Page - 448 - in The Origin of Species

448 ORIGIN OF SPECIES certain forms and not others to enter, either in greater or lesser numbers; according or not, as those which entered happened to come into more or less direct competition with each other and with the aborigines : and according as the im- migrants were capable of varying more or less rapidly, there would ensue in the two or more regions, independently of their physical conditions, infinitely diversified conditions of life, —there would be an almost endless amount of organic action and reaction,—and we should find some groups of beings greatly, and some only slightly modified, —some de- veloped in great force, some existing in scanty numbers — and this we do find in the several great geographical prov- inces of the world. On these same principles we can understand, as I have endeavoured to show, why oceanic islands should have few inhabitants, but that of these, a large proportion should be endemic or peculiar; and why, in relation to the means of migration, one group of beings should have all its species pe- culiar, and another group, even within the same class, should have all its species the same with those in an adjoining quarter of the world. We can see why whole groups of or- ganisms, as batrachians and terrestrial mammals, should be absent from oceanic islands, whilst the most isolated islands should possess their own peculiar species of aerial mammals or bats. We can see why, in islands, there should be some relation between the presence of mammals, in a more or less modified condition, and the depth of the sea between such islands and the mainland. We can clearly see why all the inhabitants of an archipelago, though specifically distinct on the several islets, should be closely related to each other; and should likewise be related, but less closely, to those of the nearest continent, or other source whence immigrants might have been derived. We can see why, if there exist very closely allied or representative species in two^ areas, however distant from each other, some identical species will almost always there be found. As the late Edward Forbes often insisted, there is a strik- ing parallelism in the laws of life throughout time and space; the laws governing the succession of forms in past times being nearly the same with those governing at the