Species and Varieties, Their Origin by Mutation.

by Hugo DeVries.

PREFACE BY THE AUTHOR

THE purpose of these lectures is to point out the means and methods by which the origin of species and varieties may become an object for experimental inquiry, in the interest of agricultural and horticultural practice as well as in that of general biologic science. Comparative studies have contributed all the evidence hitherto adduced for the support of the Darwinian theory of descent and given us some general ideas about the main lines of the pedigree of the vegetable kingdom, but the way in which one species originates from another has not been adequately explained. The current belief a.s.sumes that species are slowly changed into new types. In contradiction to this conception the theory of mutation a.s.sumes that new species and varieties are produced from existing forms by sudden leaps. The parent-type itself remains unchanged throughout this process, and may repeatedly give birth to new forms.

These may arise simultaneously and in groups or separately at more or less widely distant periods.

The princ.i.p.al features of the theory of mutation have been dealt with at length in my book "Die Mutationstheorie" (Vol. I., 1901, Vol. II., 1903.

Leipsic, Veit & Co.), in which I have endeavored to present as completely as possible the detailed evidence obtained from trustworthy historical records, and from my own experimental researches, upon which the theory is based.

The University of California invited me to deliver a series of lectures on this subject, at Berkeley, during the [vii] summer of 1904, and these lectures are offered in this form to a public now thoroughly interested in the progress of modern ideas on evolution. Some of my experiments and pedigree-cultures are described here in a manner similar to that used in the "Mutationstheorie," but partly abridged and partly elaborated, in order to give a clear conception of their extent and scope. New experiments and observations have been added, and a wider choice of the material afforded by the more recent current literature has been made in the interest of a clear representation of the leading ideas, leaving the exact and detailed proofs thereof to the students of the larger book.

Scientific demonstration is often long and enc.u.mbered with difficult points of minor importance. In these lectures I have tried to devote attention to the more important phases of the subject and have avoided the details of lesser interest to the general reader.

Considerable care has been bestowed upon the indication of the lacunae in our knowledge of the subject and the methods by which they may be filled. Many interesting observations bearing upon the little known parts of the subject may be made with limited facilities, either in the garden or upon the wild flora. Accuracy and perseverance, and a warm love for Nature's children are here the chief requirements in such investigations.

In his admirable treatise on Evolution and Adaptation (New York, Macmillan & Co., 1903), Thomas Hunt Morgan has dealt in a critical manner with many of the speculations upon problems subsidiary to the theory of descent, in so convincing and complete a manner, that I think myself justified in neglecting these questions here. His book gives an accurate survey of them all, and is easily understood by the general reader.

In concluding I have to offer my thanks to Dr. D.T. MacDougal and Miss A.M. Vail of the New York Botanical Garden for their painstaking work in the preparation of the ma.n.u.script for the press. Dr. MacDougal, by [viii] his publications, has introduced my results to his American colleagues, and moreover by his cultures of the mutative species of the great evening-primrose has contributed additional proof of the validity of my views, which will go far to obviate the difficulties, which are still in the way of a more universal acceptation of the theory of mutation. My work claims to be in full accord with the principles laid down by Darwin, and to give a thorough and sharp a.n.a.lysis of some of the ideas of variability, inheritance, selection, and mutation, which were necessarily vague at his time. It is only just to state, that Darwin established so broad a basis for scientific research upon these subjects, that after half a century many problems of major interest remain to be taken up. The work now demanding our attention is manifestly that of the experimental observation and control of the origin of species. The princ.i.p.al object of these lectures is to secure a more general appreciation of this kind of work.

HUGO DE VRIES.

Amsterdam, October, 1904.

PREFACE BY THE EDITOR

PROFESSOR DE VRIES has rendered an additional service to all naturalists by the preparation of the lectures on mutation published in the present volume. A perusal of the lectures will show that the subject matter of "Die Mutationstheorie" has been presented in a somewhat condensed form, and that the time which has elapsed since the original was prepared has given opportunity for the acquisition of additional facts, and a re-examination of some of the more important conclusions with the result that a notable gain has been made in the treatment of some complicated problems.

It is hoped that the appearance of this English version of the theory of mutation will do much to stimulate investigation of the various phases of the subject. This volume, however, is by no means intended to replace, as a work of reference, the larger book with its detailed recital of facts and its comprehensive records, but it may prove a subst.i.tute for the use of the general reader.

The revision of the lectures has been a task attended with no little pleasure, especially since it has given the editor the opportunity for an advance consideration of some of the more recent results, thus materially facilitating investigations which have been in progress at the New York Botanical Garden for some time. So far as the ground has been covered the researches in question corroborate the conclusions of de Vries in all important particulars. The preparation of the ma.n.u.script for the printer has consisted chiefly in the adaptation of oral [xii]

discussions and demonstrations to a form suitable for permanent record, together with certain other alterations which have been duly submitted to the author. The original phraseology has been preserved as far as possible. The editor wishes to acknowledge material a.s.sistance in this work from Miss A.M. Vail, Librarian of the New York Botanical Garden.

D.T. MacDougal.

New York Botanical Garden, October, 1904.

PREFACE TO THE SECOND EDITION.

THE constantly increasing interest in all phases of evolution has made necessary the preparation of a second edition of this book within a few months after the first appeared. The opportunity has been used to eliminate typographical errors, and to make alterations in the form of a few sentences for the sake of clearness and smoothness. The subject matter remains practically unchanged. An explanatory note has been added on page 575 in order to avoid confusion as to the ident.i.ty of some of the plants which figure prominently in the experimental investigations in Amsterdam and New York.

The portrait which forms the frontispiece is a reproduction of a photograph taken by Professor F.E. Lloyd and Dr. W.A. Cannon during the visit of Professor de Vries at the Desert Botanical Laboratory of the Carnegie Inst.i.tution, at Tucson, Arizona, in June, 1904.

D. T. MACDOUGAL.

December 15, 1905.

A. INTRODUCTION

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 [2] 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 [3]

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 att.i.tude. 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 [4]

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 unsurpa.s.sed 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 acc.u.mulated. Physiologic facts concerning the origin of [5] 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 [6] 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 a.s.serted.

It is [7] 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.