384 THE CHANGING GENERATIONS 



in the region beyond. A barrier of this sort may be called a barrier to 

 establishment. 



Sometimes there is a gap in the barriers that enclose a species popula- 

 tion, permitting the species to spread along a relatively narrow highway 

 into another region suitable for its existence. Many Great Plains species 

 are barred to the west by the Rocky Mountains but have been able to 

 pass through gaps, in Wyoming or in the Southwest, to reach the dry 

 grasslands of the interior basin. Many Canadian species find southward 

 leading highways along the crests of the Appalachian, Rocky Mountain, 

 and Cascade-Sierra Nevada ranges. Central America has served as a 

 highway for interchange of tropical species between northern South 

 America and southernmost North America. During the latest of the 

 geological eras the gap between Siberia and Alaska has several times been 

 bridged by land, permitting exchange of species between the Old and 

 New Worlds. 



Historical factors in distribution. In the evolutionary time scale 

 highways and barriers are shifting and ephemeral. Land connections 

 emerge and again sink beneath the sea; mountain ranges are uplifted and 

 eroded away; climates shift from arid to humid, from warm to cold, and 

 back again. Forests become grasslands and again forests; and limiting 

 biotic relationships undergo changes. This means that the longer a species 

 or genus or family has been in existence, the more opportunities its mem- 

 bers will have had to spread away from the region of origin. It also means 

 that with increasing age of the species or group the more likely it is to 

 have broken up into separately evolving populations. Spread followed by 

 the formation of new barriers, or spread through narrow gaps in barriers, 

 isolates descendants of the same stock and permits them to diverge along 

 separate evolutionary paths. 



The mingling of the species of previously isolated regions results in 

 changes in the biotic environment — new enemies, new food organisms, 

 new parasites and diseases, and new competition between species adapted 

 to similar modes of life. The result of such invasions is generally the 

 extinction of some species and always a readjustment of many biotic rela- 

 tions within the affected regions. 



The ranges of closely allied forms. The ranges of closely related 

 species and of the subspecies of a single species are found to show a rather 

 consistent and orderly spatial pattern. David Starr Jordan first formally 

 stated this as follows: "In any group of related organisms, whether species 

 or subspecies, the most closely related will be found, not in the same geo- 

 graphic area, nor in widely separated areas, but in adjacent areas separated 

 by a barrier of some sort." This generalization, known as Jordan's rule, has 

 had to be amplified as noted below, but it is found to hold true in the great 

 majority of instances. 



