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 striking 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 present time the differences in different areas.

We see this in many facts.

The endurance of each species and group of species is continuous in time; for the apparent exceptions to the rule are so few that they may fairly be attributed to our not having as yet discovered in an intermediate deposit certain forms which are absent in it, but which occur above and below: so in space, it certainly is the general rule that the area inhabited by a single species, or by a group of species, is continuous, and the exceptions, which are not rare, may, as I have attempted to show, be accounted for by former migrations under different circumstances, or through occasional means of transport, or by the species having become extinct in the intermediate tracts.

Both in time and space species and groups of species have their points of maximum development.

Groups of species, living during the same period of time, or living within the same area, are often characterised by trifling features in common, as of sculpture or colour.