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THE AMERICAN NATURALIST [Vol. XLYIII 



Wages of definite ecological type, as regards growth- 

 form, etc., are regularly accompanied by animal assem- 

 blages of similar ecological type, as regards mores. In- 

 terest attaches also to the problem whether associated 

 plant and animal assemblages show definite species 

 relations. 



One familiar with a certain association, who visits a 

 representation of that same growth in a different part 

 of the same climatic region, will be struck with the fact 

 that a large proportion of both plant and animal species 

 is well known, while a certain proportion, perhaps con- 

 siderably smaller, is new to him. The writer has been 

 impressed with the similarity of the plant and animal 

 populations of the sandhills of central Nebraska and of 

 eastern Colorado, to those of the sand prairie of central 

 and western Illinois, despite the fact that certain species 

 are not common to the two areas. Tiger-beetles, blow- 

 snake, grasshoppers, box-turtle, six lined lizard, western 

 meadow-lark, white-footed mouse, among the animals; 

 prickly-pear, lead-plant, bunch-grasses, sand-bur, sand 

 evening primrose, among the plants; are represented in 

 the two areas either by the same or by closely related 

 varieties and species. There are no yuccas or sand-sages 

 in the Illinois sand prairie, no lizard Holbrookia nor 

 lubber-grasshopper Uracln/stola; and there are certain 

 eastern species not found in the western sandhills. But 

 on the whole the species (particularly the important 

 species) common to the two areas are more numerous. 

 This is the more remarkable in view of the fact that dis- 

 tribution of sand prairie is discontinuous, the largest, 

 nearly uninterrupted gap being several hundred miles in 

 extent. Many of the animals, as well as plant species, of 

 dry mixed prairie-grass in loamy soil, are the same along 

 the mountain-front in Colorado (Vestal, 19146) as in 

 north-central Illinois. The likenesses become much more 

 impressive as distance is decreased. 



Absolute identity of species composition, where large 

 numbers of species are involved, is an ideal condition, 



