CORRELATION OF CLIMATIC CYCLES AND SUCCESSION. 343 



the composition of the community. Such changes may affect the dominance 

 and be strikingly evident, or they may vary in significance to a point where 

 they can be determined only by minute quadrat methods. 



The changes which occiu- in the clisere that marks a glacial advance or 

 retreat consist of a change of general composition as well as of dominance. 

 This is not merely the result of invasion in one direction and destruction in 

 the other. It is also in some degree a consequence of the adaptation and 

 variation of species, i. e., of evolution. When the latter exceeds invasion, 

 and finally becomes paramount and universal, a new flora results and a new 

 vegetation era is inaugurated. Pulsating invasion is characteristic of glacial- 

 interglacial cycles. Evolution appears to be the consequence of a glacial or 

 other restriction of life which is followed by a rapid differentiation of new 

 habitats and a correspondingly rapid adaptation of the vigorous but plastic 

 survivors. 



Relation to the diflferent climatic cycles. — ^A definite correlation with the 

 various cycles is impossible as yet, partly because too little is known of the 

 cycles themselves, especially as to the intensity of the changes. The chief 

 difficulty lies in the fact that we have had no experimental study of responses 

 of vegetation to cycHc phases, and only a few observational studies, such as 

 those of Douglass and Huntington on tree-growth and of Arctowski and others 

 upon crop production. In the case of both observation and experiment, exact 

 correlations will be possible only after a detailed study by means of instruments 

 and permanent and denuded quadrats of both native and culture vegetation. 

 The minor cycles of 2.5, 11, 21, and 35 years, and the volcanic episodes, must 

 clearly affect only the fxmctions, growth and reproduction of the individual 

 plant and the fimctions and development of the commimity, as well as its 

 composition. This is probably the case likewise with the major cycles of 

 150 and 300 to 400 years. The grand cycles of 1,000 and 10,000 to 20,000 

 years would probably bring about the frequent or regular change of climaxes 

 by pulsating invasion, with some concomitant evolution of new forms. Both 

 these results would be characteristic of cooled or glacial and interglacial 

 epochs. The grand deformation cycle is marked by the development of a 

 new flora and vegetation, and hence is coextensive with an era. 



With respect to succession, chmatic cycles produce seres only when they 

 result in destruction and denudation, either directly, as in the case of excessive 

 drouth or freezing, or in connection with topographic features, as in glacia- 

 tion, flooding, etc. Minor svm-spot cycles would affect the rate of successional 

 development, accelerating or retarding it merely, as is also true of cycles of 

 150 and 300 to 400 years. The cycles of 1,000 and 10,000 to 20,000 years 

 would probably produce cliseres, which are the striking characteristic of each 

 glacial and interglacial stage. Finally, the grand deformation cycle would be 

 represented by the eosere of each era, with its complement of cliseres, coseres, 

 and seres. 



