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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 simple 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.
The Struggle for Existence 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 naturalisation. 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 naturalised 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 neighbours—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 favourable 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 hedgerow, 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 favourable 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 wellbeing, that combination perhaps being determined by local conditions or changes which many years ago allowed a particular set of plants and animals to monopolise 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 hillside, 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.
Geographical Distribution and Dispersal of Organisms 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
With this view he made numerous observations and some ingenious experiments. He endeavoured 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 1000 miles, and in very exceptional cases even 3000 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 round 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 thirtyseven 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, molluscs 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 aerial currents of such plants as have very minute or very light seeds." The facts are of so anomalous and apparently contradictory a character 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 Maderia, have them rather larger than usual. Equally new and important is the Darwinian explanation of the form of the bees' cell, which is shown to
1 This series of observations and experiments, supplemented by those of other observers, have 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. See Island Life. Macmillan and Co.
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.
The Descent of Man and Later Works We must, however, pass on to the great and important work, The Descent of Man and on Selection in Relation 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 tells us that in a second work he shall 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, will 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