184 Mr. A. R. Wallace on the Law which has regulated XVIII. — On the Law which has regulated the Introduction of New Species. By Alfred R. Wallace, F.R.G.S. Every naturalist who has directed his attention to the subject of the geographical distribution of animals and plants, must have been interested in the singular facts which it presents. Many of these facts are quite different from what would have been anticipated, and have hitherto been considered as highly curious, but quite inexplicable. None of the explanations at-tempted from the time of Linnseus are now considered at all satisfactory ; none of them have given a cause sufficient to ac-count for the facts known at the time, or comprehensive enough to include all the new facts which have since been, and are daily being added. Of late years, however, a great light has been thrown upon the subject by geological investigations, which have shown that the present state of the earth, and the organisms now inhabiting it, are but the last stage of a long and uninter-rupted series of changes which it has undergone, and conse-quently, that to endeavour to explain and account for its pre-sent condition without any reference to those changes (as has frequently been done) must lead to very imperfect and erroneous conclusions. The facts proved by geology are briefly these : — That during an immense, but unknown period, the surface of the earth has undergone successive changes ; land has sunk beneath the ocean, while fresh land has risen up from it ; mountain chains have been elevated; islands have been formed into continents, and continents submerged till they have become islands ; and these changes have taken place, not once merely, but perhaps hun-dreds, perhaps thousands of times : — That all these operations have been more or less continuous, but unequal in their progress, and during the whole series the organic life of the earth has undergone a corresponding alteration. This alteration also has been gradual, but complete; after a certain interval not a single species existing which had lived at the commencement of the period. This complete renewal of the forms of life also appears to have occurred several times : — That from the last of the Geo-logical epochs to the present or Historical epoch, the change of organic life has been gradual : the first appearance of animals now existing can in many cases be traced, their numbers gradu-ally increasing in the more recent formations, while other species continually die out and disappear, so that the present condition of the organic world is clearly derived by a natural process of gradual extinction and creation of species from that of the latest geological periods. We may therefore safely infer a like grada-tion and natural sequence from one geological epoch to another.