102 KINDS OF INDICATORS. 



Tertiary times. It could hardly have become a universal process until the 

 pastoral phase became general, and its greatest extension has doubtless taken 

 place during the last 1,000 years. The primary processes involved in physio- 

 graphic and climatic changes must have had much the same indicators as 

 to-day, allowing for the differences in flora during the various eras. While 

 such changes seem much greater and more frequent during the geological 

 past than to-day, this is almost certainly the result of a short perspective. 

 With respect to factor indicators, the plant genera concerned during the 

 Cenozoic era were largely those which characterize marked differences in 

 water, light, and temperature to-day, and this was particularly true after the 

 Miocene. During the earlier eras, the genera were mostly different, but the 

 vegetation- and habitat-forms the same. 



The fragmentary nature of the fossil record makes it necessary to emphasize 

 certain existing indicator relations, as well as to employ some not needed in act- 

 ual vegetation. These are derived from the methods of interpretation already 

 discussed. The use of indicators based upon the successional sequence is 

 much the same, except that a single dominant or stage must often serve to 

 denote the presence of the entire sere. Even this is not so different from 

 conditions to-day, since there are many swamps in arid regions especially 

 in which the reed-swamp associes is represented by Scirpus or Typha alone. 

 The method of causal sequence furnishes many of the most striking and sig- 

 nificant of paleic indicators. Habitat, plant, and animal are linked together 

 in a fundamental cause-and-effect relation, in which each one serves as an 

 indicator of the other. The importance of the plant in this relation has 

 been emphasized elsewhere. It may be said to have a double indicator 

 value, since it indicates the habitat directly by its response, and the animal 

 directly by virtue of the control exerted through food and shelter. Thus, 

 while there are numerous examples of definite relations between habitat and 

 land animals, most of these take the plant community for granted. The 

 indicators of cycles comprise both those derived from succession and from 

 causal sequence. In fact, they are the indicators of the grand successions 

 recorded in the clisere and eosere, and consist chiefly of shifting formations 

 and floras. Fossil genera and families often possess great indicator values 

 which arise from their phyletic relationships. While phylogeny must long 

 remain a field for varied opinions, certain great lines of relationship receive 

 increasing recognition, and can be employed with corresponding certainty. 

 Thus, while Juncus is not recorded until the Eocene, the presence of both 

 Carex and Phragmites in the Cretaceous makes it all but certain that the more 

 primitive Juncus was already in existence. In connection with phylogeny 

 and succession, plants may indicate distribution in space and in time as well 

 as the presence of associated dominants (Plant Succession, 352). 



Since the field of indicators has been developed wholly with reference to 

 plants and with particular application to agriculture, the importance of 

 reciprocal indicators has not been recognized. However, in paleo-ecology 

 where the body of definite facts is relatively small, it is of the greatest aid to 

 secure all possible indications from every fact, and to check these by the 

 indications of related facts. Fossil plants and animals constitute the best of 

 reciprocal indicators, but topography and climate are often of great service 

 also. When all four can be employed as indicators in a particular period or 



