322 NATURE SKETCHES IN TEMPERATE AMERICA 



have been carefully studied. A great deal can be done in the 

 way of listing the fauna, or studj'ing the life habits of its species, 

 without any knowledge of plant ecology, but an adequate 

 study must be based upon a knowledge of the vegetation. 

 Although animal formations are often poorly defined, there 

 can be no doubt of their existence. Frequently they coincide 

 with plant formations, and then have very definite limits. 

 They exhibit both development and structure, and are subject 

 to laws of invasion, succession, zonation, and alteration, though 

 these are not altogether similar to those known for plants, a 

 fact really explained by the motility of animals. Considered 

 from the above j)oint of view, zoo-geography is a virgin field, 

 and it promises great things to the student who approaches it 

 with the proper training.^ 



Nature's Recl..\m.\tion of Sterile Ground 



When a portion of the surface of the earth is denuded, that 

 is, all the vegetation has been removed, the new area becomes 

 inhabited by plant life of a pioneer type. When this ground is 

 not shifting by action of the wind, certain alga?, fungi, compos- 

 ites, grasses, or weeds initiate a formation. As time goes 

 on, this new formation of vegetation may be succeeded or 

 supplemented by another or secondary vegetation, having 

 migriited from the adjacent areas. 



The ordinary .sequence of vegetation forms in succession 

 is; first: alga^, fungi, and mosses; second: annuals and biennials; 

 third: perennial herbs; fourth: bushes and shrubs; fifth and 

 lastly: trees. The stages or formations of a succession are 

 distinguished as initial, intermediate, and ultimate; the number 

 of species occupying a denuded area is small in the initial stages, 

 becomes large in the intermediate, and again decreases in the 

 ultimate formation because of the dominance of a few species 

 supposed to be created by competition. There is a universal 

 tendency to stabilization in vegetation, grass being the ultimate 

 stage of succession for prairie lands, while forest is the last stage 

 for mesophytic midlands and lowlands of the temperate region. 



* Bailey Willis has treated this subject in a recent interesting article entitled 

 "Paleogeography," in Science, Vol. XXXI, p. iol. 



