Зображення сторінки
PDF
ePub
[ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][merged small][ocr errors][ocr errors][ocr errors][merged small]

nearly of the same artiuue. Mr. Darwin showev, however, wai su lai uvm the ring of coral resting on a corresponding ridge of rock, the lagoons on

[graphic][merged small][merged small][merged small][merged small]

the contrary now occupy the place which was once the highest land. He pointed out that some lagoons, as for instance, that of Vanikoro, contain an island in the middle; while other islands, such as Tahiti, are surrounded by a margin of smooth water, separated from the ocean by a coral reef. Now, if we suppose that Tahiti were to sink slowly, it would gradually approximate to the condition of Vanikoro; and if Vanikoro gradually sank, the central island would disappear, while on the contrary the growth of the coral would neutralise the subsidence of the reef, so that we should have simply an atoll, with its lagoon. The same considerations explain the origin of the “ barrier reefs,” such as that which runs, for nearly one thousand miles, along the north-east coast of Australia. Thus, Mr. Darwin's theory explained the form and the approximate identity of altitude of these coral islands. But it did more than this ; because it showed us that there were great areas in process of a subsidence, which, though slow, was of great importance in physical geography.

His monograph of the Cirripedia or barnacles, a curious group of abnormal crustacea, long supposed to belong to the class of molluscs; and even by the older naturalists imagined, probably from their feathery légs, to stand in a mysterious connection with the barnacle-geese, is universally admitted amongst naturalists to be a most masterly work, but is, of course, of a special character. Perhaps the most interesting point is the discovery that certain minute creatures, found adhering to the female barnacles, are really the males. They are, in some species, almost rudimentary, and very short lived, being, indeed, incapable of taking any food. For these little creatures he proposed the term “complemental males."

In the year 1858, Mr. Darwin communicated to the Linnean Society a short, but most important memoir, “On the Variation of Organic Beings in a State of Nature,” in which he briefly indicated the views which, under his name, have since become so famous. Mr. Wallace, also, simultaneously and independently arrived at similar results. Mr Darwin's great work “On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life,” in which these views were developed with masterly ability, appeared in the following year, and may truly be said to have constituted an epoch in natural history.

The conclusions to which he arrived were as follows: “That the theory of descent with modification embraces all the members of the same class. I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number.” . . .." Therefore, on the principle of natural selection with divergence of character, it does not seem incredible that, from some such low and intermediate form, both animals and plants may have been developed.

And, if we admit this, we must admit that all the organic beings which have ever lived on this earth may have descended from some one primordial form." . . . . . . . . . . . . . . . . . . . .. “ There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed laws of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

These views were supported by close reasoning, and an immense array of facts. Mr. Darwin commenced by discussing the variability of animals under domestication and nature, showing the difficulty of distin. guishing between varieties and species; and the differences which man had been able to produce in such cases as that, for instance, of our domestic pigeons, all unquestionably descended from a common ancestor. He then referred to the doubtful species, showing that wide ranging, much diffused, and common species vary the most, and that species of the larger genera in each country vary more than those of the smaller. He then called attention to the effect of the struggle for existence, a phrase which has since become an household word, in killing out the individuals less perfectly adapted to their environment, thus exercising in fact a true, though unconscious selection, comparable in its effect to that exercised by man on domesticated animals and plants. He then proceeded to discuss the laws of variation, and to point out, with characteristic candour, the difficulties of his theory. The absence of intermediate varieties between species, he accounted for by the imperfection of the geological record ; and he then proceeded to show that the geographical distribution of animals and plants, the fauna and flora, for instance, of oceanic islands, the absence of batrachians and terrestrial mammals, the relation of the inhabitants of islands to those of the nearest mainland, afforded powerful arguments in support of his views. The remarkable facts presented by embryology, and the existence of rudimentary organs, were also shown to point clearly in the same direction.

No one could read this work without admiration, but, although Mr. Darwin's views from the first received the adhesion of some of the most eminent naturalists, they were so much opposed to generally-received opinions, that they naturally aroused much opposition. It is, however, not going too far to say that they have gradually gained ground, not only amongst professed naturalists, but with all those who have taken the trouble carefully to weigh the evidence. Almost all, now, would probably admit that natural selection has greatly influenced the present forms of organised life, though there would still be much difference of opinion as to how far the results have been modified by other causes. Mr. Darwin's views would probably have attracted less opposition had it not been for their obvious bearing on the origin of the human race. Mankind,

« НазадПродовжити »