136 ILLINOIS ACADEMY OF SCIENCE 



removed as to be unnoticed in Gray, it offers a puzzle in 

 distribution. (A.) 



A few like the last named and the salix are an uncertain 

 problem. (H.) 



Finally one or two seem to have actually adopted a new 

 habitat as a place of safety in the struggle for existence. Not- 

 ably is this so with Viola striata, and its remarkable home on 

 the wet cliffs of the Mississippi river bluff. Luxuriant to a 

 degree and absolutely safe from extermination, it shows how 

 survival may be brought about by change of habit. (I.) 



Summarizing the causes of peculiar or isolated distribu- 

 tion we have, 



A. Resulting from railway traffic and other commercial 



agents. 



B. Aboriginal plantings by the Indians for food or other 

 purposes. 



C. Extension out-posts of floras with growth centers far 

 removed. 



D. Production of artificial habitats resembling in essentials 

 the natural. 



E. Destruction by cultivation, of most of the suitable habi- 

 tats, isolated stations remaining. 



F. Results of the glacial ice extension and retreat. 



G. Survivals in the struggle for existence. 



H. Uncertain. 



I. Acquirements of new habitats by change in habits of 

 growth. 



