372 r. E. CLEMENTS SIGNIFICANCE OF PALEO-ECOLOGY 



space and in time and from association. The former enables ns to close 

 many a gap in the fossil record and to fill in the areas ontlined b}^ the 

 known distribution of dominants. Inference from association, for ex- 

 ample, aided by phylogeny, makes it all bnt certain that swamps of reed- 

 grass, bulrushes, and cattails existed as far back as the Cretaceous, though 

 Phragmites is the only one of the three dominants recorded for that period. 



The most recent is the method of cycles which gives promise of becom- 

 ing one of the most important. It is perhaps too soon to insist that cyclic 

 processes are universal in time and in space; but the great mass of evi- 

 dence from geology and climatology is matched by an increasing body of 

 facts from biological succession. The most fertile of all these assump- 

 tions is that climatic changes recur in cycles of various intensity and 

 duration. It is a matter of congratulation tliat climatic cycles can be 

 studied by their effects almost as well in the past as in the present. This 

 is particularly true in peat bogs and in badlands where fossil trees, alter- 

 nating or recurring deposits, and cycles of erosion furnish a wealth of 

 virgin material. Fossil wood is fortunately of the widest occurrence, and 

 it is i^roposed to study the annual rings of fossil trees from those of 

 recent peat bogs back through the Pleistocene and Tertiary into the later 

 Mesozoic. Preliminary studies in the Pleistocene, Miocene, and Eocene 

 already suggest the existence in these periods of a sun-spot cycle identical 

 with that demonstrated by Douglass, Huntington, Kapteyn, and others 

 in the trees of today. Similar cycles seem to be recorded in the rings of 

 sagebrush, saltbush, and other desert shrubs, and these have been used in 

 studying shifting cycles of erosion in badlands. 



Like its modern representative, paleo-ecology must focus its attention 

 on the three great and interrelated problem complexes, namely, the hab- 

 itat, the biotic community, and the development of the latter. While it 

 comes first causally, the habitat actually must wait on biological inter- 

 pretation, as its biological effects are about all that remain of it. Thus 

 it becomes the biome, or mass of plants and animals of a particular area 

 or habitat, on which attention must first be fixed. The direct outcome 

 of this is to reveal the successional movement, and on this as well as on 

 the adaptive features of species and genera must be based our assump- 

 tions as to geological climates and soils. 



In phytoecology the concept advanced in 1901, that the plant com- 

 munity is a complex organism, with structure and functions and with a 

 characteristic development, appears to be slowly making its way. The 

 admission of animals into the community with full rights and privileges 

 promises to open a new period in synthetic ecology. In paleo-ecology the 

 concept of the biome, or biotic community, seems to have peculiar value, 

 as it directs especial attention to the causal relations and reactions of the 



