the origin of a new species, and what as a mere modification or advance in a species already recognized, are questions for which it is difficult to find an exact answer. The theory of the "origin of species" by natural selection seems placed in an awkward perplexity as to what constitutes origin of a new order of life. And this difficulty must be regarded as a constant attendant on the scheme of thought, since "adaptive changes" must be of slow progress, and historically obscure, inasmuch as a succession of very slight differences must contribute to a general result. In this way it may even become matter of keen discussion what actually constitutes organic advance. Mr. Darwin admits serious difficulty at this point. He says, "Here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organization."[BK] Thus there is dispute among competent authorities as to which may properly be considered the highest order of plants, and which the highest order of fishes. On the other hand, it is comparatively easy to decide among the more highly organized animals, when an advance is made, by reference to increased complexity in structure, or provision of separate organs for accomplishment of distinct functions. These considerations, however, suffice to indicate how many and complicated are the subjects requiring to be examined on evidence, and adjusted in their relations to each other, before it can be possible to get beyond surmise, in order to formulate a complete scientific theory. That "adaptive changes" by natural law of evolution are not only possible, but that they frequently occur under observation, admits of no question; but whether this includes changes of structure such as imply origin of species may still be subject of grave doubt. The alterations made by Mr. Darwin in successive editions of his book, from the first edition in 1859, to the sixth edition in 1872,[BL] introduced, as he explains, "according as the evidence has become somewhat stronger or weaker," are sufficient to suggest that a vast amount of work remains to be done before a well-defined theory can be formulated. While there is universal agreement as to the possibility of "adaptive changes" to which Mr. Darwin provisionally restricted his theory on account of the investigations of Nageli as to plants, and those of Broca as to animals, there is much diversity of opinion concerning the wider application of the theory of evolution. This diversity arises in part from the varying estimate of the value of evidence as now accumulated, and in part from the varying conception of the completeness of our records of the ancient history of organism as presented by geology. There is as yet no general consensus of opinion, nor is there likely to be for a long time to come. Mr. Darwin himself is sufficiently cautious and faithful to observational methods, to admit that there are serious difficulties, of some of which he ventures only to say that they "are greatly diminished," while some have disappeared. Other writers, such as Hæckel, with greatly less caution, and with much greater alacrity in leaping over chasms, are prepared to go much further and faster than Darwin. Many more are exceedingly doubtful as to the scientific value of the evidence at command, being, as Mr. Darwin has said, "much shaken in their former belief."[BM] And of many it must be said that they are convinced that the evidence is far from warranting the conclusion that all organized existence can be traced to "only a few forms," or to "one," according to the alternatives suggested by Darwin in the closing sentence of his book.
Waiving, then, meanwhile, as the state of scientific evidence warrants us to do, the question of the probable number of primordial forms in which organized existence appeared, there is at least another definite result to be recorded as following from even a modified recognition of a theory of development, that is the rejection of belief in the simultaneous origin of all species or orders of animal life existing now in the world. The scientific conception of the history of animal life is, that there has been a historical progression in the appearance of animals, in so far as lower orders took precedence of higher, while the higher have shown large power of adaptation to the circumstances in which they have been placed. In accordance with the whole principles regulating the relations of religion and science, religious men, scientific and non-scientific, will readily acquiesce in this modification of general belief, as largely favored by evidence which geology supplies, and supported by testimony drawn from the actually existing order of things; and they will do so with clear recognition that this view involves no conflict with scriptural statement, and is so far from containing in it any thing antagonistic to the fundamental conception of the supernatural origin of existence, that it harmonizes with it, even intensifying the demand upon a transcendent cause for the rational explanation of the admitted order of things.
Having thus indicated in definite form the favorable impression made on the public mind by the theory of evolution under a law of natural selection, it will suffice to indicate very briefly the more prominent difficulties with which the theory has grappled, but from which it has not escaped. In doing so, it should be said that the careful and deliberate manner in which Mr. Darwin has faced the host of difficulties which have gathered around is deserving of highest praise, as in harmony with the scientific spirit, and in marked contrast with the light-and-go-easy style in which others, such as Hæckel, and even Schmidt, pass over the ground, announcing things as undoubted facts, and even "self-evident" truths, of which no man can speak with any degree of certainty. On the other hand, it seems a reasonable ground of complaint against many opponents of the theory, which Mr. Darwin urges specially against Mr. St. George Mivart, that it is no part of their plan "to give the various facts and considerations opposed to their own conclusions," while marshalling the difficulties against an evolution theory. And yet it should be remembered that a great service is done to science in a period of transition, when difficulties are powerfully urged against a popular hypothesis, as an injury is done to science by precipitate and ill-considered arguments in support of such a hypothesis.