CHAPTER XXIV. THE EXTERNAL RELATIONS OF ORGANISMS. Groups of organisms differ greatly as to their relations to past time and their geographical distribution, while the life of individual organisms depends to a large extent upon other organisms living at the same time; and not only are individual organisms affected by their environment, but such affections may be inherited. Relations to time and space-Inter-relations of organisms-Effects produced on organisms by their environment. SOME of the external relations of organisms have been incidentally noticed in our last chapter. Without such notice, the functions of organisms could not have been considered; for respiration † and the formation of organic matter, take place through interactions between living organisms and their inorganic environment. The processes which are necessary to effect the nutrition of organisms § which feed upon other organisms, and the phenomena of parasitism, also involve external relations between living creatures of different kinds. But these matters need now to be considered somewhat more at length. As the relations which exist between organisms and the inorganic world have been already sufficiently explained for the purpose of the present work, the other external relations of organisms may be treated of under three heads-(1) their relations to time; (2) to space; and (3) to one another. For an exhaustive notice of the influence upon organisms of their environment, together with a complete list of books and memoirs bearing on the subject, see a paper by J. Arthur Thomson, M.A., F.R.S.E., in the "Proceedings of the Royal Physical Society of Edinburgh," vol. ix., part 3, 1888. See above, p. 161. § See above, p. 336. See above, p. 333. Relations to time It is generally known that different kinds of animals and space. and plants have successively inhabited the surface of this planet. The order of that succession has been far from regular; the past duration of different groups of organisms having varied greatly. Many kinds of animals have become extinct in certain regions in historic times, and some have become also absolutely and universally extinct. Thus wolves have disappeared from England since the time of Henry VIII., while only seventy years ago the great bustard wandered over the South Downs and Salisbury Plain. The dodo, the rhytina, the great auk, and the dinornis have, more or less recently, become absolutely extinct. The only evidences we possess of past life, is afforded us by the five kinds of fossils before described.* This record is an exceedingly imperfect one; remains of animals and plants having been only here and there exceptionally preserved by some favouring accidents, and often in a very fragmentary manner. In most cases such remains seem to have been transported by water for greater or less distances, and buried in mud; and creatures which live habitually in or near water, form a very large proportion of those the hard parts of which have been thus preserved. We have no space for even the briefest catalogue of the most generally interesting fossils. One or two, however, must be mentioned which bear upon problems of special interest treated of in this work. The Laurentian rocks, in spite of their enormous thickness, have as yet afforded little evidence of life, and that only of very lowly organisms; but when even the lower Cambrian rocks were deposited, many Echinoderms and Brachiopods, Mollusks and Arthropods, were already in existence, while sharks and sturgeons are Devonian, and huge Batrachians, called Labyrinthodonts, characterize the coal measures. In the secondary epoch there existed a multitude of large reptiles, such as the marine Ichthyosauri and Plesiosauri, the terrestrial Dinosauria, and the ærial Pterosauria, which then flew as bats do now. The tertiary strata introduce us to the remains of creatures belonging See above, pp. 313, 314. to various orders of beasts which exist at present, and, during a portion of it, a very man-like, long-armed ape (Dryopithecus) dwelt in Europe. Organisms have evidently inhabited this planet through untold ages. It is possible, indeed, that even the Laurentian deposits may not lead us more than a minute step towards the commencement of life on the earth, although they seem to indicate a very considerable one towards that commencement. * It would be rash to deny that our own species may have appeared in Miocene times; and many naturalists date man's origin approximately from somewhat before the Glacial epoch. But it is certain that man has witnessed great geographical changes, and the extinction of a considerable number of beasts, such as the mammoth, the woolly rhinoceros, cave bears, lions and hyænas, and other forms. Plants and animals have very definite relations to space as well as to time. Each large portion of the earth's surface has its special plant-population, or " flora,” as it has its special animal population, or "fauna." This is not the place to set out the details of organic geography; it will be enough to note that (1) South America, (2) Africa south of the Sahara, (3) Australia, and (4) India with its Archipelago, have each an interesting peculiar fauna, and that (5) North America on the one hand, and (6) Europe, with North Africa and Asia, on the other, are similarly, though less strikingly, characterized. Various species which now inhabit one or other of these areas, closely resemble certain tertiary fossils also found therein. tions of We may next consider the inter-relations of organisms. Inter-claOne great organic inter-relation, already noticed,† under- organisms. lies all others, for animals cannot subsist without feeding directly or indirectly upon plants, while plants are nourished by the carbonic acid which the animal world gives forth in breathing. Every animal which lives on other animals is therefore restricted in its range, not only by the supply of animals fit for its food, but also by the supply of vegetal life fit for the nourishment of the animals on which it lives. The drainage of our fens kills plants on which the † See above, p. 333. *See above, p. 312. grubs of the swallow-tailed butterfly feeds, and that insect is therefore rapidly becoming extinct. Its extinction would. necessarily carry with it the extinction of any animal which. absolutely needed that insect for its subsistence. Certain plants are benefited by ants, which are not only inimical to caterpillars and slugs, but serve even to protect such plants. from cattle or from man himself. Thus the bull's-horn acacia maintains a standing army of these insects, providing them with lodging in certain special cavities, and with rations in the form of special nutritive outgrowths; and plants of the genus Triplaris are also similarly protected by ants, which lodge within its cavities and rush out in multitudes if the tree be touched or shaken, and bite furiously. One organism may benefit another by destroying the indirect enemies, or rivals, of such other. It is thus that around herdmen's huts in the Alps we find species of aconite, dock, chenopodium, and different plants which have gained an extensive footing through the destruction of other vegetation by the grazing herds which find the aconite, etc., unpalatable, and so, by constantly sparing them, unintentionally extend their range. Animals benefit others in very curious ways. Thus barnacles which fix themselves to and are carried about by whales are provided thereby with a larger supply of food, and lobsters, in the midst of the eggs which load their abdomen at certain seasons of the year, often carry about with them a sort of leech which feeds on such eggs as may die. A small fish has also been found to live within the interior of a sea-anemone, feeding on portions of the latter's food. Some ants are benefited by ants of a different species, which the former take captive when young, and which when full grown, turn to and do the domestic work of their captors, and even feed the latter and carry them about. Some organisms, again, benefit others by involuntarily disseminating their eggs or seeds. Thus birds notoriously disseminate seeds which they have swallowed, and sometimes do so by carrying them—or the eggs of small animals-in the mud which may adhere to their feet. Every one now knows how important, or indispensable, may be the aid given by the visits of insects to the fertilization of the flowers they visit. We find also very many curious arrangements by which the visits of useful insects at favourable times are facilitated by the opening, shutting, colours, or odours of flowers-or those of noxious creatures, or of any at unfavourable times, are avoided. Thus snails, slugs, and caterpillars may be kept from flowers merely by means of a group of prickles placed on those parts of the plant which have to be traversed to reach the flower. Insects visit a flower to eat the nectar it secretes, and its nectaries are often so placed as to make it certain that the insects which obtain the nectar should either fertilize the flower, or should go away with pollen grains so adhering to them as to cause them to involuntarily fertilize the next flower they visit. For it is often advantageous that a flower should not be fertilized by its own pollen, and many flowers are so constructed as to favour their fertilization by the pollen of other flowers. no plants are the arrangements to ensure insect fertilization so curious and complex as in orchids. In their flowers the nectar is so stored that insects, in order to reach it, must carry away the pollen on their heads, so that they are forced to apply it to the stigma of the next flower they visit, which is thus fertilized. Plants the pollen of which is only carried accidentally by the wind, have sometimes each pollen grain furnished with a membranous expansion which greatly facilitates its carriage. Some islands where insects are rare, as Tahiti and the Galapagos Islands, have few flowering plants with conspicuous flowers. Yet in Juan. Fernandez, where insects are very scarce, showy flowers are by no means deficient. Honey-eating birds with brushlike tongues, act much as insects do in fertilizing flowers. In In spite, however, of the manifest inter-relations between the forms of insects and the shapes and colours of flowers, a number of instances are to be found in which their shapes and colours do not answer any purpose of this kind. That a very prejudicial influence must be exercised by some kinds of animals on other animals and plants is obvious, since they feed on them. The Pacific Islands are exceptionally rich in pigeons, and this may be due to the absence from those islands of monkeys and all other arboreal mammals which devour |