History of Forest Ecology 389 



by many too involved to come into general use. Perhaps the 

 best course open to botanists and foresters is to select such terms 

 as may appear to be helpful. Schimper (10) made a distinct ad- 

 vance in the science when he distinguished between physical and 

 physiological dryness of the soil. He made practically the same 

 divisions of water-content associations as Warming did in 1895. 

 Schimper, likewise, divided vegetation into three great climatic 

 formations, namely : forest, grassland, and desert, which is merely 

 another expression of the water-content of the soil. He classified 

 the vegetation of the earth's surface into four districts based 

 upon temperature, and these into groups of climatic formations. 

 These groups are simply vegetation units whose physiognomy is 

 related largely to climatic conditions, but principally to soil 

 moisture. 



Warmings' classifiation based upon the water-content of the 

 soil is doubtless the best possible classification if but one factor 

 is considered. Graebner's (29, 30) classification based upon soil 

 characters includes the advantages of Warming's classification 

 and adds desirable new features. This scheme is based in the main 

 on the chemical and physical properties of the soil. The primary 

 divisions are chemical, depending on the richness or poverty of 

 the soil in plant food materials, and the secondary divisions are 

 based upon soil moisture. This is interesting in the light of Thur- 

 mann's ideas, and simply points to the beginning of the two 

 schools of thought that are developing, namely, one claiming 

 that the physical properties of the soil are most important in de- 

 termining local vegetation, and the other school adhering to the 

 chemical point of view. 



Invasion zvas defined by Goeze (31), in 1882, as the movement 

 of plants from an area of one character into one of a different 

 character and the establishment of the plants in the latter area. 

 Two distinct ideas are involved, first the movement from one 

 place to another and secondly the germination, growth, adjust- 

 ment, and final establishment of the plant in the new environ- 

 ment. The former is often called migration, the latter ecesis (the 

 making of a home). Migration consists of at least four distinct 

 processes, namely: mobility, agency, proximity and topography. 

 Mobility has to do primarily with the character, size and weight 

 of the seed; agency with the means for transporting the seed, 

 such as winds, water, birds, mammals, etc.; and proximity and 



