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Show 284 GEOLOGICAL SUCCESSION, [CHAP. X. The fact of the forms of life changing simultaneously, in the above large sense, at distant parts of the world, h~s reatl struck those admirable observers, MM. d.e Verneuil ~nd d~rchiac. After referring to the parallehsm of the alreozoic forms of life in various parts of Europe, they idd, " If struck by this strange sequen~e, we turn o?-r attention to North America,. and there diSC?Yer a senes of 10 Ous phenomena it will appear certain that all these manoad if)gic ations of speci'e s, thm· r ext~· nctl· on, and the I· n t ~?- duction of new ones, cannot be owing to mere changes In marine currents or other causes more or less local and temporary, but depend on general laws which govern .the whole animal kingdom." M. Ba~·rande h~s ~ade form~le remarks to precisely the same effect. It ~s, Indeed, quite futile to look to changes of currents, chmate, or o~her physical conditions, as the cause of these great mutat1o:o.s in the forms of life throughout the world, under the most different climates. We must, as Barrande has remarked, look to some special law. We shall see this more clearly when we treat of the present distribution of organic beir:gs, and find how slight is the r.elation between the phJ:SI~al conditions of various countries, and the nature of theu In-habitants. . This great fact of the par~llel su?cession of the fonns of life throughout the world, IS exphcable on the theory of natural selection. New species are formed by new varieties arising which have some advantage over older forms· and those forms, which are already dominant, or have ~orne advantage over the other forms in their own country, would naturally oftenest give rise to new yari~ties or incipient species ; for these ~atter must be vwtorwus in a still higher degr~e _in ord~r to be pr~served ~nd to survive. We have distinct evidence on th1s head, In the plants which are dominant, that is, ;vhich ~re commor:est in their own homes, and aremost w1dely diffused, having produced the greatest number of new varieties. It is ~lso natural that the dominant, varying, and far-~pread1ng species, which already have invaded to a certain ext~nt the territories of other species, should be those whwh would have the best chance of spreading still further, and CuAP. X.] THROUGHOUT THE WOI{LD. 285 of giving rise in n~w c?untries to new varieties and species. The process of diffusion may often be very slow being dependent ~n climatal a;nd geographical changes: or on strange accidents, but In the long run the dominant forms will generally succeed in spreading. The diffusion, would, it is probable, be lower with the terrestrial inhabitants of distinct continents than with the marine inhabitants of the continuous sea. We might therefore expect to find, as we apparently do find, a less strict degree of parallel succession in the productions of the land than of the sea. Dom~ant species spreading f~om any region might encounter still more domrnant species, and then their triumphant course, or even their existence, would cease. We know not at all precisely what are all the conditions most favourable for the multiplication of new and dominant species; but we can, I think, clearly see that a number of individuals, from giving a better chance of the appearance of favourable variations, and that severe competition with many already existing forms, would be highly favourable, as would be the power of spreading into new territories. A certain amount of isolation, recurring at long intervals of time, would probably be also favourable as before explained. One quarter of the world may have ?een most !avourable for the production of new and domInant species on the land, and another for those in the waters of. the sea. If two great regions had been for a long period favourably circumstanced in an equal degree whenever their inhabitants met, the battle would be pro: longed and severe ; and some from one birthplace and some from. the ·other might be victorious. But in the course of time, the forms dominant in the· highest degree, wherever prod1_1ced, would tend everywhere to prevail. As they prevailed, they would cause the extinction of other and inferior forms ; and as these inferior forms would be allied in gro?-ps by inheritance, whole groups would tend slowl:t to disappear; though here and there a single member might long be enabled to survive. Thus, as it seems to me, the parallel, and, taken in a large sense, simultaneous, succession of the same forms of |