32 CONCEPT AND HISTORY. 



experience extending over a period of years. It is desirable if not essential 

 that this period be 12 to 15 years, in order to cover the range of conditions 

 from the wet phase to the dry phase of a climatic cycle. This is particularly 

 true in the use of indicators for land classification, in which grazing, forestation, 

 and crop production must all be taken into account. 



Direction of indication. — The increasing attention paid to plants as indicators 

 during the past decade has largely arisen from practical considerations. While 

 this is highly desirable, it must be recognized that indicators have also a wide 

 range of scientific application. Moreover, the more important and certain 

 practical values are made possible only through the ecological study of indi- 

 cators. It is in the ecological sense that every plant is an indicator. The 

 indicators of actual practice will be obtained by the selection of those which 

 are the most distinctive and dependable. Thus, while the indicators for graz- 

 ing, forestry, agriculture, and land classification will be established by more 

 and more exact study, many indicators will find their chief use in ecology and 

 related fields, winch must lay the foundation for the scientific agriculture and 

 forestry of the future. 



For these reasons, it is necessary to recognize that every dominant can be 

 used as an indicator of past and future as well as of present conditions. This 

 is due, of course, to the fact that every dominant or subdominant has a definite 

 position in succession. As a consequence, it is an indicator not only of the 

 plants which precede and follow it, but also of the soil conditions in which they 

 grow. At the same time the definite existence of a climatic cycle makes it 

 possible to relate growth and successional movements to climatic changes, 

 both past and future, and to extend the application of indicators correspond- 

 ingly. On the one hand, this enables us to greatly broaden and definitize the 

 use of plants as indicators of soil, climate, and vegetational movements in the 

 geological past; on the other, it permits us to look ahead and anticipate the 

 changes due to climatic cycles and the development and movements of vegeta- 

 tion and habitat. 



Scope. — A complete understanding of the broad significance of indicator 

 studies must rest upon a recognition of the aims and methods of modern 

 ecology. In the early characterization of this field (Clements, 1905 : 1) it 

 was emphasized that ecology is the central and vital part of botany and that 

 all the questions of botanical science lead sooner or later to the two ultimate 

 facts, plant and habitat. These statements appear even truer to-day in the 

 light of the progress made during the past twelve years. The one essential 

 amplification is the inclusion of zoology, due to the growing conviction that 

 the real unit of response to the habitat is the biological community. Further- 

 more, it is desirable to place all possible emphasis upon the fact that ecology 

 must fix its attention upon habitat and community in their natural relation. 

 Finally, there must be the clearest recognition of the fact that the plant or 

 animal must be the final arbiter in ecology, except of course in the vast field 

 of human ecology. Fascinating and valuable as they are, instruments and 

 quadrats are useful only in so far as they tell us what the plant, animal, or 

 community is doing. The most complete records of climate, for example, 

 have no merit in themselves. They acquire value only as the plant or animal 

 discloses by its responses the factors or quantities which are effective or con- 

 trolling. 



