CLIMATES AND HABITATS. 359 



Deformational and climatic cycles. — In accordance with the above, the 

 climax habitats of the Cenophytic era corresponded to the climates. The dif- 

 ferentiation of the latter must have begun with the Permian deformation and 

 glaciation, but it was carried further by the major deformational cycles of 

 the Cretaceous-Eocene, Oligocene-Miocene, and PUocene-Pleistocene. All of 

 these cycles were attended by cooled or glacial climates, and from the stand- 

 point of vegetation they mark the main divisions of the Cenophytic era. 

 Each doubtless exhibited grand sun-spot cycles, but the latter have left a 

 clear record only in the six glacial-interglacial cycles of the Phocene-Pleisto- 

 cene. Similarly, climate and vegetation must have shown lesser changes 

 throughout the era, due to major and minor sun-spot cycles and to volcanic 

 episodes. Apart from such volcanic stases as those of the Yellowstone, there 

 is no decipherable record of these, until we come down to 50 b. c, when the 

 big trees of California began to form their remarkable record. If we assume 

 for the whole era the cycles known from the Pleistocene to the present day, we 

 obtain the most plausible picture of its climates and the climatic changes. 

 This can best be understood by extending the cycles of the Human period back 

 over the preceding ones as far as the Cretaceous. It appears probable that 

 cooler and warmer phases, corresponding to the Pleistocene cycles, also 

 occurred in the two earlier deformation cycles. In so far as the climates of 

 North America during the Eocene and Miocene were wanner than at present, 

 the effect of sun-spot cycles was probably less than it is to-day. 



As to the presence of the three causes of climatic change, viz, deformational, 

 solar, and volcanic, there is abundant geologic evidence of all of them, except 

 in the case of major and minor sun-spot cycles. As a summation of aU the 

 climatic effects, the flora has been reUed upon to a large degree, and it seems 

 certain that more exact ecological analysis will give it much greater value. 

 A very interesting beginning in this direction has been made by Sinnott and 

 Bailey (1914:547; 1915:1), and Bailey and Sinnott (1915:831), and their 

 suggestive conclusions furnish the ecologist with promising working hypothe- 

 ses. These, however, must stand the test of development as well as of floristic. 



Periods and epochs. — ^While the geological periods and epochs are useful 

 landmarks because of their general acceptance, they are of secondary impor- 

 tance in tracing the course of succession. This is due partly to the fact that 

 they were not grounded upon vegetation, and partly to the fact that it is the 

 genera rather than the species which are of primary significance in the develop- 

 ment of the eosere (Chamberlin and Salisbury, 1906: 3: 193, 221, 226). The 

 outstanding events from the standpoint of the eosere are the deformation and 

 resultant cooling or glaciation, characteristic of each of the grand deforma- 

 tional cycles (p. 304). It seems probable that each of these was marked by 

 a differentiation and especially a shifting of climatic and vegetational zones. 

 This was the striking feature of the Pleistocene; it was much less noticeable 

 in the Ohgo-Miocene, and at the beginning of the Eocene. The shifting of 

 climax zones and alternes marked the major changes in eoseral development 

 and from the standpoint of succession divides the Cenophytic era into three 

 successional periods, which correspond exactly to the three major deformational 

 cycles. Each of these stages is characterized by a clisere and its corresponding 

 eosere, though we can only infer their com-se for the two earlier cycles. The 

 great abundance of accessible Pleistocene horizons and their vigorous study 



