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ALL students of the growth and other physical activities of plants during the past twenty years have found that any consideration of the manner by which a plant retains its rigidity, plumpness or firmness, or executes movements, inevitably brings one to follow the experimental studies 01 Hugo de Vries, who published his first paper upon the mechanicaL causes of turgidity as early as 1877. A plant cell may be roughly likened to a sac of living matter which holds water in its interior under a pressure which may amount to as much as 25 or 30 atmospheres, and this is possible only by reason of the impermeability of the outer wall, which is capable of undergoing a wide range of variation as to its physical properties. Outside of the living membrane of a cell is another of cellulose, and this is fully permeable. The differential behavior of the living and non-living membranes when the cell is placed in solutions of osmotically active substances, such as sugar and potassium nitrate, form the basis of the phenomena of plasmolysis and afford a clue to the conditions of turgescence within the cell. The systematic use of solutions in producing plasmolysis, and a rational interpretation of the facts, were frst given by De Vries in 1877. These results have their direct and immediate importance chiefly in plant physiology, but their wider application in the establishment of the isotonic co-efficients of various substances, together with the experiments of Pfeffer, another botanist, in osmosis, form the basis of the electrolytic dissociation theory of Arrhenius, as well as the law of van't Hoff that “dilute solutions obey the same law as gases,” both conceptions of the greatest importance in the physics and chemistry of to·day. With achievements of such magnitude to his credit within eight years after receiving his doctor's degree from the University of Leyden, and at the age of 29 (born in 1848, at Haarlem, Holland), the attainment of a foremost place among the botanists of the world, and the earned recognition as the greatest evolutionist after Darwin, seem a logical and natural development of a master mind in biological science. His developing powers of research can be shown to be coincident with a movement in all natural sciences which occurred late in the last century by which attention was directed more and more to the transformations of energy in general and to the activities, functions, qualities and capacities of organisms in particular. Habits and performance were recognized as biologically more important than form or even structure. It was in consonance with this trend of science that De Vries, who had long been concerned with variability in organisms, should formulate a prothesis as to the mechanism of heredity, which should put forward a physiological rather than a morphological explanation of heredity, which was done with the presentation of his Pangenesis in 1889. This splendid contribution was not solely an abstract product of the study, but represented the working hypothesis of a brilliant experimentalist intent on visualizing the mechanism of heredity and providing a working hypothesis by which a rational interpretation of the continuation of qualities from generation to generation through germ-cells could be made. Co· incidently with its appearance, De· Vries began to publish the results of his investigations upon variability, and mutations in plants, upon which he finally erected his Mutation Theory of Descent, in final and formal shape, about the beginning of this new century. The extensive experimental cultures organized by deVries in the Botanical Garden of Amsterdam, yielded results of importance with regard to the inheritance of unusual characters in plants, such as torsions or twistings of stems, fascinations, bandings or cristations, as well as statistics upon the curves of variation of single characters. It was in these studies in the behavior of pairs in balanced characters in crossing, that De Vries re-discovered independently the Mendelian principles of alternative inheritance, which had remained unnoticed for a half century. The simplest illustration of Mendelianism is to be seen when a red and a white variety of the same species are crossed, the color qualities being considered as “balanced” or paired. The seeds from such a cross give rise to a progeny of plants all of which bear red flowers, that color being dominant over white. The seeds produced by this first generation of red-flowered individuals, however, generally produce a progeny, three· fourths of which are red-flowered while the remainder bear white flowers. Similar behavior is exhibited by many other qualities, although the division of the progeny does not always follow the simple formula noted. In addition, he brought to light many important things concerning xenia, atavism, derivation of economic races of plants, and ef. ects of selection, of prime interest to a wide range of scientists, and to horticulturists, agriculturists and plant breeders. As may be seen, however, these results are to be consid- PROF. HUGO DE VRIES ered as no more than by-products of his cultures, since much more important things were the center of attention. Early in the eighties, a series of observations were begun in which . the successive generations of about a hundred species of plants were followed in order to ascertain exactly what resemblances might be found between parent and progeny in guarded and pure lines of descent. The behavior of one plant, a large evening primrose, Oenothera Lamarckiana, which had been introduced into Europe in the seventeenth century, offered phenomena of unusual interest, since it was seen to give rise to several sports, or salts, or mutants which differed distinctly from the ancestral form, and these new forms bore distinct qualities which were maintained in direct descent. It was upon such facts that De Vries founded his theory of the origin of new species by the sudden origination or disappearance of qualities in organisms, The biological public received these generalizations with indifference. In addition to the inertia of old ideas, some curious nationalistic prejudices found expression. As many English naturalists and scholars in other branches fought the idea of natural selection, when presented by Darwin, with vicious unfairness, so now a similar element was responsible for much criticism, resentful of anything which might modify the attitude of the world toward Darwinism. an att- tude still recognizable in current publications. The writer of this note had the pleasure of presenting a summary of the mutation idea of evolution, with an account of the experimentation upon which it was based, to a meeting of botanists in the New York Botanical Garden in 1902, at which time some cultures of the Oenothems from seeds furnished by Prof. de Vries were begun, which, it is believed, constitute the first demonstrations of the main thesis outside of the Botanical Garden at Amsterdam. These experiments in New York confirmed the facts obtained in Amsterdam in every impo. particular, and” their pUblication in 1903, found a rapidly growing interest in the subject in America. Prof. de Vries was consequently invited to give a series of lectures in various institutions from the Atlantic to the Pacif'c, receiving a full measure of recognition in academic honors, which have been followed by similar expressions of appreciation from learned bodies all over the world. A full, non-technical exposition of the mutation theory was given at the University of California in the summer of 1904, and to the writer fell the pleasure of putting these lectures into book form under the title of “Species and Varieties: Their Origin by Mutation.” This book came to its second edition within a few months, being styled the “most notable scientific book of the year” by a competent reviewer, a statement apparently justified by the fact that it has since been translated into Dutch ' , French, and German. A large number of the investigations which were begun as a result of his first visit to America, profited by his second visit in 1905, and he has had the exceptional opportunity of seeing his observations repeated under the widest variety of climatic and other environmental conditions. These confirm the significance of the facts upon which the mutation theory was founded. In addition to the sports or mutants of the evening primroses which formed such an important part of the original evidence upon the subject, sinilar phenomena have been observed in many other seed-plants and among the lower forms including the bacteria. The origination of new races or species in animals is not so easily observed, but still authenticated instances are accumulating. That new qualities suddenly appear in lines of descent is no longer a question, but it is still to be determined how large a part such action plays in the general scheme of evolutionary development. Any discussion of Prof. de Vries's work raises the question at once as to the relation of the new ideas he has formulated to the older conceptions of Darwin. One theological author has gone so far wrong as to write “The death-bed of Darwinism” by a misapp;n of the meaning of mutation, Darwin would have evolutionary progress by the selection and survival of infinitely minute divergences through thousands or hundreds of thousands of generations, arriving finally at types widely different from the original. De Vries holds that the organism fluctuates steadily about its average or norm, from which it does not depart beyond a certain limit, but the line of descent may, at any time, include individuals possessing new qualities not shown in any degree by the parental strain, Selection decides between these forms and those previously existing, the fittest of the types surviving. Instead of supplanting the theory of natural selection, the mutation conception has the actual force of coming to its support at a time when many of its generalizations were being recognized as notably inadequate to the full interpretation of known facts. The mutation theory defines more accurately, the manner in which selection may act, in addition to offering an explanation of the manner in which new capacities or qualities may arise. Although now in his sixty·third year, and nearing the age of academic retirement, Prof. de Vries is deeply engaged in experimentation, and the cultures in the Botanical Garden at Amsterdam may yet yield results of a theoretical and Dractical importance in evolutionary science, scarcely less than that of pangenesis :l mutation.