348 GEOKGE E. NICHOLS 



further subdivided the plant formations of the earth into four- 

 teen formation-classes and seventeen form.ation-groups. They 

 do not, however, restrict the use of these terms to climatic for- 

 mations : under prata, for example, are included the salt marshes 

 and fresh marshes of forested regions, as well as the cHmatically- 

 conditioned grassland formations. The word formation-type, 

 from the standpoint of regional physiographic ecology, is here 

 used only with reference to climatic formations. Schimper 

 (19) has used the term in this sense, distinguishing three chief 

 types of climatic formations (woodland, grassland, and desert) 

 as well as various types of subordinate rank. Climatic forma- 

 tion-types, like climatic formations, are best designated in 

 terms of the ecological aspect of the vegetation, as has been 

 done in the illustrations cited above. 



The climatic forrnation-complex. In treating the vegetation 

 of any large area, such as the continent of North America, where 

 more than one type of climate and correspondingly numerous 

 different climatic formations are represented, the term climatic 

 formation-complex may be used to include collectively the cli- 

 matic formations of the entire area. The chmatic formations 

 comiprising the complex may be genetically related in point 

 of geologic time, as in the glaciated parts of eastern North 

 America, but the term does not necessarily imply such a rela- 

 tionship. The chmatic formation-complex of the earth, taken in 

 its entirety, might be regarded as forming a terrestrial formation. 



The application of the law of the minimum in regional physio- 

 graphic ecology. In applying the Law of the Minimum to prob- 

 lems in local physiographic ecology, climatic factors need not 

 be taken into account since they are essentially constant 

 throughout the region. The variable factors are edaphic; they 

 are due, directly or indirectly, to variations in either soil or 

 topography. In dealing with problems in regional physio- 

 graphic ecology, however, where the climatic factors also are 

 variable, these of course are of paramount importance. Pre- 

 cipitation, the evaporating power of the air, temperature, and 

 light afford the chief hmiting factors, the factors which deter- 

 mine the general aspect of the vegetation in a climatic formation. 



