No. 571] TERRESTRIAL ASSOCIATIONS 439 



never actually attained. No one can say just what pro- 

 portion of species-in-common is necessary for two 

 growths to be' said to represent the same association. In 

 addition to likenesses and differences of environment, of 

 aspect, and of history, which must be weighed, the differ- 

 ent plant and animal species vary so much in importance 

 in the association, in physiological variation and in range 

 of environmental tolerance, that associations can hardly 

 be separated or placed together on a statistical basis. A 

 comparison of species is fair if the following kinds of 

 plants and animals are left out of consideration; (1) 

 those of limited range within the climatic region or prov- 

 ince, including species belonging more properly to other 

 provinces; (2) those of very indefinite habitat-relations, 

 which are found in nearly any kind of habitat; (3) those 

 of special restricted habitats, which may be scattered 

 about in many kinds of associations, as moist dead wood, 

 in which particular fungi, beetles, perhaps snails, myrio- 

 pods and pill-bugs, are usually found ; or as excrement of 

 grazing animals, in which certain molds, certain dipter- 

 ous and scarabaeid larvae, etc., regularly occur, irrespect- 

 ive of surrounding conditions; (4) invaders from near- 

 by associations; (5) ruderal and introduced species; and 

 possibly one or two other groups. The second and third 

 groups may be called the irregular element; the fourth 

 and fifth may be known as the derived element. While 

 these groups make a formidable list, their representa- 

 tives constitute usually a very small proportion of the 

 organisms of the association. The other organisms, and 

 some of these, follow habitat-differences, as represented 

 in different associations, very closely. 



Since hardly any two species are identical in habitat- 

 relations, geographic and even local variation must be 

 looked for, but since many species resemble each other 

 more or less closely in general ecological relations, there 

 come to be recognized certain ecological groups of spe- 

 cies, each characterized by a general type of growth-form 

 in plants, or by a general kind of mores in animals, and 



