So, our four species of wild pigeons--the ring-dove, stock-dove, rock-pigeon, and turtle-dove--are not closely allied to each other, but each of them belongs, according to some ornithologists, to a separate genus or subgenus, and has its nearest relatives in distant parts of Asia and Africa.

In mammalia the same thing occurs.

Each mountain region of Europe and Asia has usually its own species of wild sheep and goat, and sometimes of antelope and deer; so that in each region there is found the greatest diversity in this class of animals, while the closest allies inhabit quite distinct and often distant areas.
In plants we find the same phenomenon prevalent.

Distinct species of columbine are found in Central Europe (Aguilegia vulgaris), in Eastern Europe, and Siberia (A.glandulosa), in the Alps (A.Alpina), in the Pyrenees (A.pyrenaiea), in the Greek mountains (A.ottonis), and in Corsica (A.Bernardi), but rarely are two species found in the same area.

So, each part of the world has its own peculiar forms of pines, firs, and cedars, but the closely allied species or varieties are in almost every case inhabitants of distinct areas.