Charles Darwin: The Origin of Species by means of Natural Selection

CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION--continued. 5. SUMMARY OF THE LAST AND PRESENT CHAPTERS. (continued)

If the difficulties be not insuperable in admitting that in the long course of time all the individuals of the same species, and likewise of the several species belonging to the same genus, have proceeded from some one source; then all the grand leading facts of geographical distribution are explicable on the theory of migration, together with subsequent modification and the multiplication of new forms. We can thus understand the high importance of barriers, whether of land or water, in not only separating but in apparently forming the several zoological and botanical provinces. We can thus understand the concentration of related species within the same areas; and how it is that under different latitudes, for instance, in South America, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are linked together in so mysterious a manner, and are likewise linked to the extinct beings which formerly inhabited the same continent. Bearing in mind that the mutual relation of organism to organism is of the highest importance, we can see why two areas, having nearly the same physical conditions, should often be inhabited by very different forms of life; for according to the length of time which has elapsed since the colonists entered one of the regions, or both; according to the nature of the communication which allowed 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 immigrants were capable of varying more or less rapidly, there would ensue in the to 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 developed in great force, some existing in scanty numbers--and this we do find in the several great geographical provinces of the world.