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Show 350 GEOGRAPHICAL DISTRIBUTION. CHAP. XI. the American con tin en t and in the American seas. We see in these facts some deep organic bond, prevailing throughout space and time, over the. same .areas of land and water, and independent of their physical conditions. The naturalist must feel little curiosity, who is not led to inquire what this bond is. This bond, on my theory, is simply inheritance,.that cause which alone, as far as we positively know, produces organisms quite like, or, as we see in the case of varieties nearly like each other. The dissimilarity of · the inhabitants of different regions may be attributed to modification through natural selection, and in a quite subordinate degree to the direct influence of different physical conditions. The degree of dissimilarity will depend on the migration of the more dominant forms of life from one region into another having been effected with more or less ease, at periods more or less remote ;-on the nature and number of the former immigrants ;and on their action and reaction, in their mutual struggles for life ; -the relation of organism to organism being, as 1 have already often remarked, the most im- . portant of all relations. Thus the high importance of barriers comes into play by checking migration; as does time for the slow process of modification through natural selection. Widely-ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely-extended hmnes will have the best chance of seizing on new places, when they spread into new countries. In their new homes they will be exposed to new conditions, and will frequently undergo further modification and improvement; and thus they will become still further victorious, and will produce groups of modified descendants. On this principle of inheritance with modification, we can understand how it is that sections of genera, whole genera, CH.AP. XI. SINGLE CENTRES OF CREATION. 351 and even families are confined to the same areas, as is so commonly and notoriously the case. I believe, as was remarked in the last chapter, in no law of necessary development. As the variability of each species is an independent ·property, and will be taken advantage of by natural selection, only so far as it profits the individual in its complex struggle for life, so the degree of modification in different species will be no uniform quantity. If, for instance, a number of species, which stand in direct con1petition with each other, migrate in a body into a new and afterwards isolated country, they will be little liable to modification; for neither migration nor isolation in themselves can do anything. These principles come into play only by bringing organisms into new relations with each other, and in a lesser degree with the surrounding physical conditions. As we have seen in the last chapter that some forn1s have retained nearly the same character from an enormously remote geological period, so certain species have migrated over vast spaces, and have not become greatly modified. On these views, it is obvious, that the several species of the same genus, though inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they have descended from the same progenitor. In the case of those species, which have undergone during whole geological periods but little modification, there is not much difficulty in believing that they may have migrated from the same region; for during the vast geographical and climatal changes which will have supervened since ancient times, almost any amount of migration is possible. But in many other ·cases, in which we have reason to believe that the species of a genus have been produced within comparatively ~ecent times, there is great difficulty on this head. It |