CONCEPT. 29 



logical and for the most part can be determined only by experiment. Such 

 complex physiological processes as growth and reproduction are exceptions 

 inasmuch as they are subject to direct observation. Consequently they are 

 among the most valuable of indicator evidences. Structural responses are 

 the most visible of all, but their exact use is the most difficult since they stand 

 at the end of the process initiated by the causative factors. Structure also 

 possesses a well-known inertia, as a result of which it may register the impact 

 of factors but partially or slightly. Moreover, the adaptation to the habitat 

 may be made in the tissues of the leaf without affecting the gross features to 

 an appreciable degree. A plant may show the most exact response to chang- 

 ing conditions by the behavior of chlorenchyma or stomata, and yet reveal 

 no sign of this in its outward appearance (E. S. Clements, 1905). 



The interpretation of indicators is profoundly affected also by the double 

 complex of factors and plants. The species of a community do not always 

 register the same response, nor do they respond to any one factor in the same 

 degree. The habitat itself is still largely a puzzle, and it is often difficult to 

 assign well-marked effects to definite causes. The behavior of individuals, 

 though manifestly of less importance, is not without its difficulties. It is 

 impossible to tell at present whether the varying behavior of individuals of 

 the same species is due to individuality or to minute differences in the habitat. 

 Hence, the problem of indicator values is chiefly one of analyzing the factor- 

 complex, the habitat, and of relating the functional and structural responses 

 of both the plant and community to it. This then makes possible the accurate 

 employment of indicators in practical operations. 



Animals as indicators. — Since their response is direct, plants are the best 

 indicators of physical processes and factors. They are by no means unique 

 in this respect. Animals likewise show direct responses to physical conditions 

 and to this extent serve as indicators of them. For a number of reasons they 

 are inferior to plants, however. The chief reason is that their significance is 

 subordinate to that of plants because the latter as food-supply usually con- 

 stitute the controlling factor. In other words, animals are as a rule indicators 

 of plants more directly than of physical conditions. Their mobility makes 

 the control of a particular habitat or set of conditions less absolute, especially 

 with land animals. With the exception of insects, land animals are much less 

 abundant than plants, and the indications of an animal community are much 

 less complete and definite. Finally, our knowledge of the ecology of animals is 

 much less than that of plants, especially with reference to factor control and 

 succession. In spite of all this, however, animals do have great indicator 

 value, second only to that of plants. While the time has not yet come for an 

 adequate treatment of them in this connection, they are taken into account 

 at various points in the text. Indeed, any other course would be illogical in 

 view of the conviction that the complete response to habitat is the biome, or 

 community of both plants and animals. 



Plant and community. — It has already been suggested that the individual, 

 the species, and the community are all involved in the indicator concept. 

 Each of these has its own value, while all of them must be taken into account 

 sooner or later. Up to the present, the species has almost monopolized the 

 role, though the work of Shantz (1911) in particular has emphasized the impor- 

 tance of the community as an indicator. In constructing a complete scale of 



