292 



PRINCIPLES OF ANIMAL BIOLOGY 



species in any given type of habitat tend to follow one another in a cei-tain 

 order. This order of change of species is correlated with the order of 

 change in the environment and in general is a change from instability 

 toward a condition of equilibrium. 



Plant communities of certain kinds have advantages for a study of 

 succession, because in them the remains of earlier species are preserved. 

 Thus, when peat is dug from bogs for fuel, successive layers of the mate- 

 rial are well enough preserved to indicate what plants produced them 

 (Fig. 259). The general order in such places seems to be aquatic plants, 

 sedges, grasses, bog shrubs or alders, bog trees (larches), dry-ground 



Fk;. 259. — Section through peat bed. The type.s of phmts that produeed it at the 

 (iiffeieiit levels can be determined from the remains. {From Weaver and Clements, "Plant 

 Ecology r) 



forest. Successions starting on bare land begin with herbs and pass 

 through shrubs to forests. 



Animal successions are less easily ascertained and less simply described, 

 l^rief cycles may be demonstrated, such as the succession of protozoan 

 types in laboratory cultures. These cultures are at first dominated by 

 flagellate protozoa, then several types of free-swimming ciliates (nearly 

 always in a given order), then the stalked ciliate \'orticella, and finally 

 the amoebalike species. Following these protozoa come the simple 

 plants or algae. Another .succession much longer than the above, but 

 short as compared with the plant series described, is that \\'hich is started 

 by the wood-boring beetles that live in the trunks of living oak trees and 

 gradually kill them. Larvae of another family, the long-horned beetles, 

 enter the dying trees, utilizing the burrows of their predecessors. Dai-k- 

 ling beetles come next and leave the bark separated from the wood by 



