Hugo De Vries (1848-1935), Dutch botanist and geneticist, is the author of the mutation theory of evolution. His work led to the rediscovery and establishment of Mendel's laws.

Hugo de Vries was born on Feb. 16, 1848, in Haarlem. His father, Gerrit De Vries, had been prime minister of the Netherlands and came from a family of Baptist ministers and businessmen.. His mother, Maria Everardina Reuvens, came from an ancestry of scholars and statesmen. Educated first at a private Baptist school in Haarlem, young De Vries attended gymnasium (or highschool) in the Hague, matriculating to the University of Leiden in 1866. Here, he read two works that greatly stimulated his interest in botany: Darwin's Origin of Species (1859) and Julius Sachs' Textbook of Botany (1868). The first book raised De Vries' curiosity about variation and its relationship to the process of evolution, particularly the diversification of species. The latter publication aroused De Vries' enthusiasm of quantitative, experimental work, as opposed to the old-style taxonomy that made up so much of the field of botany at the time.

Pursuing physiological studies at Leiden, De Vries earned his doctorate in plant physiology in 1870, but felt stifled by the university, where conditions for experimental work were crude and where there was open hostility to Darwinism. He therefore decided to continue his education in Germany, first at Heidelberg (1870) and then at Würzburg (1871) with Sachs who took a great interest in De Vries' career, helping him refine his experimental techniques and nominating him for several important posts over the next few years. Under Sachs' guidance De Vries began a series of detailed studies of osmosis, plasmolysis, and the effects of salt solutions on plant cells. He carried out these experiments at Würzburg, at Amsterdam while teaching in a gymnasium (1871-1877), and finally at the University of Amsterdam where he was appointed lecturer in plant physiology in 1877 and professor in 1881. He was at the same time director of the Botanic Gardens at the University of Amsterdam. De Vries remained at Amsterdam until compulsory retirement in 1918, when he moved to the small village of Lunteren.

De Vries made his first notable contributions to science in the 1880s in the field of plant physiology. While investigating the movement of fluids in plants, he confirmed Jacobus Hendricus Van't Hoff's theory of osmosis and Svante Arrhenius's theory of ionic diffusion. During the 1870s De Vries had carried out a series of studies for the Prussian Ministry of Agriculture involving the problems of plant breeding and hybridization. The results of this research were published in monographs on clover, the sugarbeet, and the potato. After his appointment as professor, he turned his attention more and more to questions concerned with the theory of evolution and the ways in which new species might evolve.

Evolutionary Theory in the Late 19th Century

To understand the significance of De Vries's research, it is important to place his investigation in the context of the scientific debates of the period. Charles Darwin's theory of evolution by natural selection held that species evolved or changed in form from generation to generation because some members of the species lived for a longer time than others and were able to produce more offspring than their less fit fellows. In the long run, this would result in a species becoming more like the favored variation and less like the unfavored variations. In his Origin of Species Darwin did not establish how variations occurred or how they were inherited. Subsequently, the area of heredity and variation became a recognized field of research for biologists interested in evolutionary theory.

Darwin had put forward the idea that variations between different individuals in a species were usually of a continuous nature. He believed that because of natural selection certain ranges of this continuous variation would be more favored in the struggle for survival and the species would become changed toward those ranges. However, by the late 1880s and the 1890s some biologists were becoming convinced that evolution depended on the effect of natural selection on discontinuous variations, not on continuous variations.

Biologists were at the same time involved in much debate and research about the nature of heredity. Darwin realized that one of the gaps in his theory of evolution was an adequate explanation of the mechanism of heredity. To fill this gap, he proposed his theory of pangenesis: Each character in a mature organism was determined by a minute particle, or pangene, passed on from the parental organisms via the sex cells. The pangenes passed from all parts of the parental body through the bloodstream to the sex cells and then determined the character of the appropriate parts of the offspring by similar diffusion as the offspring grew.

One aspect of Darwin's theory of pangenesis caused much debate among biologists. How, they asked, could the pangenes, which were discrete particles, give rise to continuous variations? For this to occur, there would probably have to be some blending of the pangenes from different parents into one pangene. Some biologists preferred to believe that if heredity did depend on the passing of discrete units from parents to offspring, these units would remain discrete in the offspring and give rise to discontinuous variations in the mature offspring.

Another area of research was of great importance in the overall picture of evolution. This was the question of the structure of the cell and its nucleus and the analysis of the behavior of cell and nucleus during division. During the last quarter of the 19th century cytologists established a fairly detailed picture of what happened to the nuclear material during cell division. The material was chemically identified, and biologists began to speculate on the connection between the nucleic acids of the chromosomes and the mechanism of inheritance. De Vries played an important role in pointing out the connection between the nuclear material and the particles which controlled the inheritance of characteristics from generation to generation.

De Vries's Pangenesis Theory

De Vries published his theory of pangenesis in Intracellular Pangenesis (1889; trans. 1910). He took the name "pangenesis" from Darwin and, like Darwin, he held that characters were passed from parent to offspring through the medium of small particles or elementary units. These units he called "pangenes." De Vries held that the pangenes were located in the nucleus of each cell and that every nucleus contained a complete set of the pangenes for that particular individual. The complete set of pangenes represented all the potential characters of the mature organism. He further maintained that at the time of cell division the whole set of pangenes also divided so that every daughter cell contained a complete set of pangenes. By placing his pangenes in the nucleus and suggesting that they were present in the chromosomes, he was able to tie his theory of pangenesis much more closely to cytological observations than Darwin was.

Although De Vries was not able to outline in any detail how the pangenes determined the character of an organism, he suggested that a pangene left the nucleus of the cell, entered the surrounding cytoplasm, and thus controlled the activity of the cell. In Intracellular Pangenesis, a critical review of the hereditary theories of Darwin, Herbert Spencer, August Weismann, and Carl von Nageli, he stated that each pangene represented "a special hereditary character ... The pangenes are not chemical molecules, but morphological structures each built of numerous molecules ... they assimilate and take nourishment and thereby grow, and then multiply by division; two new pangenes, like the original one, usually originate at each cleavage. Deviations from this rule form a starting point for the origin of variations and species."

De Vries's theory of pangenesis put forward a hereditary mechanism which did not allow for any possibility of environmental or Lamarckian influence on heredity. His theory was also capable of fitting in with the findings of the contemporary cytologists on the nature of cell division and the role of the nucleus. The most important area for further work seemed to him to be the whole question of the source and nature of biological variation.

De Vries's Work on Variation

Darwin's theory of evolution maintained that new species were formed by the action of natural selection on variations which always occurred among the members of a species. In the mid-1880s De Vries did a great deal of work on the inheritance of the different characteristics of marigolds. He was impressed by the constancy of the species over several generations and became convinced that the ordinary or continuous variations were not the source of the new forms needed for new species.

In 1886 near Hilversum, outside of Amsterdam, De Vries came across some evening primroses (Oenothera lamarckiana) growing in a field and noticed that they showed great variations in height, form of leaves, and pattern of branching. By 1889 he had examined over 53,000 of these primrose plants from eight generations. In that time he found eight completely new types, which he felt were different enough from the original plants to be called new species. These new types bred true, that is, they had offspring similar to themselves, when they were cross-pollinated. He felt that he had at last uncovered the secret of the origin of new species, which he put forward in The Mutation Theory (1901-1903; trans. 1909).

De Vries's Mutation Theory

In his theory of mutation De Vries combined his theory of pangenesis, which explained heredity, with his theory that new species could arise only from a very large and completely spontaneous variation, which he called a "mutation." This mutation was the result of a new pangene or several new pangenes. In The Mutation Theory he said that the adoption of this new theory "influences our attitude toward the theory of descent [or evolution] by suggesting to us that species have arisen from one another by a discontinuous, as opposed to a continuous, process. Each new unit, forming a fresh step in the process, sharply and completely separates the new form as an independent species from that from which it sprang. The new species appears all at once; it originates from the parent species without any visible preparation and without any obvious series of transitional forms."

De Vries noted several types of mutations that occurred in plants: progressive (introducing a wholly new character, and usually making the plant a new species); retrogressive (loss of a trait); and degressive (activation of a trait long-latent in the species). While he saw retrogressive and degressive mutations as following Mendel's laws (progressive mutations did not), he made little of the point. His focus lay less in the problem of heredity and more in that of the origin of species.

De Vries contrasted his mutation theory with the Darwinian theory of selection, emphasizing that he saw the origin of species through mutation whereas Darwin had seen it through the selection of ordinary or fluctuating variation. The mutation theory was widely accepted in the years immediately after it was published. Investigators felt the theory met many of the difficulties they saw in the Darwinian theory: lack of sufficient geological time for the slow and haphazard process of natural selection to produce new species; the problem of new traits being swamped or blended out by backcrossing with the parents; and the reliance of Darwinians on the heritablility of slight, individual (as opposed to large-scale) variations as the raw material on which selection could act. In 1904, 1906, and again in 1916, he made a lecture tour of the United States, where he expounded his theory and encouraged many scientists to seek in other organisms, including animals, large-scale mutations of the sort he had found in Oenothera. While no such mutations were forthcoming, De Vries' work did stimulate much interest in the experimental study of evolution, as scientists sought ways to produce mutations artificially and to detect their presence through experimental breeding. One result of De Vries' influence was that in 1908 Thomas Hunt Morgan, Columbia University, began to search for mutations in the fruit fly Drosophila melanogaster, an organism whose favorable breeding characteristics were to become a major focus for experimental genetics in the 20th century.

Rediscovery of Mendel's Work

During the 1890s De Vries carried out many experiments in breeding plants. He crossed plants with different characteristics (for example, hairy and smooth stems) and counted the numbers of plants in succeeding generations which had the different parental characteristics. By the end of the 1890s he had gathered much evidence to show that there were definite rations which kept recurring among the offspring (for instance, hairy and smooth stems would occur in the ratio 3 to 1). By late 1899 he had obtained similar results in more than 30 different species and varieties. De Vries reasoned that the obtaining of fixed ratios supported his theories of pangenesis and mutation. The pangenes, which determined the characters of the plants, were seen as units which must separate and recombine according to regular patterns during breeding; these regular patterns would give rise to the fixed ratios he had discovered. Mutations would arise from the loss or great change of some of the pangenes.

Sometime in 1900, before De Vries published his new findings about the fixed ratios of characters among the offspring in cross-breeding experiments, he discovered a paper by Gregor Mendel which included an account of the same laws about the regular patterns of inheritance. Mendel's paper had been published in 1866 and had been ignored by the scientific world. The laws which Mendel had originally discovered and which De Vries had independently rediscovered became the basis of the modern study of genetics. Simultaneously, two other European biologists, Karl Correns and Eric Tschermak, rediscovered Mendel's work. De Vries incorporated a discussion of Mendel's results in his own work on the poppy, published in 1900. This publication appears to have triggered both Carl Correns and Erich von Tschermak-Seysengg to read Mendel's work and recognize its importance.

There has been some controversy about De Vries's role in the rediscovery of Mendel's work, including the suggestion that he did not want to acknowledge Mendel's priority in the discovery of the basic laws of genetics. However, it would seem that De Vries never felt that the Mendelian laws were as significant as his own mutation theory, so that his apparent lack of recognition for Mendel could stem from a feeling that biologists were placing too much emphasis on Mendel's laws and not paying enough attention to De Vries's mutation theory.

Among his many honors, De Vries was the recipient of eleven honorary degrees and became a corresponding member of many foreign academies of science. His world-wide esteem was reflected in invitations to give the major lectures at the opening of the Station for Experimental Study of Evolution at Cold Spring Harbor, Long Island (1904), and at the dedication of Rice Institute in Houston, Texas (1960).

From 1900 until he retired in 1918 De Vries spent most of his energy trying to find further evidence for his mutation theory. It drew less support as geneticists found more evidence to support Darwin's original theory that the source of evolutionary change was the normal variations that occurred among all numbers of a species. Between 1907 and 1915 various cytogeneticists showed that heredity in Oenothera involved a number of unusual chromosomal phenomena (polyploidy, or increased numbers of chromosomes; two groups of chromosomes attached end-to-end, each transmitted as a whole from parent to offspring) that only gave the illusion of new species. In reality the mutants of Oenothera were explicable not by De Vries' pet mutation theory but by the very Mendelian theory De Vries had helped to recover. By the time of De Vries's death in Amsterdam on May 21, 1935, the action of natural selection on ordinary variations had again become the accepted version of evolutionary theory and the term "mutation" was used to apply to any new character of a plant or animal--not only very large and striking variations.

Further Reading

• There is no standard biography of De Vries in English. However a biographical sketch of De Vries written by Peter van der Pas for the Dictionary of Scientific Biography includes a lengthy bibliography. For the reception of De Vries' work, see Garland E. Allen, "Hugo de Vries and the reception of the 'mutation theory'," Journal of the History of Biology (1969). For the relationship between De Vries and evolutionary problems, see: Lindley Darden, "Reasoning in scientific change: Charles Darwin, Hugo de Vries, and the discovery of segregation," in Studies in History and Philosophy of Science (1976) and Peter van der Pas, "Correspondence of Hugo de Vries and Charles Darwin," Janus (1970). De Vries' role in modern genetics is discussed in J. Heimans, "Hugo de Vries and the gene concept," Human Implications of Scientific Advance, E.G. Forbes, editor (Edinburgh, 1978); in Malcom Kottler, "Hugo de Vries and the rediscovery of Mendel's laws," Annals of Science (1979); and in Peter van der Pas "Hugo de Vries and gregor Mendel," Folia Mendeliana (1976). For a general account of his work the best books are L. C. Dunn, A Short History of Genetics (1965), and A. H. Sturtevant, A History of Genetics (1965). For De Vries's part in the rediscovery of Mendel see Robert C. Olby, Origins of Mendelism (1966).