Popular Science Monthly/Volume 74/April 1909/Darwin and Botany
|DARWIN AND BOTANY|
By Dr. NATHANIEL LORD BRITTON
NEW YORK BOTANICAL GARDEN
CONSIDERING the fact that Charles Darwin disclaimed the title of botanist, his contributions to the knowledge of plant life and its phenomena were certainly extraordinary. His investigations extended over a great range of topics, at one time or another practically covering the whole field of botanical research. In repeatedly stating that he was not a botanist, he evidently meant to imply that he was not a systematist, and it is true that his knowledge of plant taxonomy was the least of his scientific acquirements. In his first letter to Dr. Asa Gray, written in 1855, which was the commencement of a long correspondence, he almost apologized for asking questions! During that year he became keenly interested, however, in knowing more about the kinds of plants growing wild in the vicinity of his home, and in a letter to Dr. Hooker he complains about the dreadful difficulty of naming plants, though he apparently became quite enthusiastic in this pursuit and advised Dr. Hooker, "If ever you catch quite a beginner and want to give him a taste of botany tell him to make a perfect list of some little field or wood." The facts just stated seem to indicate the extent of his taxonomic studies. He accepted, for the most part, the names of plants which he studied from the determinations of others.
Darwin was attracted to observations of natural objects as a young boy and he early considered plants; his juvenile collections were entomological, and his earlier investigations were mainly zoological and geological. As a pupil of Professor Henslow at Cambridge University he attended botanical lectures and took part in field excursions; he greatly enjoyed the field work, and from it his inspiration for investigation was doubtless derived.
As naturalist of the voyage around the world of the ship Beagle (1831-1836) his collections of plants made in South America and on the islands of the Pacific Ocean, and his observations upon the botanical features of the countries visited, contributed greatly to the knowledge of the flora of those regions. They were extensively utilized by Dr. Hooker in his "Flora Antarctica" and in his "Flora of the Galapagos Archipelago," as well as by other authors in various contributions. Darwin's valuable herbarium is preserved in the museum of Cambridge University. That he collected assiduously at times during portions of this expedition, is evidenced by his having brought home specimens of 193 species of the 225 species which, after his specimens had been studied, were known to inhabit the Galapagos Islands and by the fact that about 100 species new to science were represented in his Galapagos collection. He noticed the extraordinary distribution of species or races on the several islands of this group, many of them inhabiting only a single island, and he laid the foundation for all subsequent study of insular floras. The narrative of observations and experiences during this memorable voyage is replete with interesting facts and suggestions concerning plants, and his conclusion that "Nothing can be more improving to a young naturalist than a journey in distant countries," is one that should be reiterated by all teachers of natural science, and such experience should be sought by all students who propose engaging in investigation. Darwin is commemorated in botanical taxonomy by many species named in his honor. The beautiful barberry, Berberis Darwinii of Hooker, native of Chiloe, is occasionally seen in cultivation. Darwinia, an Australian genus of the myrtle family, named by Rudge in 1813, commemorates his grandfather, Erasmus Darwin.
The beginnings of Darwin's theory of descent of animals and plants from preexistent species, with modifications, were made during the voyage of the Beagle, and from the year after his return to England, when, he tells us, he opened the first note-book on the subject. For twenty-two years he was interrogating gardeners and breeders, botanists and zoologists, and diligently observing plants and animals. He first thought of publishing on the theory of descent in 1839, but delayed for twenty years. During the studies which led up to the publication, in 1859, of "The Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life," Darwin closely observed a great number of wild and cultivated plants, with reference to variation in nature and under domestication, the struggle for existence due to competition for food and sunlight, the facts of geographic distribution, the succession of plant life on the earth as indicated by the fossils of successive geologic periods, and a great range of other facts and phenomena. The recorded observations of other botanists were also freely utilized and discussed. Nearly all the chapters of this epoch-making work contain conclusions drawn from his own botanical observations. He was especially impressed by the divergent views of different botanists relative to the taxonomic treatment of highly polymorphic genera such as Hieracium (hawk-weeds), Rubus (blackberry), Quercus and Rosa, and he employed this consideration to great advantage in his argument for derivation during descent. Rudimentary organs were considered with much interest and readily explained by Darwin as vestiges of structures which were useful to the plant in earlier stages of its existence. The facts of geographic distribution were eagerly examined as bearing on the theory of descent and Darwin's writings abound in speculations relative to their significance. He was inclined to combat the geologic theory of former land connections of present existing continents, as not satisfactorily accounting for many features of geographic distribution, though he ultimately agreed with this theory to some extent. He closely studied the natural means by which seeds are transported over great distances and also inquired into the vitality of seeds.
The title of the "Origin" was a subject of considerable doubt in his mind, and in 1857, two years before it was printed, he had proposed to call it "Natural Selection." The title "Origin of Species by Means of Natural Selection," is, if taken literally, somewhat misleading, and has occasioned considerable discussion. The subtitle—" Or the Preservation of Favored Races in the Struggle for Life "—is a more accurate statement of his theory. On November 23, 1856, he wrote to Dr. Hooker:
Darwin's great contribution to the subject of evolution was the incontrovertible proof adduced by him that living species are modified descendants of preexisting species, and that the modifications are brought about by natural causes. His observations led him to the conclusion that the modifications were all minute, gradual and cumulative. We know that they may also be considerable and abrupt and that they are cumulative because favorable changes are preserved.
How, then, do the modifications or primordial variations, either large or small, arise? Is variation an innate essential quality, or is it induced by external environmental factors? Proof of environmental agencies having at least something to do with it in plants seems to be accumulating, as the experimental work carried on by MacDougal and by Gager at the New York Botanical Garden appears to imply.
I think that we may now safely outline the methods of formation of species somewhat as follows: Through causes which are not yet at all well known, but by means of which agencies external to the germ-cells certainly may have a part, the offspring of a plant grown from seed differ more or less from the parent (variation). The thus modified offspring, subjected to natural selection, ultimately perish if they are unadapted, but survive if they are adapted to their surroundings. Repetitions of this process finally bring the descendants of plants to differ materially from their ancestors (evolution). The end of the process seems to be the development of organisms which are little or not at all subject to variation (monotypic genera). All genera of plants containing a large number of species are evidently subject to continued variation and their species and races almost defy classification. Just what part the phenomena of hybridism take in the final result is not clear, but it may be pointed out that they are evidently unnecessary, because great groups, whole orders, in fact, of the fungi, are devoid of sexuality, and hybridism is therefore impossible among them; yet they are subject to variation like other plants and are quite as difficult to classify.
Observations on insectivorous plants occupied Darwin at intervals from 1860 until the publication of his volume on that subject in 1875. He commenced with the round-leaved sundew (Drosera rotundifolia) while staying at Ashdown Forest, and was soon intensely interested in the exquisite sensitiveness of the leaf-glands to nitrogenous substances. His studies were continued over most of the plants of the sundew family, and to others known to entrap insects or other small animals. He discovered that the leaves of Drosera and of Dionæa secreted a ferment when supplied with various kinds of nitrogenous food and he closely observed the movements of their glands and tentacles and recorded them in detail. Experiments were also made on these plants with a great variety of non-nitrogenous substances. Darwin pointed out the remarkable parallelism between the digestive powers of the secretions of the Droseraceæe and those of the gastric juices of animals. The sacs of the aquatic bladder-worts (Utricularia) and the leaves of butterworts (Pinguicula) were also closely studied. His book is replete with records of careful observations and ingenious deductions. Nepenthes had already been shown by Dr. Hooker to secrete digestive fluids in its pitcher-like leaves, and Sarracenia was suspected of similar activity by Darwin and by others, although he did not regard this as proved.
As early as 1838 or 1839 Darwin was attracted to observe the processes of pollination and noticed the dimorphic flowers of Linum flavum. He had concluded at that time that cross-fertilization was potent in holding species stable and constant. He obtained a great deal of information on this topic in 1841 by reading Sprengle's "Entdeckte Geheimniss der Natur," which stimulated him to continued investigations during summers and he became especially interested in the methods of pollination of the wild orchids growing about his home. This study of pollination of orchids resulted in the publication, in 1862, of his book on that subject, and in it his detailed observations are recorded. Some of his closest observational work was done on this subject of cross-pollination, and he examined a great many species and grew thousands of plants from seed, reaching the broad generalization that cross-fertilization is beneficial to a species and self-fertilization is injurious. The phenomena do not now, however, appear to have as important a relation to evolution as they were formerly supposed to have, and Darwin later expressed regret that he had not given more attention to the processes of self-fertilization.
His interest in showing that cross-fertilization was beneficial led him to closely investigate the various structural features of flowers which necessitate this process to a greater or less degree, such as diœcism, monœcism, polygamy and heterostyly; his observations and speculations are presented in the volume entitled "Different Forms of Flowers and Plants of the Same Species," published in 1877. He records that making out the meaning of heterostyled flowers gave him very great pleasure. A chapter of the book is devoted to cleistogamic flowers, which are necessarily self-fertilized and produce seed abundantly. This work is largely a revision and rearrangement of several papers previously published in the Journal of the Linnæan Society.
"The Variation of Animals and Plants under Domestication," Darwin's largest work, appeared in 1868, published in two volumes. As bearing on this topic he had studied, among plants for many years, the cereal grains, garden vegetables, edible fruits, ornamental trees and ornamental flowers. In the preface he again discusses natural selection and defines it as "This preservation, during the battle for life of varieties which possess any advantage in structure, constitution or instinct," noting that Herbert Spencer had well termed the same process "The Survival of the Fittest." But the bulk of the work is given to the consideration of selection by man—artificial selection, by which races useful to us, economically or esthetically, have been preserved and modified, some of them having originated in very remote times and been taken advantage of by uncivilized man. A chapter is devoted to the phenomena of bud-variation, in which many cases of branches of plants different in one respect or another from other branches on the same plant are described in detail. Many of these have been taken advantage of by horticulturists for the propagation of valuable races. He did not reach any definite conclusion as to the cause of these interesting occurrences; but recently acquired knowledge of mutation seems to indicate that they are of that category, differing from seminal mutations in that a cell in the axil of a leaf is affected rather than a germ-cell. In these volumes we find Darwin's most detailed discussion of heredity of variability and of hybridism and the last chapter outlines his provisional hypothesis of Pangenesis, an ingenious supposition, applying to living matter the general features of the atomic theory, with an additional inherent power of reproduction of the atoms or "gemmules" as he termed the hypothetical ultimate particles.
The movements of plants and of their various organs were also studied by Darwin for many years. His first essay on this topic appeared in 1865 and ten years later he revised and enlarged it as a book under the title "The Movements and Habits of Climbing Plants," using, as always, not only his own detailed and extensive observations, but also the published writings of other botanists, among them the paper on tendrils by Hugo de Vries, who was subsequently destined to throw such a flood of light on the phenomena of variation. Darwin grouped climbing plants into twiners, leaf-climbers, tendril-bearers, hook-climbers and root-climbers. He maintained that the climbing habit has been developed to enable vines to reach the light and free air; tropical forests show conclusively that this is the case. He showed that circumnutation, the bending of growing tips successively to all points of the compass, is a general phenomenon among flowering plants, and he thought it of high importance to them. The sensitiveness of tendrils to external influences interested him deeply and he made many original experiments upon them. Following the subject much further, he published in 1880 the work entitled "The Power of Movement in Plants," a treatise abounding in records of original observations on seedlings and parts of mature plants, including further studies of circumnutation, of the sensitiveness of plants to light and to other forces, and of the phenomena of geotropism and apogeotropism, which he regarded as modified phenomena of circumnutation.
The value of the impulse given by Darwin to botanical investigation in all its branches is beyond estimation; his power of exact observation and record has seldom been equaled and certainly never excelled; his deductions were highly philosophical and most of them have stood the test of thirty years' inquiry and criticism; he was searching for truth and his absolute honesty in research is plainly evidenced by his repeated criticism of his own conclusions.
The immense number of plant species which had been described and named, and the lack of any complete index to them led Darwin to provide in his will for complete enumeration of the names of published species of flowering plants. This great work was prepared at the library of the Royal Gardens, Kew, England, and published in 1895 in four large quarto volumes, to which several supplements have since been added. This "Index Kewensis" is a great boon to all investigators, and is quite indispensable to those who have to take plant names into consideration.
- An address given at the American Museum of Natural History on February 12.