100 KINDS OF INDICATORS. 



land animals, and these modified plant and community as a result of various 

 kinds of distrubance. The conception of the biome, or biotic social unit, 

 seems even clearer for past periods than for the present, owing to the lack of 

 confusing detail, especially in the remoter eras. Finally, there is positive 

 evidence of the minor climatic cycles, such as the 11-year sun-spot cycle, in 

 the rings of fossil trees, and of greater cycles in the coseres of peat-bogs. 

 Paleo-ecology is characterized, moreover, by great changes of flora and vege- 

 tation such as are unknown for ecology to-day. These are expressed in great 

 successions, such as the clisere and eosere, which correspond with the grand 

 deformational cycles. 



Nature of paleic indicators. — While all the types of indicators now recog- 

 nized must have existed in the past, especially if the Recent period is included, 

 paleic indicators show one essential difference. This lies in the fact that com- 

 munities were but rarely fossilized, and that the community itself must be 

 inferred often from the merest fragments of its total population. Fortunately, 

 the conception of the community as a complex organism with characteristic 

 parts and processes furnishes an adequate method of interpretation. The 

 great majority of species not only play a definite r6le in the climax or in its 

 development as a dominant, subdominant, or concomitant, but each species 

 also bears distinct relations to other species. When its r61e is interpreted in 

 the light of its vegetation-form and habitat-form, it can be placed in the 

 vegetation with something of the certainty possible in existing communities. 

 As a consequence, the indicator values which have been taken for granted in 

 all the preceding discussion, namely, the indication of other species, or even a 

 whole community or sere by a single dominant or subdominant, play a para- 

 mount part in paleo-ecology. The smallest fragment of a fossil may thus be- 

 come an indicator of the greatest significance, providing only that its generic 

 identification be certain. In the case of plants at least, even this is not 

 absolutely necessary if the vegetation-form or habitat-form be sufficiently 

 distinctive to determine its habitat, and consequent position in climax or sere. 



The methods of interpretation employed in paleo-ecology have been dis- 

 cussed in "Plant Succession" (p. 280), and summarized in a later paper 

 (1918 : 371). Because of its importance for the understanding of paleic 

 indicators, this summary is quoted in full : 



"The methods by which the ecological results of to-day can be carried back 

 into the past have been briefly discussed in 'Plant Succession' and it will 

 suffice to pass them in review here. For the most part these are methods with 

 which the paleontologist is already familiar, since they have to do primarily 

 with the translation of facts from the present to the past. The foremost is the 

 method of causal sequence, already mentioned, with its basic relation of habitat, 

 plant, and animal. This is well illustrated by the occurrence of Stipa in the 

 Miocene of Florissant, which indicates not merely the existence of prairie, but 

 also, of course, a grassland climate and a grazing population. A similar but 

 even more fundamental sequence begins with deformation and passes through 

 gradation, climate, and vegetation to exhibit its final effects in the fauna. The 

 method of phytogeny which has been the most serviceable of taxonomic tools is 

 likewise of great value in the reconstruction of the life-forms and communities 

 of the past. It shares with the method of succession the credit of permitting 

 us to give more and more detail to the bold outlines of past vegetations and 

 vegetation movements. The method of succession is based on the great strides 



