A DEFINITION OF EVOLUTION 



oceanic islands are usually few in number of species, but a large propor- 

 tion of these are peculiar to each island, and are similar to the inhabitants 

 of the nearest continent. Also, amphibians and terrestrial mammals are 

 usually not found on oceanic islands. 



Thus, the magnolias at present occur naturally only in the southeastern 

 portion of the United States and of Asia. The physical and biotic environ- 

 ments of these areas are similar. In the vast expanse which separates these 

 two populations, magnolias do not occur naturally, although there are 

 areas, such as southern California, in which they flourish when introduced. 

 The predecessors of Darwin interpreted such facts as indicating that one 

 and the same species had been created independently in more than one 

 place. Many species, however, have very \\ ide ranges, and Darwin pointed 

 out that, if a wide-ranging species were to become extinct in the inter- 

 mediate portions of its range, the result would be widely separated popu- 

 lations of the same species. Actualh', fossil evidence indicates that, during 

 a much warmer age, magnolias ( and the associated flora and fauna ) were 

 distributed continuously over the vast area which now separates the extant 

 populations. During the glacial ages, the climate became too severe for 

 these sul)tropical plants o\'er most of this vast range, with the result that 

 thev became extinct except for the mildest parts of their total range. 



One of the most striking examples of widely separated populations of 

 the same or closely related species is the common case of the inhabitants 

 of high mountains, which may be identical even though the)' are sepa- 

 rated bv great expanses of lowland in which alpine species could not pos- 

 siblv sur\i\'e. Or again, such mountains may have a flora and fauna closely 

 similar to that of lowlands far to the north (Figure 2). Thus Dar\\in 

 pointed out that the plants of the White Mountains of New England are 

 the same as those of Labrador, and similar to those of the highest peaks 

 of Europe. It was not difficult to explain this on the basis of a single 

 origin for each plant species, with subsecpient migration and modification. 

 For, as the glacial ages advanced, arctic plants would progress e\-er fur- 

 ther southward, replacing the temperate plants which, in turn, would 

 migrat(> closer to the tropics. At the height of the glaciation, an essentially 

 arctic flora would prevail o\ c>r the entire northern United States, lowlands 

 as well as mountains. As the glaciers retreated, arctic plants would again 

 move northward in the lowlands and upward on the mountains, while the 

 temperate plants mo\ed out from their southern refuges to recapture thc'ir 

 former territories. Thus, mountain plants would be expected to be the 

 same as those of the lowlands to the north. And the close similarity of the 

 alpin(> plants of Europe and America is understandable because of the ob- 

 served fact that circumpolar plants are (juite uniform in all places. Similar 

 phenomcMia are known for animals. Thus the ptarungan and the varying 

 hare are found in the higher moimtains of western United Stat(\s and in 

 the arctic and subarctic lowlands of Canada and Alaska. In Europe, the 

 mouulain haic is found in mountains lioiii llic west coast eastward to 

 the Caucasus and Ural Mountains, and also in the lowlands of arctic 

 Europe, but it docs not inhabit the intcr\(Miing lowlands. 



16 



