THE SUCCESSIONAL BASIS. 51 



Secondary species. — This is here used as an inclusive term to comprise all the 

 autonomous species of a community outside of dominants and subdominants. 

 Their subordinate importance has caused them to receive relatively little 

 attention, but their correlation with habitat factors has gone far enough to 

 show that they all possess indicator value to some degree. In a sense, this is 

 thrice removed from the habitat, since in climax communities in particular the 

 conditions to which secondary species respond have been modified by the 

 dominants and then by the subdominants. Secondary species either make 

 minor communities such as clans, e. g., Antennaria dioeca, Meriolix serrulata, 

 Anemone caroliniana, Delphinium carolinianum, etc., or they occur jas scattered 

 individuals in society or consociation. When they form more or less extensive 

 clans which recur throughout an association, their indicator value approxi- 

 mates that of a subdominant. In fact, it must be recognized that some of the 

 most important clans might well be regarded as societies. Or to put it more 

 clearly, some subdominants vary sufficiently in abundance and control from 

 place to place and year to year that they may form societies at one place or time, 

 and clans at another. Apart from these, clans and scattered species have 

 their chief importance in revealing minor differences of habitat within the con- 

 sociation or society. They are often due to small disturbances and to succes- 

 sion in minute areas, and derive their indicator significance from this fact. 

 It is probable that the careful study of secondary species will disclose some 

 indicators of much sensitiveness and usefulness. 



Plant and animal association. — It is desirable for many reasons to consider 

 animals an intrinsic part of the community as a biological unit. The great 

 value of this is that it insures an adequate and correlated treatment of both 

 plants and animals. It does not change in the least the basic relations between 

 physical factors, plants, and animals, upon which their mutual indicator sig- 

 nificance depends. Just as the plant indicates the factors and processes to 

 which it responds, so does the animal serve as an indicator of the plant or 

 community which furnishes it food, shelter, or building materials. The 

 animal also indicates physical factors in so far as they affect it directly. The 

 plant, however, has a double indicator relation by virtue of its response to 

 factors on the one hand and of its control of animals on the other. Since 

 animals are mobile for the most part, the control and the indications afforded 

 by plants are necessarily less definite and exact. While the study of animal 

 communities has gone far enough to provide a qualitative basis for plants and 

 animals as reciprocal indicators, there has been no conscious endeavor to 

 investigate this relation as yet. This is not true of paleontology, however, in 

 which such causal relations as that between grassland and grazing animals 

 have been much used. Even here an adequate and comprehensive system 

 must await a fuller development of indicator values in present-day communi- 

 ties. A preliminary attempt at such a system in both ecology and paleo- 

 ecology is made in Chapter III. 



THE SUCCESSIONAL BASIS. 



Scope. — Since the nature of the habitat and the character of the population 

 are constantly changing in all serai areas, succession is of profound importance 

 in connection with indicators. While the basic rule that plants respond to the 

 controlling factors holds for developmental as well as climax communities, the 

 indicators change as the succession advances. Each stage of the succession is 

 marked by factors which act upon species which react in turn. Hence the 



