36 



of the enA'ironment are wind, which tends to move the sand, and 

 vegetation, which tends to stabiHze it. These two opposing forces 

 are primarily responsible for the present location of every association. 

 In the forested portion of the sand the water factor is apparently 

 the most important, but it depends chiefly upon the influence of the 

 vegetation rather than upon any truly physical condition. In a 

 broader way, the presence of a particular flora in the sand is due 

 partly to the selection from the surrounding- associations of various 

 species with certain physiolog^ical requirements, and partly to climatic 

 changes in the past. The latter can only be conjectured; the fomier 

 are not known for any plants in this state and for very few in any 

 place. According- to these views, physical factors are relatively little 

 concerned in the development of vegetational structures in this re- 

 gion, while the demands of the plant and the efi:'ects of its growth are 

 of chief importance. 



The delimitation of the various associations concerned is also a 

 matter which must depend, for the present at least, upon direct ob- 

 sen^ation. Jaccard {ipo2) has given a method for comparing dif- 

 ferent associations and stating- numerically the degree of difference 

 between them. This has been used frec[uently and has given some 

 comparisons which are interesting rather than important. It can not, 

 however, be used successfully in the field. The chief difficulty in sep- 

 arating associations lies in the idea of the association itself, which 

 has never been expressed with sufficient clearness.* By some this idea 

 has never been received with favor. It is true that the distinctness 

 of the associations is lost and their character greatly modified by 

 the effects of civilization, but experience in natural conditions jus- 

 tifies the statement that associations are definite organized units 

 and that all vegetation is composed of them, either mature and fully 

 differentiated or in process of organization (cf. Harper, igo6: 33, 

 34). It is as difficult to formulate a satisfactory definition of an 

 association as of a species, and as unnecessary. For the present it 

 may be considered that it is a homogeneous area of vegetation in 

 which the interrelations of the component individual plants permit 

 them to endure the physical environment. 



In this work the recognition of associations has been based upon 

 the idea of unifomiity, and those areas, whether large or small, 



* The concepts of the association as expressed by authors are very variable 

 and frequently conflicting. Some demand that each association shall occupy a 

 definite habitat (Clements, 1905: 292), others allow a wide range in environment 

 (Cowles, igoi: yg) ; some consider that the change of a single species affects the 

 nature of the association (Harshberger, 7900; 652), while others permit a large 

 variation in the flora (Warming, /pop.' 145, 146). 



