| Looking to geographical distribution, if we admit that there has been during the long course of ages much migration from one part of the world to another, owing to former climatal and geographical changes and to the many occasional and unknown means of dispersal, then we can understand, on the theory of descent with modification, most of the great leading facts in Distribution. We can see why there should be so striking a parallelism in the distribution of organic beings throughout space, and in their geological succession throughout time; for in both cases the beings have been connected by the bond of ordinary generation, and the means of modification have been the same. We see the full meaning of the wonderful fact, which
struck every traveller, namely, that on the same continent, under the most diverse conditions, under heat and cold, on mountain and lowland, on deserts and marshes, most of the inhabitants within each great class are plainly related; for they
→are the
descendants of the same progenitors and early colonists. On this same principle of former migration, combined in most cases with modification, we can understand, by the aid of the Glacial period, the identity of some few plants, and the close alliance of many others, on the most distant mountains,
→and in the northern and southern temperate zones;
and likewise the close alliance of some of the inhabitants of the sea in the northern and southern temperate
though separated by the whole intertropical ocean. Although two
may present
physical conditions
→as closely similar as the same species ever require,
we need feel no surprise at their inhabitants being widely different, if they have been for a long period completely
from each other; for as the relation of organism to organism is the most important of all relations, and as the two
will have received colonists
→OMIT
at various periods and in different proportions,
→from some other country or from each other, the
course of modification in the two areas will inevitably
different. |
