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As Natural Selection which works so slowly is a main element in Mr. Darwin's theory, it is necessary to understand distinctly what he means by it. On this point he leaves us no room for doubt. On p. 92, he says: "This preservation of favorable variations, and the destruction of injurious variations, I call Natural Selection, or, the Survival of the Fittest." "Owing to the struggle (for life) variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by their offspring. The offspring also will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to man's power of selection. But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest, is more accurate, and sometimes is equally convenient." (p. 72). "Slow though the progress of selection may be, if feeble man can do so much by artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the co-adaptations between all organic beings, one with another, and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection, or the survival of the fittest." (p. 125). "It may be objected that if organic beings thus tend to rise in the scale, how is it that throughout the world a multitude of the lowest forms still exist; and how is it that in each great class some forms are far more highly developed than others? … On our theory the continuous existence of lowly forms offers no difficulty; for natural selection, or the survival of the fittest, does not necessarily include progressive development, it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life. … Geology tells us that some of the lowest forms, the infusoria and rhizopods, have remained for an enormous period in nearly their present state." (p. 145). "The fact of little or no modification having been effected since the glacial period would be of some avail against those who believe in an innate and necessary law of development, but is powerless against the doctrine of natural selection, or the survival of the fittest, which implies only that variations or individual differences of a favorable nature occasionally arise in a few species and are then preserved." (p. 149)

This process of improvement under the law of natural selection includes not only changes in the organic structure of animals, but also in their instincts and intelligence. On entering on this part of his subject, Mr. Darwin says, "I would premise that I have nothing to do with the origin of the primary mental powers, any more than I have with that of life itself. We are concerned only with the diversities of instinct and of other mental qualities within the same class." (p. 255) He shows that even in a state of nature the instincts of animals of the same species do in some degree vary, and that they are transmitted by inheritance. A mastiff has imparted courage to a greyhound, and a greyhound has transmitted to a shepherd-dog a disposition to hunt hares. Among sporting dogs, the young of the pointer or retriever have been known to point or to retrieve without instruction. "If," he says, "it can be shown that instincts do vary ever so little, then I can see no difficulty in natural selection preserving and continually accumulating variations of instinct to any extent that was profitable. It is thus, as I believe, that all the most complex and wonderful instincts have arisen." (p. 257) He was rather unguarded in saying that he saw no difficulty in accounting for the most wonderful instincts of animals. He admits that he has found very great difficulty. He selects three cases which he found it specially hard to deal with: that of the cuckoo, that of the cell-building bee, and of the slave-making ant. He devotes much space and labor in endeavoring to show how the instinct of the bee, for example, in the construction of its cell, might have been gradually acquired. It is clear, however, that he was not able fully to satisfy even his own mind; for he admits that "it will be thought that I have an over-weening confidence in the principle of natural selection, when I do not admit that such wonderful and well established facts do not annihilate the theory." (p. 290) This remark was made with special reference to the instincts of the ant, which he finds very hard to account for. He adds, "No doubt many instincts of very difficult explanation could be opposed to the theory of natural selection: cases in which we cannot see how an instinct could possibly have originated; cases in which no intermediate gradations are known to exist; cases of instinct of such trifling importance that they could hardly have been acted upon by natural selection; cases of instincts almost identically the same in animals so remote in the scale of nature, that we cannot account for their similarity by inheritance from a common progenitor, and consequently cannot believe that they were independently acquired through natural selection. I will not here enter on those cases, but will confine myself to one special difficulty which at first appeared to me insuperable, and actually fatal to the whole theory. I allude to neuters, or sterile females in insect communities; for these neuters often differ widely in instinct and structure from both the males and the fertile females, and yet, from being sterile, they cannot propagate their kind." (p. 289) He is candid enough to say, in conclusion, "I do not pretend that the facts given in this chapter (on instinct) strengthen in any great degree my theory; but none of the cases of difficulty, to the best of my judgment, annihilate it." (p. 297) When it is remembered that his theory is, that slight variations occurring in an individual advantageous to it (not to its associates), in the struggle for life, is perpetuated by inheritance, it is no wonder that the case of sterile ants gave him so much trouble. Accidental sterility is not favorable to the individual, and its being made permanent by inheritance, is out of the question, for the sterile have no descendants. Yet these sterile females are not degenerations, they are in general larger and more robust than their associates.

We have thus seen that, according to Mr. Darwin, all the infinite variety of structure in plants and animals is due to the law of natural selection. "On the principle of natural selection with divergence of character," he says, "it does not seem incredible that, from some such low and intermediate form, both animals and plants have been developed, and if we admit this, we must likewise admit that all the organized beings which have ever lived on this earth may be descended from some one primordial form." (p. 573) We have seen also that he does not confine his theory to organic structure, but applies it to all the instincts and all the forms of intelligence manifested by irrational creatures. Nor does he stop there; he includes man within the sweep of the same law. "In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history." (p. 577)

The "distant future" was near at hand. In his introduction to his work on the "Descent of Man," he says, he had determined not to publish on that subject, "as I thought that I should thus only add to the prejudices against my views. It seemed to me sufficient to indicate, in the first edition of my 'Origin of Species,' that by this work 'light would be thrown on the origin of man and his history;' and this implies that man must be included with other organic beings in any general conclusion respecting his manner of appearance on this earth. Now the case wears a wholly different aspect. When a naturalist like Carl Vogt (we shall see in what follows what kind of a witness he is) ventures to say in his address as President of the National Institution of Geneva (1869), 'Personne, en Europe au moins, n'ose plus soutenir la création indépendante et de toutes piéces, des espéces,'—it is manifest that at least a large number of naturalists must admit that species are the modified descendants of other species; and this especially holds good of the younger and rising naturalists. … Of the older and honored chiefs in natural science, many unfortunately are still opposed to evolution in every form." Carl Vogt would not write thus. To him no man is honored who does agree with him, and any man who believes in God he execrates.

In 1871, Mr. Darwin ventured on the publication of his "Descent of Man." In that work, he endeavors to show that the proximate progenitor of man is the ape. He says "there is less difference of structure between the two, than between the higher and lower forms of apes themselves." Not only so, but he attempts to show that the mental faculties of man are derived by slight variations, long continued, from the measure of intellect possessed by lower animals. He even says, that there is less difference in intelligence between man and the higher mammals, than there is between the intelligence of the ant and that of the coccus, insects of the same class.[7]

In like manner he teaches that man's moral nature has been evolved by slow degrees from the social instincts common to many animals. (pp. 68, 94) The moral element, thus derived, he admits might lead to very different lines of conduct. "If men," he says, "were reared under the same conditions as hives-bees, there can hardly be a doubt, that our unmarried females would, like the worker-bees, think it a sacred duty to kill all their brothers, and mothers would strive to kill their fertile daughters; and no one would think of interfering. (vol. i. p. 70)

"Lower animals, especially the dog, manifest love, reverence, fidelity, and obedience; and it is from these elements that the religious sentiment in man has been slowly evolved by a process of natural selection." (vol. i. p. 65)

The grand conclusion is, "man (body, soul, and spirit) is descended from a hairy quadruped, furnished with a tail and pointed ears, probably arboreal in its habits, and an inhabitant of the Old World." (vol. ii. p. 372) Mr. Darwin adds: "He who denounces these views (as irreligious) is bound to explain why it is more irreligious to explain the origin of man as a distinct species by descent from some lower form, through the laws of variation and natural selection, than to explain the birth of the individual through the laws of ordinary reproduction." (vol. ii. p. 378)