41 



resulting community coefficient of different areas would probably be 

 above 0.900 (in the black oak association virtually i.ooo). Unfor- 

 tunately no practical method for this has been devised. 



The more widely the different areas of an association are sepa- 

 rated, the greater are the fioral discrepancies. The dominant species, 

 however, remain constant, and the change lies almost wholly in 

 the secondary species. Many of these are the results of selective 

 migration from neighboring associations, so that a variation in the 

 general nature of the vegetation of an area affects the specific struc- 

 ture of each association. This phenomenon has been discussed briefly 

 by Warming {lOop: 145, 146) under the name of geog'raphical va- 

 riation. It is well illustrated in the sand areas of Illinois by the sec- 

 ondary species in the black oak association. In the Havana area are 

 found some typically southern species, as Qiiercus inarilaiidica and 

 Galhivi pilosuiu, while in the Winnebago area some species of 

 northern or eastern distribution occur, as Pyrits ainericaiia and 

 Lupiniis pcrcnnis. In comparing areas of such wide geographic 

 separation emphasis must be placed upon the dominant species, which 

 are the fundamental cause of the general physiognomy of the asso- 

 ciation. 



The areal distribution of an association may be compared to the 

 distribution of a species. Both are irregular in outline, although 

 coextensive with certain combinations of environmental factors. 

 Both consist of scattered members, independent of each other, but 

 related by a common genesis and common demands upon the environ- 

 ment. Both show minor local and broad geographical varieties. The 

 former are illustrated in the association by the consocies ; the latter, 

 in the species by the subspecies, which in their typical form occupy 

 outlying arms or peninsulas but toward the center of distribution 

 intergrade with the main body of the species. Taxonomic work has 

 shown that the interpretation and classification of these forms is a 

 matter of great difficulty. Proper treatment of the geographical 

 varieties of an association will be a matter of much greater difficulty, 

 since the necessary comparisons must be based entirely upon written 

 description or photographic record. 



Because of this geog'raphical variation and consequent difficulty 

 of comparison, few correlations of associations in different parts of 

 America have been made or attempted. Ecological literature con- 

 tains numerous descriptions taken from the few representatives of the 

 associations in a limited locality, but as yet no one has given a gen- 

 eral description of an association, compiled from observations taken 



