Species and Varieties, Their Origin by Mutation

Lecture I

DESCENT: THEORIES OF EVOLUTION AND METHODS OF INVESTIGATION

Newton convinced his contemporaries that natural laws rule the whole universe. Lyell showed, by his principle of slow and gradual evolution, that natural laws have reigned since the beginning of time. To Darwin we owe the almost universal acceptance of the theory of descent.

This doctrine is one of the most noted landmarks in the advance of science. It teaches the validity of natural laws of life in its broadest sense, and crowns the philosophy founded by Newton and Lyell.

Lamarck proposed the hypothesis of a common origin of all living beings and this ingenious and thoroughly philosophical conception was warmly welcomed by his partisans, but was not widely accepted owing to lack of supporting evidence. To Darwin was reserved the task of bringing the theory of common descent to its present high rank in scientific and social philosophy.

Two main features in his work have contributed to this early and unexpected victory. One of them is the almost unlimited amount of comparative evidence, the other is his demonstration of the possibility of a physiological explanation of the process of descent itself.

The universal belief in the independent creation of living organisms was revised by Linnaeus and was put upon a new foundation. Before him the genera were supposed to be created, the species and minor forms having arisen from them through the agency of external conditions. In his first book Linnaeus adhered to this belief, but later changed his mind and maintained the principle of the separate creation of species. The weight of his authority soon brought this conception to universal acceptance, and up to the present time the prevailing conception of a species has been chiefly based on the definition given by Linnaeus. His species comprised subspecies and varieties, which were in their turn, supposed to have evolved from species by the common method.

Darwin tried to show that the links which bind species to genera are of the same nature as those which determine the relationship of subspecies and varieties. If an origin by natural laws is conceded for the latter, it must on this ground be granted for the first also. In this discussion he simply returned to the pre-Linnean attitude. But his material was such as to allow him to go one step further, and this step was an important and decisive one. He showed that the relation between the various genera of a family does not exhibit any features of a nature other than that between the species of a genus. What has been conceded for the one must needs be accepted for the other. The same holds good for the large groups.

The conviction of the common origin of closely allied forms necessarily leads to the conception of a similar descent even in remote relationships.

The origin of subspecies and varieties as found in nature was not proved, but only generally recognized as evident. A broader knowledge has brought about the same state of opinion for greater groups of relationships. Systematic affinities find their one possible explanation by the aid of this principle; without it, all similarity is only apparent and accidental. Geographic and paleontologic facts, brought together by Darwin and others on a previously unequalled scale, point clearly in the same direction. The vast amount of evidence of all comparative sciences compels us to accept the idea. To deny it, is to give up all opportunity of conceiving Nature in her true form.

The general features of the theory of descent are now accepted as the basis of all biological science. Half a century of discussion and investigation has cleared up the minor points and brought out an abundance of facts; but they have not changed the principle. Descent with modification is now universally accepted as the chief law of nature in the organic world. In honor of him, who with unsurpassed genius, and by unlimited labor has made it the basis of modern thought, this law is called the "Darwinian theory of descent."

Darwin's second contribution to this attainment was his proof of the possibility of a physiological explanation of the process of descent itself. Of this possibility he fully convinced his contemporaries, but in indicating the particular means by which the change of species has been brought about, he has not succeeded in securing universal acceptation. Quite on the contrary, objections have been raised from the very outset, and with such force as to compel Darwin himself to change his views in his later writings. This however, was of no avail, and objections and criticisms have since steadily accumulated. Physiologic facts concerning the origin of species in nature were unknown in the time of Darwin. It was a happy idea to choose the experience of the breeders in the production of new varieties, as a basis on which to build an explanation of the processes of nature. In my opinion Darwin was quite right, and he has succeeded in giving the desired proof. But the basis was a frail one, and would not stand too close an examination. Of this Darwin was always well aware. He has been prudent to the utmost, leaving many points undecided, and among them especially the range of validity of his several arguments. Unfortunately this prudence has not been adopted by his followers. Without sufficient warrant they have laid stress on one phase of the problem, quite overlooking the others. Wallace has even gone so far in his zeal and ardent veneration for Darwin, as to describe as Darwinism some things, which in my opinion, had never been a part of Darwin's conceptions.

The experience of the breeders was quite inadequate to the use which Darwin made of it. It was neither scientific, nor critically accurate. Laws of variation were barely conjectured; the different types of variability were only imperfectly distinguished. The breeders' conception was fairly sufficient for practical purposes, but science needed a clear understanding of the factors in the general process of variation. Repeatedly Darwin tried to formulate these causes, but the evidence available did not meet his requirements.

Quetelet's law of variation had not yet been published. Mendel's claim of hereditary units for the explanation of certain laws of hybrids discovered by him, was not yet made. The clear distinction between spontaneous and sudden changes, as compared with the ever-present fluctuating variations, is only of late coming into recognition by agriculturists. Innumerable minor points which go to elucidate the breeders' experience, and with which we are now quite familiar, were unknown in Darwin's time. No wonder that he made mistakes, and laid stress on modes of descent, which have since been proved to be of minor importance or even of doubtful validity.

Notwithstanding all these apparently unsurmountable difficulties, Darwin discovered the great principle which rules the evolution of organisms. It is the principle of natural selection. It is the sifting out of all organisms of minor worth through the struggle for life. It is only a sieve, and not a force of nature, not a direct cause of improvement, as many of Darwin's adversaries, and unfortunately many of his followers also, have so often asserted.

It is only a sieve, which decides what is to live, and what is to die. But evolutionary lines are of great length, and the evolution of a flower, or of an insectivorous plant is a way with many sidepaths. It is the sieve that keeps evolution on the main line, killing all, or nearly all that try to go in other directions. By this means natural selection is the one directing cause of the broad lines of evolution.

Of course, with the single steps of evolution it has nothing to do. Only after the step has been taken, the sieve acts, eliminating the unfit. The problem, as to the manner in which the individual steps are brought about, is quite another side of the question.

On this point Darwin has recognized two possibilities. One means of change lies in the sudden and spontaneous production of new forms from the old stock. The other method is the gradual accumulation of those always present and ever fluctuating variations which are indicated by the common assertion that no two individuals of a given race are exactly alike. The first changes are what we now call "mutations," the second are designated as "individual variations," or as this term is often used in another sense, as "fluctuations." Darwin recognized both lines of evolution; Wallace disregarded the sudden changes and proposed fluctuations as the exclusive factor. Of late, however, this point of view has been abandoned by many investigators, especially in America.

The actual occurrence of mutations is recognized, and the battle rages about the question, as to whether they are be regarded as the principal means of evolution, or whether slow and gradual changes have not also played a large and important part.

The defenders of the theory of evolution by slow accumulation of slight fluctuations are divided into two camps. One group is called the Neo-Lamarckians; they assume a direct modifying agency of the environment, producing a corresponding and useful change in the organization. The other group call themselves Darwinians or selectionists, but to my mind with no other right beyond the arbitrary restriction of the Darwinian principles by Wallace. They assume fluctuating variations in all directions and leave the choice between them to the sieve of natural selection.

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