SHORTER ARTICLES AND DISCUSSION 



BARRIERS TO DISTRIBUTION AS REGARDS 

 BIRDS AND MAMMALS 

 The geographical range of any species of animal may be 

 likened to a reservoir of water in a mountain canyon. The eon- 

 fining walls are of varying nature. A concrete dam, absolutely 

 impervious, may retain the water at one end. Along either side 

 the basin's walls differ in consistency from place to place. The 

 substratum varies in porosity, at some points being impervious 

 like the dam, at others permitting of seepage of water to a greater 

 or less distance from the main volume. The water continually 

 presses against its basin walls, as if seeking to enlarge its area. 

 And it may succeed in escaping, by slow seepage through such 

 portions of its barrier as are pervious or soluble, or by free flow 

 through a gap in the walls, if such offers. The area occupied 

 by the water will extend itself most rapidly along the lines of 

 least resistance. 



Every species has a center or centers of abundance in which 

 favoring conditions usually give rise to a rate of reproduction 

 more than sufficient to keep the critical area stocked. A tendency 

 to occupy a laruer space results, because of competition within the 

 species: individuals and descent-lines multiply and travel radi- 

 ally, extending those portions of the frontier where least resist- 

 ance is offered. Such radial dispersal takes place slowly in some 

 directions, more rapidly in others, according to the degree of 

 pas.sabilitv of the opposing barriers. These barriers consist of 

 any sort of conditions less favorable to the existence of the 

 species than those in the center of abundance. 



Theoretically, sooner or later and in all directions, every 

 species is absolutely stopped. But as a matter of undoubted 

 fact mast barriers are continually shifting, and the adaptability 

 of the animals themselves may be also undergoing continual 

 modification ■ so that perfect adjustment is beyond the limits of 

 possibility so lontr as topography and climate keep changing. 

 The ranges of species may thus he constantly shifting. Descent- 

 lines may move about repeatedly over the same general region, 

 like sparks in the soot on the back of a brick fireplace. 



Yet, in all of our studies, of but a few years' duration, the 

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