In considering the value of Charles Darwin’s work and its permanent effect upon the thought of mankind, we must be careful to distinguish between two phases of his effort. It was his aim to prove two propositions: first, that there is such a process as evolution; second, that he had discovered the method by which evolution is accomplished. Before his time there was no general agreement as to the fact of evolution. People generally thought the idea absurd, as well as irreligious. All previous efforts on the part of advanced thinkers to persuade mankind of the truth of evolution had been nearly without effect. Among the early philosophers the whole idea was purely speculative. They made no attempt to prove it, and the conception was without influence upon the thinking of the ordinary man. This remains true until the time of Lamarck. This French genius succeeded in persuading not a few people of the validity of the idea of evolution. He probably could have convinced many more had it not been for the hostility of Cuvier. Accordingly, Charles Darwin’s “Origin of Species” fell upon a world entirely hostile to the idea, when it thought of it at all. Within fifty years of the publication of this wonderful book, probably the entire scientific world is agreed that evolution, in some form or other, is the undoubted solution of the mystery of creation. The materialist may think of it as a mechanical process relentlessly working itself out without design or purpose. The theist will accept it as the plan by which Eternal Power steadily works. The devout Christian or Jew will see in it God’s method of creation. The idea of development has penetrated every science that has to do with animals or man. It is even beginning to influence such inorganic sciences as Physics and Chemistry. We now hear of the evolution of the elements, and the evolution of forces. The world has been persuaded that evolution is true, and this is primarily the result of the work of Charles Darwin. It is astonishing that so great a revolution should have come in so short a time.

The other phase of Darwin’s work was his attempt to find the agent which is bringing about the actual transformation of animals and plants. As we have seen in the preceding chapters, it was his idea that natural selection was the efficient agent which constantly eliminated all unfit variations, leaving only the best to carry on the work of the world and to reproduce their own fit kind. Many biologists since his time have doubted whether unaided Natural Selection will account for the constant advance in organisms. This is the part of the work which is often seriously questioned.

Weissman and his co-workers have contended that this unaided principle will serve. Most biologists have asked for some more efficient cause, and assert that selection does not account for the appearance of variations, but only for their preservation, and that any valid theory of evolution must show how variations originate. It is chiefly in this respect that Darwin’s work has failed to satisfy many later biologists. When we hear a scientist speak of Darwinism as being dead, this is what he means. He does not think evolution false, but believes that Natural Selection is not sufficient to account for evolution. There are three main difficulties involved in Darwin’s theory. The chief defect lies in the fact that selection cannot originate varieties. In all his earlier works Darwin simply accepted variations as he found them. He was content to say that all species varied constantly, and in every direction. He gave no theory to account for variation. Whenever he took measurements of the dimensions of any large series of objects of the same kind he found these measurements to vary, apparently, in all directions. Upon the facts of these variations, and without accounting for them, he built his own theory of evolution. He realized his weakness, and acknowledged it in his book. He probably did not anticipate how insistently later biologists would demand an explanation that would account for this variation. In his later work, responding to this criticism, Darwin originated a theory which he called Pangenesis. He believed that when an adult animal had responded to his environment and acquired a new character he could transmit this character to his offspring. At that time no one doubted this fact. The whole theory of Lamarck was based on the assumption that this could be done. Darwin suggested that every organ of the body threw off minute particles, which he called pangenes. These little bodies, carried by the blood, were taken up by the egg cells or sperm cells, and the latter cells determined the future development. Consequently, the character of the new individual was determined by the parental pangenes. In this way the gain acquired by one generation could be passed on to the next. This theory was purely speculative. He never pretended that there was the faintest corroborating evidence visible to the microscope in the organ, in the blood, or in the germ cell. It was not an accounting for what is, but for what it seemed possible to him might be.

This theory of Pangenesis, in the shape in which Darwin promulgated it, has dropped out of consideration almost entirely. DeVries of recent years has revised it, but with distinct modifications, and most biologists pay no attention to it.

There is a school of biologists, headed by Weissman, who have come to be known as Neo-Darwinians. These men have insisted that Natural Selection, if properly understood and developed, is quite sufficient to account for the fact of evolution, including the appearance of variations. Weissman himself is a microscopist of more than common skill. He is thoroughly accomplished in the most modern methods of killing, fixing, staining, and mounting. This worker’s acquaintance with the intimate structure of the cell is probably as great as that of any other man in the world. Weissman asserts that he has seen inside the nucleus all the machinery necessary to explain how the father hands over his qualities to his children. He insists, equally strongly, that this process is such that no father can hand to his child any qualities which he himself did not have at least in potentiality at his birth. Everything the individual acquires during his lifetime is his own possession, which he may use and develop to the utmost extent, but it dies with him. His children, born after he possesses it, can no more inherit it than those born before. Weissman expressed this in his famous statement that “There is no inheritance of acquired characters.” The biological world has had no shock equal to this since Darwin’s time, and there are few other questions to which scientists to-day return with such constant vigor.

If what Weissman says is true, that no variation or development which comes to an animal during his lifetime can be transferred into his own germ cells and handed on to his children, then it becomes evident that we must find some cause of variation that acts within the germ cells. This is the difficulty which Weissman meets. He says that there are small particles in the nucleus of each cell; that these particles which he calls determinants decide the form and the course of development of that cell; that when that cell divides to produce another cell it gives to this other cell one-half of each determinant. As a result the second cell grows to be like the first. This tells us why offspring are like their parents. There is nothing in the theory thus far to show us why offspring are not exactly like their parents. In other words, there is no accounting, thus far in the theory, for variation. When the biologist studies carefully the history of an egg while it is being formed, he sees that at one stage in its development it throws away not one-half of each determinant, but one-half of the determinants. When an egg does this, it deliberately casts aside one-half of the possibilities of its own development. This throwing away is quite as effective for all its descendants. Any ancestral quality now lost is lost from the line forever. In the formation of the sperm cell set free by the male a similar throwing away of one-half the characters has taken place. The egg cell and the sperm cell fuse together. There are as many possibilities now as there were in either parent, but not all the potentialities of both parents. Half the possibilities of each have been thrown away, and hence cannot appear in the offspring. By this constant process we get, in every generation, new combinations of qualities. This is the main cause, says Weissman, for variations.

There is, however, another possible cause. Each cell has enough determinants in it for many individuals, and it seems to be more or less a matter of accident which qualities shall come out. It has been suggested that as an egg lies within the gland, a blood vessel may bring blood to it in such way that a determinant, lying in a certain position in the egg, may get the richest supply of blood, and hence develop at the expense of the less nourished determinant. By these two methods variation comes into an animal’s life, if Weissman and his school are to be believed.

This is a serious blow, if true, to many theories of evolution. The great mass of evolutionists still feel that somehow there is an influence by which the environment produces variation. How the influences of the surrounding world can get down into the body of the parent and affect the egg is unknown. This is freely confessed by every biologist. All are agreed that Weissman’s work has made us cautious, and prevented our lightly accepting a belief in the influence of the environment. Yet it is felt by many that slowly and gradually, in the long run, the germ is affected in the same manner as is the body of the parent. In other words, even those who are not followers of Weissman, have accepted the idea that there is little inheritance of acquired characters. Yet they return to the belief that somehow, in some way as yet unexplainable, the main cause for variation in animals lies in the situation in which they live, and tends toward better adaptation to that situation.

Whether men with this conviction are merely reactionaries whose confidence is returning, or bold thinkers whose views will ultimately prevail, time alone can tell.

A second strong objection was brought against the theory of Natural Selection. Darwin declared that small variations in favorable directions are selected and become the starting point of new and better things. It is soon seen, however, that the effect of unaided Natural Selection would be but to mix new departures with the old forms, and soon swamp out any progressive tendency. Whenever a genius appeared, instead of finding a corresponding genius with which to pair, it mated with the average of its own species. Hence its offspring were nearer the average than it was, and their offspring still nearer. Thus whatever advantage the genius originally possessed gradually sank into the common level.

It was Moritz Wagner, a German naturalist, who first insisted that if favorable variations were to amount to anything these possessors must not only mate with others of their same kind, but must also be prevented from mating with the old average group. Accordingly, the belief arose that, under ordinary circumstances, variations returned to the common level. Wherever a varying group became separated by any barrier from mating with the rest of its species, and had only its own kind to pair with, a new species sprang up. This barrier might be a desert, or an impassable mountain range, an arm of the sea, or anything else that the animal could not, or would not, cross. Isolated in this way, the little group that had an advantage in a different direction could develop its tendencies, and a new species would be made of what had been previously only a geographical race. In this matter of geographical isolation Wagner is very strongly supported by the American zooelogist, David Starr Jordan, who believes that no two closely related species of animals ever occupied the same geographical area. Both Wagner and Jordan are ardent admirers of Darwin and his theory of natural selection, but both believe that it is necessary to add the idea of isolation in order to make natural selection effective.

George John Romanes, a British naturalist, has added to Wagner’s idea of isolation, the expanded conception that there may be isolations that are not geographical. For this phase, Romanes has coined the term physiological isolation. Something in the structure or habit of the animals with the new variation prevents them from mating with the older type. Occasionally it is a difference in the structure of the reproductive organs themselves. This, however, is not the only possible divergence. The mating season in one group may come earlier than that of the other, or may come during the day, while the main group is in the habit of mating at night. Anything which keeps some members of a species separate in their mating from the rest, will result in the course of a longer or shorter time, says Romanes, in the formation of a new species.

A third great objection was raised against Darwinism. The theory said that only useful variations were selected by nature. It was asserted by objectors that the earliest beginnings of any variation must be too slight to be useful, or as the term went, to have selective value.

It has been noticed by a number of naturalists that certain animals seem to carry the development of a peculiarity altogether too far. It is seen for instance that in the Irish Elk, which has for some time been extinct, the horns were so enormous as to be a source of danger rather than of assistance to their owner. It was said that the tendency to produce heavy horns had gained, as it were, a sort of momentum, and that this impulse had carried the development beyond a safe limit. The Irish Elk became extinct because his horns were too heavy. During the Mesozoic period the reptiles grew too large. They seemed to have carried size to a point at which it became a danger instead of a help. They completely passed out of existence, leaving behind them only very much smaller reptiles.

Eimer, of Germany, has based on facts like these his theory of Orthogenesis. He says that variations in animals are not indefinite and in every direction, but that they follow along clear and definite lines. These lines, in the case of the elk and of the Mesozoic reptiles, developed too far, but ordinarily the effect of such a tendency is distinctly beneficial to the animal. It particularly assists in carrying on for a time the variations which have not yet become useful to the animal. It has always been difficult on Darwinian principles to understand how the beginnings of the useful variations could be selected before they were strong enough to be of actual value to the animal. This tendency to variations in certain directions instead of at random would account for such early development. This theory of Orthogenesis has not figured very strongly in the history of the movement, but it recurs at intervals.

Both in America and France there is a constant tendency on the part of zooelogists to return to the Lamarckian idea that it is the use of an organ that develops it, its disuse that makes it fade away. This is undoubtedly true of the individual, and although Weissman insists that it is useless to the species as a whole, many zooelogists are slow to relinquish entirely the idea that somehow these favorable developments become reproduced in the offspring.

Professor Cope, the American paleontologist, was a strong believer in the effect of activity, both upon the individual and upon his descendants. He believed that the insistent beating of the foot of an animal upon the hard soil of the drying Tertiary plateau, had influenced the production of a firmer nail, which spread around the entire end of the toe and made the hoof of the ungulate. He believed that the use of the teeth in grinding produced a stronger and better molar tooth, and that the offspring shared in this advantage. Since Weissmann’s time, however, every Lamarckian feels it necessary to suggest some method by which the altered body of the parent can produce modifications in the germ plasms from which the young are to spring. One of our later biologists begins to talk of some effect comparable with wireless telegraphy or induced electricity. He believes that organs in the adult, not necessarily by direct action, but by action from a distance, may alter the germ. Of this, there is no proof at present. Others have suggested that just as the ductless glands pour into the blood chemical substances which materially affect the growth and development of other portions of the body, so similar enzymes, or other chemical substances, may be sent into the blood, which subsequently bathes the germ cells of the coming generation and produces the change. But of this, again, there is no proof. We may believe that acquired characters are transmitted, but we certainly do not have a very clear idea as to how it can be done.

One of the strongest objections to Darwin’s idea of evolution by natural selection of small and favorable variations, is that the process is too inconceivably slow to account for the enormous progress which has been made. The answer has always been that our observation ran back so short a time that we really have no clear idea of how rapid evolution may have been. Again, it has been answered that transitional geological periods, in which there is much change in the physical geography of a country, will produce more rapid evolution than we at present are experiencing.

Hugo DeVries, of Amsterdam, believes he has found the answer to this difficulty. Outside of his botanical garden an American species of Evening Primrose had run wild. In looking over a number of these plants he found, every here and there, certain peculiar members of the species. They differed noticeably to the practiced eye from the rest of the group. When they were planted and crossed with each other, and the resulting seeds were again planted, the peculiarity remained constant in all the members of the collection. Here then we have a true variation, not large in amount, but at the same time quite definite, and which from the first remains true. Here are the beginnings, says DeVries, of new species. They are true from the first; they can live among other members of the species and still come true; they do not need isolation, at least in Wagner’s geographical sense. These forms DeVries calls mutations. It is his thought that a species may run along uniformly for a long time when, from some cause which he has not determined as yet, instability comes into the species and it varies in quite a number of directions. Each of these variations may be the starting point of a new species. DeVries believes that he has at least half a dozen mutants of his new Evening Primrose.

This theory of Mutation has been eagerly seized upon by many botanists. The zooelogists have not accepted it quite so enthusiastically. If this is the chief method by which species transform, it seems strange that we do not find more mutations than we do. Perhaps we do not look carefully enough; perhaps we shall find them a little later. Just at present it seems premature to believe that all evolution is by mutation, although quite possibly some of it is. The main apparent advantage of mutation is that it hastens the time in which a new species may arise.

There are certain difficulties which run back into the problem, and which must first be reasonably solved before a clear understanding of the idea of evolution is possible. The first of these is as to the nature of life. What is life? The reply of the biologist will probably be that so far as its material side is concerned, it must be answered in terms of physics and chemistry. As to any side not material, if it have any such side, science says that the chemist can have nothing to say. The chemist may have an opinion of his own based on some other ground than his chemistry, but so far as he is a chemist, he has no opinion. The chemical side of life is being very carefully and very fully investigated. We are certainly being brought nearer to the borders of the living substance. We are rapidly gaining fuller knowledge of the physical and chemical processes which constitute life, or with which life is always associated. If we gain this knowledge we shall be in better position to solve many of our other problems. Even then there is a problem which preceded and which will possibly always defy solution. How did life originate? Has it developed out of chemical and physical activities which we know as heat, light or electricity? If so, what were the conditions under which it developed? If we understand the nature of life, and the conditions under which it developed, we may be able to produce it at will.

A few scientists may hope dimly that this will be attained. I suspect a great majority believe it to be impossible, and that the question as to whether life evolved upon this planet, or this planet became infected with life through meteoric dust from some other center, will forever remain an unsolved problem.