EMPLOYMENT OF PREVIOUS ECOLOGICAL RESULTS 373 



three elements — habitat, plant, and animal. Fortunatety this viewpoint 

 is so new that there are no landmarks or traditions to handicap. It is 

 possible to deal with causes and reactions from a single vantage ground 

 of developmental processes. As already stated, the plant community 

 appears to have unique advantages in tracing the concomitant develop- 

 ment of habitat and biome and in determining the structural responses of 

 the latter. Here, again, ecology is fortunate in that zoologists have but 

 recently turned to the development and structure of animal communities. 

 It is thus necessary to follow the causal sequence and to base the treat- 

 ment primarily on vegetation as the effect of habitat and as a cause in 

 relation to animal communities. The opportunity is also given to test 

 the successional method of vegetation study in its application to develop- 

 ment when animals are regarded as an intrinsic part of the community. 

 This application has already begun both in ecology and paleo-ecology, and 

 this use of successional methods gives every promise for the future. 



The developmental method is based on the universal fact that bare 

 areas of rock, soil, or water, and areas denuded of vegetation by fire, 

 cultivation, erosion, etcetera, become occupied by pioneer plants and ani- 

 mals. These react on the habitat in such a way as to change it in favor 

 of organisms of greater requirements, which then invade and replace the 

 pioneers. This process of reaction and successive invasion continues 

 through more or less definite stages until a final population appears and 

 the climax is reached. The climax once reestablished will maintain itself 

 indefinitely unless a change of climate occurs or the climax is destroyed 

 wholly or in part as a result of external forces. One of the most familiar 

 examples of such a unit succession, or sere, is afforded by a pond or lake 

 in which the submerged plants and associated animals are gradually re- 

 placed by floating plants, and these in succession by reeds, sedges, grasses, 

 and scrub, until, in a forested region, the final forest is reached. Similar 

 cases of biotic succession occur in dunes, badlands, lava flows, burns, 

 fallow fields, etcetera, throughout the world. Similar seres also must 

 liave been abundant throughout geological periods since the Devonian, at 

 least, except for those due to the agency of man. For certain periods, 

 such as the Pleistocene in particular, the Miocene, and the Triassic, the 

 plant remains have recorded the unit successions beyond any question. 

 This is most graphically shown in the peat bogs of Scandinavia and 

 Britain, where two or three successive seres have left a complete record 

 of their plants. Such a successional series may be termed a cosere. It is 

 of the first importance in connecting succession in the present with the 

 same developmental process in the past, and hence in putting the suc- 

 cessional study of ])aleo-ecology on a firm basis. Tlie peat cosere furnishes 

 the best evidence of population sliifting tlirougli climatic clianges, and 



XXVIII— Bull. Geol. Soc. A.m., Vol. 2J), 1017 



