Books and Current Literature. 



213 



certain fundamental conceptions that it must be reckoned, in 

 spite of its brevity, as an altogether useful contribution to the 

 formative subject of which it treats. 



Regarding the forest as an organism showing both develop- 

 ment and structure, the logical course of procedure in ecology 

 and in forestry, is fundamentally the same. In one as in the 

 other, the habitat, or sum of physical and biological factors, is 

 taken as the cause, the development of forest as of individual 

 tree the respons?. and the structure of forest as well as of tree 

 the final formal expression. Between the tree and the forest 

 there is, however, the important difference that the latter re- 

 quires so long a period to reach maturity that first-hand investi- 

 gation of the forest development can only be fragmentary. It 

 must be studied in motion, as it were, but this movement can 

 not be easily traced to its conclusion. In consequence it is 

 necessary to use the method of reconstruction in which the va- 

 rious stages of structure are shown in their proper sequence, 

 and give a picture of the life movement of a forest much as the 

 individual films give motion in a living picture. Thus recog- 

 nizing that the development of a forest gives its structure, we 

 must depend upon the latter to re-establish for us as much as 

 possible of the development that has already disappeared. 



Carrying out this thought, the structure of vegetation may 

 be viewed from three different standpoints, all equally an out- 

 growth of its development. These are (1) its composition as to 

 species. (2) the fundamental response as to the action of physical 

 factors, (3) the subdivisions or plant associations within the 

 vegetation itself. Physical factors act upon plants individually 

 and in groups, hence the species which are acted upon, as well as 

 the habitat or sum of causes, must be reckoned with in any 

 analysis. 



The strict correspondence between vegetative and physical 

 factors gives rise to a vegetative unit or plant formation for each 

 habitat. Though difficult of exact demarcation at present, 

 the general kinds of formation are distinguished by the terms 

 forest formation, chaparral formation, prairie formation, etc. 

 Societies, communities, and other more or less definite elements 

 of the formation are recognized, and a knowledge of their struc- 

 ture and development becomes necessary to its correct under- 

 standing. 



