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 requrre 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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