EVOLUTION—ITS MEANING
of species. No sound discussion of species as they exist in nature can ignore geography.
Two general facts relating to the origin of species are often disregarded by those who are engaged in experimental work. The first fact, just referred to, concerns the relation of forms to geographical conditions; the second fact is that related species seldom differ in any survival trait or character by which one is better fitted to live than another. The “survival of the fittest” is a process that operates within the species rather than between one group as a whole and another group. Most species have one or more twins or geminates, which differ in minor features and do not inhabit the same region. This rule of geminate species, accepted by the ornithologist Dr. Joel A. Allen and called by him “Jordan’s Law,” was stated by the present writer in 1904, as follows:
“Given any species in any region, the nearest related species is not likely to be found in the same region, nor in a remote region, but in a neighboring district, separated from the first by a barrier of some sort, or at least by a belt of country the breadth of which gives the effect of a barrier.”
Illustrations among plants, animals, races of men, and human speech appear on every hand. On either side of most barriers geminate species and subspecies (that is, species in the making) occur in every group of organisms, some so different as to require separate names, some barely distinguishable from their associates. Take those well-known birds the flickers, for instance. They belong to the genus Colaptes, a group of woodpeckers. On the east side of the Rocky Mountains we have the form called “yellow-hammer” (Colaptes auratus), with the shafts of its quills bright yellow. On the west side of the mountains we have
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