We may look back to past ages, as shown in the last chapter, and we find American types then prevailing on the American continent and in the American seas.

We see in these facts some deep organic bond, throughout space and time, over the same areas of land and water, independently of physical conditions.

The naturalist must be dull who is not led to inquire what this bond is.
The bond is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like each other, or, as we see in the case of varieties, nearly alike.

The dissimilarity of the inhabitants of different regions may be attributed to modification through variation and natural selection, and probably in a subordinate degree to the definite influence of different physical conditions.
The degrees of dissimilarity will depend on the migration of the more dominant forms of life from one region into another having been more or less effectually prevented, at periods more or less remote--on the nature and number of the former immigrants--and on the action of the inhabitants on each other in leading to the preservation of different modifications; the relation of organism to organism in the struggle for life being, as I have already often remarked, the most important 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.