to a style of the same length on another plant, and there is thus a triple chance of the maximum of fertility. Some other species of lythrum, of oxalis, and pontederia, were also found to have three-formed stamens and styles; and in the case of the oxalis, experiments were made showing that crosses between flowers with stamens and styles of unequal length were always nearly barren. During these experiments twenty thousand seeds of Lythrum Salicaria were counted under the microscope. For several years a further supplementary series of experiments was carried out, showing that the seeds produced by the illegitimate crosses (as he terms them) were not only very few, but, when sown, always produced comparatively weak, small, or unhealthy plants, not likely to exist in competition with the stronger offspring of legitimate crosses. There is thus the clearest proof that these complex arrangements have the important end of securing both a more abundant and more vigorous offspring.

Perhaps no researches in the whole course of the study of nature have been so fertile in results as these. No sooner were they made known than observers set to work in every part of the world to examine familiar plants under this new aspect. With very few exceptions it is now found that every flower presents arrangements for securing cross-fertilization, either constantly or occasionally, sometimes by the agency of the wind, but more frequently through the mediation of insects or birds. Almost all the irregularity and want of symmetry in the forms of flowers, which add so much to their variety and beauty, are found to be due to this cause; the production of nectar and the various nectarsecreting organs are directly due to it, as are the various odors and the various colors and markings of flowers. In many cases flowers which seem so simply constructed that the pollen must fall on the stigma and thus produce self-fertilization, are yet surely cross-fertilized, owing to the circumstance of the stigma and the anthers arriving at maturity at slightly different periods, so that, though the pollen may fall on the stigma of its own flower, fertilization does not result; but when insects carry the pollen to another plant the flowers of which are a little more advanced, cross-fertilization is effected. There is literally no end to the subjects of inquiry thus opened up, since every single species, and even many varieties of flowering plants, present slight peculiarities which modify to some extent their mode of fertilization. This is well shown by the remarkable observations of the German botanist Kerner, who points out that a vast number of details in the structure of plants,

hitherto inexplicable, are due to the necessity of keeping away "unbidden guests," such as snails, slugs, ants, and many other kinds of animals, which would destroy the flowers or the pollen before the seeds were produced. When this evident principle is once grasped, it is seen that almost all the peculiarities in the form, size, and clothing of plants are to be thus explained-as the spines or hairs of the stem and branches, or the glutinous secretion which effectually prevents ants from ascending the stem, the drooping of the flowers to keep out rain or to prevent certain insects from entering them, and a thousand other details which are described in Kerner's most instructive volume. This branch of the inquiry was hardly touched upon by Darwin, but it is none the less a direct outcome of his method and his teaching.

But we must pass on from these seductive subjects to give some indication of the numerous branches of inquiry of which we have the results given us in the "Origin of Species," but which have not yet been published in detail. The observations and experiments on the relations of species in a state of nature, on checks to increase and on the struggle for existence,—were probably as numerous and exhaustive as those on domesticated animals and plants. As examples of this we find indications of careful experiments on seedling plants and weeds, to determine what proportion of them were destroyed by enemies before they came to maturity; while another set of observations determined the influence of the more robust in killing out the weaker plants with which they come into competition. This last fact, so simple in itself, yet so much overlooked, affords an explanation of many of the eccentricities of plant distribution, cultivation, and naturalization. Every one who has tried it knows the difficulty or impossibility of getting foreign plants, however hardy, to take care of themselves in a garden as in a state of nature. Wherever we go among the woods, mountains, and meadows of the temperate zone, we find a variety of charming flowers growing luxuriantly amid a dense vegetation of other plants, none of which seem to interfere with each other. By far the larger number of these plants will grow with equal luxuriance in our gardens, showing that peculiarities of soil and climate are not of vital importance; but not one in a thousand of these plants ever runs wild with us, or can be naturalized by the most assiduous trials; and if we attempt to grow them under natural conditions in our gardens, they very soon succumb under the competition of the plants by which they are surrounded. It is only by constant attention, not so much to

them as to their neighbors,-by pruning and weeding close around them so as to allow them to get a due proportion of light, air, and moisture, that they can be got to live. Let any one bring home a square foot of turf from a common or hill-top, containing some choice plant growing and flowering luxuriantly, and place it in his garden, untouched, in the most favorable conditions of light and moisture, and in a year or two it will almost certainly disappear, killed out by the more vigorous growth of other plants. The constancy of this result, even with plants removed only a mile or two, is a most striking illustration of the preponderating influence of organism on organism, that is, of the struggle for existence. The rare and delicate flower which we find in one field or hedge-row, while for miles around there is no trace of it, maintains itself there, not on account of any specialty of soil or aspect, or other physical conditions being directly favorable to itself, but because in that spot only there exists the exact combination of other plants and animals which alone is not incompatible with its well-being, that combination perhaps being determined by local conditions or changes which many years ago allowed a different set of plants and animals to monopolize the soil and thus keep out intruders. Such considerations teach us that the varying combinations of plants characteristic of almost every separate field or bank, or hill-side, or wood throughout our land, is the result of a most complex and delicate balance of organic forces-the final outcome for the time-being of the constant struggle of plants and animals to maintain their existence.

ANOTHER valuable set of experiments and observations are those bearing on the geographical distribution of animals and plants a branch of natural history which under the old idea of special creations had no scientific existence. It is to Darwin that we owe the establishment of the distinction of oceanic from continental islands, while he first showed us the various modes by which the former class of islands have been stocked with life. By a laborious research in all the accounts of old voyages, he ascertained that none of the islands of the great oceans very remote from land possessed either land mammalia or amphibia when first visited; and on examination it is found that all these islands are either of volcanic origin or consist of coral reefs, and are therefore presumably of comparatively recent independent origin, not portions of submerged continents, as they were formerly supposed to be. Yet these same islands are fairly stocked with plants,

insects, land-shells, birds, and often with reptiles, more particularly lizards, usually of peculiar species, and it thus becomes important to ascertain how these organisms originally reached the islands, and the comparative powers different groups of plants and animals possess of traversing a wide extent of ocean.

With this view Darwin made numerous observations and some ingenious experiments. He endeavored to ascertain how long different kinds of seeds will resist the action of salt water without losing their vitality, and the result showed that a large number of seeds will float a month without injury, while some few survived an immersion of one hundred and thirty-seven days. Now, as ocean currents flow on the average thirty-three miles a day, seeds might easily be carried a thousand miles, and in very exceptional cases even three thousand miles, and still grow. Again, it is known that drift-timber is often carried enormous distances, and some of the inhabitants of the remote coral-islands of the Pacific obtain wood by this means, as well as stones fastened among the roots. Now, Darwin examined torn-up trees, and found that stones are often inclosed by the roots growing around them so as to leave closed cavities containing earth behind; and from a small portion of earth thus completely inclosed, he raised three dicotyledonous plants. Again, the seeds that have passed through the bodies of birds germinate freely, and thus birds may carry plants from island to island. Earth often adheres to the feet of aquatic and wading birds, and these migrate to enormous distances and visit the remotest islands, and from earth thus attached to birds' feet several plants were raised. As showing the importance of this mode of transport, an experiment was made with six and three-fourths ounces of mud taken from the edge of a little pond, and it was found to contain the enormous number of five hundred and thirty-seven seeds of several distinct species! This was proved by keeping the mud under glass and pulling up each plant as it appeared, and at the end of six months the result was as given above. It was also found that small portions of aquatic plants were often entangled in the feet of birds, and to these as well as to the feet themselves mollusks or their eggs were found to be attached, furnishing a mode of distribution for such organisms. Experiments were also made on the power of land-shells to resist the action of sea-water; and we have already referred to the observations on volcanic dust carried far out to sea illustrating the facilities for the wide extension by aërial currents of such plants as have very minute or very light seeds. This series of observa

tions and experiments, supplemented by those of other observers, has been applied by the writer of this article to explain in some detail the remarkable phenomena presented by the distribution of animals and plants over the chief islands of the globe ("Island Life"). The facts are of a character so anomalous and apparently contradictory that, on the old hypothesis of the special independent creation of each species, no rational explanation of them could be found; and we may fairly claim that the clear and often detailed explanation which can be given by means of the theories and investigations of Darwin lend a powerful support to his views, and go far to complete the demonstration of their correctness.

Our space will not permit us to do more than advert to the numerous ingenious explanations and suggestions with which the "Origin of Species" abounds, such as, for example, the strange fact of so many of the beetles of Madeira being wingless, while the same species, or their near allies on the continent of Europe, have full powers of flight; and that this is not due to any direct action of climate or physical conditions is proved by the equally curious fact that such species of insects as have wings in Madeira have them rather larger than usual. Equally new and important is the Darwinian explanation of the form of the bee's cell, which is shown to be due to a few simple instincts which necessarily lead to the exact hexagonal cell with the base formed of three triangular plates inclined at definite angles, on which so much mathematical learning and misplaced admiration have been expended; and this explanation is no theory, but is the direct outcome of experiments on the bees at work, as original as they were ingenious and convincing.

We must, however, pass on to the great and important work, "The Descent of Man and on Selection in Relations to Sex," which abounds in strange facts and suggestive explanations; and for the reader who wishes to understand the character and bearing of Darwin's teachings, this book is the fitting supplement to the "Origin of Species" and the "Domesticated Animals and Plants." To give any adequate account of this most remarkable book and the controversies to which it has given rise, would require an article to itself. We refer to it here in order to point out what is not generally known, that its publication was entirely out of its due course, and was not anticipated by its author three years before. In the introduction to "Domesticated Animals" (published in 1868), after explaining the scope of that work, he told us that, in a second work, he should treat of " Variation Under Nature," giving copious

facts on variation, local and general, on races, sub-species, and species, on geometrical increase, on the struggle for existence, with the results of experiments showing that diversity of forms enables more life to be supported on a given area, while the extermination of less improved forms, the formation of genera and families, and the process of natural selection, would be fully discussed. This work would have given all the facts on which Chapters II. to V. of the " Origin of Species" were founded. In a third work he proposed to show, in detail, how many classes of facts natural selection explains, such as geological succession, geographical distribution, embryology, affinities, classification, rudimentary organs, etc., etc., thus giving the facts and reasonings in full on which the latter part of the "Origin of Species" was founded. Unfortunately, neither of these works has appeared, and thus the symmetry and completeness of the body of facts which Darwin had collected have never been made public. The cause is well known to have been the continued pressure of ill-health. The work on "Domesticated Animals was thus delayed many years, after which came the labor of bringing out a much enlarged edition of the "Origin of Species." The "Descent of Man" was, apparently, at first intended to be a comparatively small book, but a difficulty connected with the origin of the distinctive peculiarities of the two sexes led to an investigation of this subject throughout the animal kingdom. This was found to be of such extreme interest, and to have such important applications, that its development with the completeness characteristic of all the writer's work led to the production of two bulky volumes, followed by another volume on the " Expression of the Emotions in Man and Animals," not less instructive. None of Darwin's works has excited greater interest or more bitter controversy than that on man; and the correction of the numerous reprints, and of a final enlarged edition in 1874, was found to be so laborious a task as to convince him that any such extensive literary works as those projected and announced six years previously must be finally abandoned. This, however, by no means implied cessation from work. Observation and experiment were the delight and relaxation of Darwin's life, and he now continued and supplemented those numerous researches on plants we have already referred to. A new edition of an earlier work on the "Movements of Climbing Plants" appeared in 1875; a thick volume on "Insectivorous Plants" in the same year; "Cross and SelfFertilization" in 1876; the " Forms of Flowers" in 1877; the "Movements of Plants,"