342 PAST CLIMATES AND CLIMAXES. 



the poles after the glacial periods of the Proterozoic, though during times of 

 maxiniuin sea-invasion and greatest oceanic circulation they must have been 

 compressed into narrow belts lying beyond 75° or 80°. The differentiations 

 of climates in space and in time seem to bear the same essential relation to 

 each other that is found in the case of climax zones and succession. The 

 development of climates, so to speak, produces a zonation of climates which 

 in its turn is an indicator of new climates. This is strikingly shown in the 

 shifting of storm-tracks and climatic belts, as described by Kullmer, Hunting- 

 ton, and Penck (1914 : 281). Thissubject is further discussed in Chapter XIII. 



CORRELATION OF CLIMATIC CYCLES AND SUCCESSION. 



General relations. — Changes of climate may affect vegetation directly or 

 indirectly through their action upon topography or animal life. This indirect 

 effect appears when an increase of rainfall, for example, brings about greater 

 erosion and deposition, and correspondingly increases bare areas for invasion. 

 Topographic initial causes must have produced in the past essentially the 

 same effects discussed in detail in Chapter III. This is likewise true of biotic 

 initial causes, though their influence is felt chiefly in changes of vegetation 

 rather than in denudation, except for the action of man in the historic 

 period. Climatic changes have also been shown to produce destruction and 

 consequent denudation, though such action is exceptional at present. This 

 was probably more frequent with the recvurence of each glacial epoch in the 

 Pleistocene and Permian glacial periods, quite apart from the destructive 

 action of the ice-mass, which was essentially topographic. The primary action 

 of climatic cycles upon vegetation has been the direct modification of the 

 latter without destruction. This effect has been exerted through the climatic 

 control of rainfall, evaporation, temperature, and winds. The first three 

 factors determine in large degree the conditions of ecesis, competition, and 

 reaction, as well as of adaptation, while the winds are largely controlling in 

 the case of migration. 



Responses of vegetation. — ^The changes of climate which constitute a cycle 

 affect vegetation in one or more of the following respects: (1) function and 

 growth of the plant; (2) fimctions of the community, ecesis, competition, 

 reaction, etc., felt chiefly in changes of composition or dominance; (3) change 

 of climax; (4) change of the dominant flora. Fluctuation in the function and 

 growth of individuals is of the least importance in native vegetation, but it 

 is the outstanding response of crop plants. Moreover, the climatic record 

 furnished by the variations in growth of annual rings is of the highest scientific 

 importance. Finally, the amotmt of growth reacts upon competition and 

 dominance, and it is also expressed more or less directly in the seed-production 

 with its intimate relation to migration and ecesis. Hence, the manner in 

 which communities are modified by climatic cycles becomes clear. Changes 

 which increase or decrease seed-production have a corresponding but indirect 

 effect upon ecesis. Ecesis may itself be directly increased or decreased by 

 climatic factors, and competition in its turn may be affected in consequence, 

 as well as influenced directly by the same factors. Reaction is modified as an 

 outcome of these changes, and it may also be independently accelerated or 

 retarded by climatic action. The final consequence is recorded in changes in 



