III.
We pass to a consideration of Darwin's masterworks, the "Origin of Species," the "Variation of Animals and Plants under Domestication," and the "Descent of Man." Before indicating the conclusions reached in the first of these works, we should point out to what extent Darwin had been preceded by dissenters from the belief once almost universally entertained by biologists that species were independently created, and, once created, were immutable. Lamarck was the first naturalist whose divergent views upon the subject excited much attention. In writings published at various dates from 1801 to 1815, he upheld the doctrine that all species, including man, are descended from other species. He pronounced it probable that all changes in the organic, as well as in the inorganic world, were the result of law, and not of miraculous interposition. He seems to have been led to his opinion that the change of species had been gradual by the difficulty experienced in distinguishing species from varieties by the almost perfect gradation of forms in certain groups, and by the analogy of domestic productions. With respect to the means of modification, he attributed something to the direct action of the physical conditions of life, something to the crossing of already existing forms, and much to use and disuse, or, in other words, to the effect of habit. Finally, he held that characters acquired by an existing individual might be transmitted to its offspring.
In 1813 Dr. W.C. Wells read before the Royal Society "An Account of a White Female, Part of whose Skin resembles that of a Negro." In this paper the author distinctly recognized the principle of natural selection, but applied it only to the races of man, and in man only to certain characters. After remarking that negroes and mulattoes enjoy an immunity from certain tropical diseases, he observed, first, that all animals tend to vary in some degree, and, secondly, that agriculturalists improve their domesticated animals by selection. He added that what is done in the latter case by art seems to be done with equal efficacy, though more slowly, by nature in the formation of varieties of mankind fitted for the countries which they inhabit. Again in 1831 Mr. Patrick Matthew published a work on "Naval Timber and Arboriculture," in which he put forth precisely the same view concerning the origin of species as that propounded by Mr. Wallace and by Darwin. Unfortunately for himself, the view was cursorily suggested in scattered passages of an appendix to a work on a different subject, so that it remained unnoticed until Mr. Matthew himself drew attention to it in 1860, after the publication of the "Origin of Species." We observe finally that Mr. Herbert Spencer, in an essay published in 1852, and republished six years later, contrasted the theories of the creation and the development of organic beings. He argued from the analogy of domestic productions, from the changes which the embryos of many species undergo, from the difficulty of distinguishing species and varieties, and from the principle of general gradation, that species have been modified; and he attributed the modification to the change of circumstances.
The two volumes comprising the "Origin of Species" constitute, as the author said, one long argument. It is, of course, impossible in the space at our command to recapitulate in detail even the leading facts and inferences which are brought forward to prove that species have been modified during a long course of descent. We must confine ourselves to a succinct statement of the author's general conclusions. What he undertakes to prove is that the modification of species during a long course of descent has been effected chiefly through the natural selection of numerous successive slight favorable variations, aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner,--that is, in relation to adaptive structures, whether past or present, by the direct action of external conditions, and by variations which seem to us, in our ignorance, to arise spontaneously. It should be observed that Darwin does not attribute the modification exclusively to natural selection. What he asserts is: "I am convinced that natural selection has been the main, but not the exclusive, means of modification." He submits that a false theory would hardly explain in so satisfactory a manner as does the theory of natural selection the several large classes of facts marshalled in the two volumes now under review. If it be objected that this is an unsafe method of arguing, Darwin rejoins that it is a method usual in judging of the common events of life, and has often been used by the greatest natural philosophers. The undulatory theory of light, for instance, has thus been arrived at; and the belief in the revolution of the earth on its own axis was, until lately, supported by scarcely any direct evidence. It is no valid objection to the Darwinian theory of the origin of species that science as yet throws no light on the far higher problem of the essence or origin of life. Neither has any one explained what is the essence of the attraction of gravity, though nobody now objects to following out the results consequent on this unknown element of attraction.
Why, it may be asked, did nearly all the most eminent naturalists and geologists until recently decline to believe in the mutability of species? Darwin replies that the belief that species were immutable productions was almost unavoidable as long as the history of the world was thought to be of short duration. Even now that we have acquired some idea of the lapse of time, men are too apt to assume without proof that the geological record is so perfect that it would have afforded plain evidence of the mutation of species if they had really undergone mutation. The chief cause, however, of the once-prevalent unwillingness to admit that one species has given birth to other and distinct species is the fact that men are slow to admit great changes of which they do not see the steps. The difficulty is the same which was experienced by many geologists when Lyell first insisted that long lines of inland cliffs had been formed and great valleys excavated, not by catastrophes, but by the slow-moving agencies which we see still at work. The human mind cannot grasp the full meaning of the term of even a million years; cannot add up and perceive the full effects of many slight variations accumulated during an almost infinite number of generations.
When the first edition of the "Origin of Species" was published in 1859, Darwin wrote that he by no means expected to convince experienced naturalists whose minds were stocked with a multitude of facts, all regarded during a long course of years from a point of view directly opposite to his. He looked forward with confidence, however, to the future, to young and rising naturalists, who would be able to view both sides of the question with impartiality. He predicted that, when the conclusions reached by him and by Mr. Wallace concerning the origin of species should be generally accepted, there would be a considerable revolution in natural history. Naturalists, for instance, would be forced to acknowledge that the only distinction between species and well-marked varieties is that the latter are known or believed to be connected at the present day by intermediate gradations, whereas species were formerly, though they are not now, thus connected. It might thus come to pass that forms generally acknowledged in 1859 to be merely varieties, would thereafter be thought worthy of specific names; in which case scientific and common language would come into accordance. In short, Darwin looked forward to the time when species would have to be treated in the same manner as genera are treated by those naturalists who admit that genera are merely artificial combinations made for convenience.
Darwin also foresaw that when his theory of the origin of species should be adopted, other and more general departments of natural history would rise greatly in interest. The terms used by naturalists--such terms as affinity, relationship, community of type, paternity, morphology, adaptive characters, rudimentary and abortive organs, etc.--would cease to be metaphorical, and would have a plain signification. "When," he wrote, "we no longer look at an organic being as a savage looks at a ship, as something wholly beyond his comprehension; when we regard every production of nature as one which has had a long history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, in the same way as any great mechanical invention is the summing up of the labor, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting--I speak from experience--does the study of natural history become." Once more: "When we can feel assured that all the individuals of the same species, and all the closely allied species of most genera, have within a not very remote period descended from one parent, and have migrated from some one birthplace; and when we better know the many means of migration, then, by the light which geology now throws, and will continue to throw, on former changes of climate and of the level of the land, we shall surely be enabled to trace in an admirable manner the former migrations of the inhabitants of the whole world."
When Darwin published the "Origin of Species," he was aware that theologians and philosophers seemed to be fully satisfied with the view that each species had been independently created, and was immutable. To his own mind, however, it accorded better with what was known of the laws impressed on matter by the Creator that the production and extinction of the past and present inhabitants of the world should have been due to secondary causes like those determining the birth and death of the individual. "When I view," he said, "all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Cambrian system was deposited, they seem to me to become ennobled." And again: "As all the living forms of life are the lineal descendants of those which lived long before the Cambrian epoch, we may feel certain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world. Hence we may look with some confidence to a secure future of great length. And as natural selection works slowly by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection."
For his own part, Darwin could see no good reason why the views propounded in the two volumes comprising the "Origin of Species" should shock the religious feelings of any one. Touching the likelihood of such a result, he reassured himself by recalling the fact that the greatest discovery ever made by man--namely, the law of the attraction of gravitation--was attacked by Leibnitz "as subversive of natural, and inferentially, of revealed, religion." Darwin was confident that, if any such impressions were made by his theory, they would prove but transient, and that ultimately men would come to see that it is just as noble a conception of the Deity to believe that He created a few original forms capable of self-development into other and needful forms as to believe that it required the fresh act of creation to supply the voids caused by the action of His laws.