4 CONCEPT AND HISTORY. 



the several divergent views are later considered briefly for the sake of a clearer 

 appreciation of existing opinion. Finally, the many studies of foresters upon 

 the tolerance of trees to shade had large elements of indicator value, but these 

 were never brought together into a system. 



Studies of the relation of plants to soil were based upon the response of the 

 individual or species. The first serious attempt to organize these into a sys- 

 tem of indicator plants was made by Hilgard (1860, 1906). In a similarly 

 virgin region, Bessey (1891, 1901) also recognized the indicator value of native 

 plants, and especially vegetation for the proper development of agriculture. 

 His ideas of the practical value of vegetational studies stimulated the develop- 

 ment of ecology as recorded in the " Phytogeography of Nebraska" (Pound 

 and Clements, 1898, 1900) and the " Development and Structure of Vegetation" 

 (Clements 1904: 1). In the latter the need of quantitative studies of habitat 

 and community and the importance of succession were first emphasized, and 

 these were made the basis of a definite quantitative system in " Research Meth- 

 ods in Ecology" (Clements, 1905). As a consequence, the way was prepared 

 for the use by Shantz (1911) of the plant community as an indicator with 

 particular reference to succession. In another direction, E. S. Clements (1905) 

 made a searching investigation of the relation of leaf structure to different 

 factors and habitats and laid the foundation for the use of habitat-forms and 

 ecads as indicators. 



The development of the idea that plants are indicators of climate is more 

 difficult to trace. Tournefort (1717) probably furnished the first recorded 

 instance of the idea, when he pointed out that the slopes of Mount Ararat 

 showed many species of southern Europe, while still higher appeared a flora 

 similar to that of Sweden, and on the summit grew arctic plants such as those 

 of Lapland. Perhaps the most important studies of climatic zones of vegeta- 

 tion were those of Humboldt and Bonpland (1805:37), Kabsch (1855:303), 

 Koppen (1884:215), Drude (1887:3), and Schimper (1898, 1903:209). In 

 none of these is there a distinct recognition of the indicator concept. This is 

 likewise true of the formulation of life zones and crop zones on the North 

 American continent by Merriam (1898). His applications of the indicator 

 idea are so numerous and definite, however, that he must be given the credit 

 for organizing the first system of climatic indicators. As to the soil, Hilgard 

 is to be regarded as the pioneer in recognizing the great possibilities of systems 

 of indicators and applying this on an adequate scale, and Shantz as the inves- 

 tigator who has placed the whole matter upon an adequate scientific basis. 



HISTORICAL. 



In a general account of the important steps in the spread of the indicator 

 concept, it appears best to deal only with those studies in which the concept is 

 either evident or actually stated. Even with these the details are reserved for 

 discussion under the various climaxes or applications. There are numerous 

 books and papers on plant-geography, forestry, and agriculture, which have 

 some general relation to the idea. Most of these have contributed nothing 

 tangible or important and for the most part are ignored. A few are considered 

 or mentioned in the proper special sections. Entire justice might demand 

 consideration of the work of Bonnier, Fliche and Grandeau, and Contejean at 

 this point, but for many reasons it has proved undesirable to treat these in 



