OCR Text |
Show 412 RECAPITULATION. [CnA.P. XIV. support the theory of descent with modification. New species have come on the stage slowly and at successive intervals; and the amount of change, after equal intervals of time, is widely different in different groups. The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection ; for old forms will be supplanted by new and im:proved forms. Neither single species nor groups o£ species reappear when the chain of ordinary generation has once been broken. The gradual diffusion of dominant forms, with the slow modification of their descendants, causes the forms of life, after long intervals of time, to appear as if they had changed simultaneously throughout the world. The fact of the fossil remains of each formation being in some degree intermediate in character between the fossils in the formations above and below, is simply explained by their intermediate position in the chain of descent. The grand fact that all extinct organic beings belong to the same system with recent beings, falling either Into the same or into intermediate grouptt, follows from the living and the extinct being the offspring of common parents. As the groups which nave descended from an ancient progenitor have generally diverged in character, the progenitor with its early descendants will often be intermediate in character in comparison with its later descendants; and thus we can see why the more ancient a fossil is, the oftener it stands in some degree intermediate between existing and allied groups. Recent forms are generally looked at as being, in some vague sense, higher than ancient and extinct forms; and they are in so far higher as the later and more improved forms have conquered the older and less improved organic beings in the struggle for life. Lastly, the law of the long endurance of allied forms on the same continent,-of marsupials in Australia, of edentata in America, and other such cases,-is intelligible, for within a confined country, the recent and the extinct will natu· t·ally be allied by descent. Looking to geographical distribution, if we admit that CHAP. XIV.] RECAPITULATlON. 413 th~re ~as been during the long course of ages much migratlon.from one part of the world to another, owing to former chn;atal and geographical changes, and to the many occasional and unknown means of dispersal, then we ~an understand, on the theory of descent with modification, most of the great leading facts in Distribution. Yf e can ~ee _wh:y: there should be so striking a parallelism In t~e dist~Ibution ?f organic beings throughout space, ~n?- In their geological succession throughout time; for In both. cases the bei~gs have been connected by the bond of ordinary generation, and the means of modification have been tlie same. We see the full meaninO' of the wonderful fact, which must have struck every traveller nam~l:y, that on the same continent, under the most divers~ conditions, under heat and cold, on mountain and lowland, on deserts and marshes, most of the inhabitants within each great class are plainly related; for they will generally b~ descendan~s of the ~a~e progenitors and early colon~ sts. . On this same pnn01ple of former migration combined In most cases with modification we can under~tand by the aid of the Glacial period, the identity of som~ few plants, and the close alHance of many others on the most distant mountains, under the most different climates · and likewise the close alliance of some of the inhabitant~ of the sea in the northern and southern temperate zones though separated by the whole intertropical ocean. Al: t~ough t:vo areas may present the same physical conditio! ls of .hfe, w~ need f~el no surprise at their inhabitants bmng widely different, If they have been for a long period completely separated from each other ; for as the Telation of organism to organism is the most important of all relations, and as the two areas 'vill have received colonists from som~ th~rd source or f~om each other, at various periods and 1n different proportions, the course of modification in the two areas will inevitably be different. . On this view of migration, with subsequent modification, we can see why oceanic islands should be inhabited ~t few species, but of these, that many should be peculiar. vv. e can clearly see why those animals which cannot cross Wide spaces of ocean, as frogs and terrestrial mammals, |