THE PLEISTOCENE CLISERES AND COSERES. 373 



able to distinguish the static series of zones from their developmental shifting 

 by a separate term. When it is remembered that movement of varying 

 intensity is always going on between two contiguous zones, and that all the 

 zones are constantly oscillating backward and forward with minor and major 

 climatic cycles, it becomes clear that a distinction into static and dynamic 

 cliseres does violence to the unity of the process. So closely does the clisere 

 correspond to the climate, both in terms of climatic differentiation in time as 

 well as in space, that the oscillation typical of climatic cycles is also typical of 

 it. Thus, each major cycle as represented in a glaeial-interglacial sequence 

 shows two movements of the cUsere, the first from colder to warmer climatic 

 zones, and the second, a reciprocal movement from warmer to colder zones, 

 whether of latitude or of altitude. Developmentally, the initial shifting 

 replaces each climax by successive preclimaxes, and the course of each sere is 

 correspondingly shortened. The reciprocal shifting restores the lost stages 

 as postcUmaxes, and the course of serai development in each region is brought 

 back to that which existed before shifting began. As will be at once recog- 

 nized, the initial movement of the clisere is actual retrogression, since forest 

 is replaced by scrub, bog, and timdra, and the latter is overwhelmed to become 

 a bare area. As was repeatedly insisted in Chapter VIII, true retrogression, 

 an actual backward movement from climax to bare area, from the highest to 

 the lowest life-forms, and from a climax habitat due to reaction to an initial 

 bare area without reaction, can occur only vmder the pressure of great climatic 

 cycles. Hence the two phases of the cKsere might be distinguished as pro- 

 gressive and regressive. The distinction already proposed, viz, into preclisere 

 and postclisere, seems less subject to confusion or misunderstanding. The 

 term "preclisere" is applied to the developmental movement as the ice 

 advances, and "postclisere" to the zonal migration as the ice retreats. 



In general, the successional effects of the six glaeial-interglacial cycles 

 would be similar or identical. The only important difference would arise 

 from variations in the amount of advance or retreat. But, while the southern- 

 most advance of the ice in the Mississippi Valley varied to the extent of several 

 hundred miles, the front edges of the different drift-sheets were practically 

 all in the same climax region. Similarly there is evidence that the various 

 retreats were into the same general region, since all the evidence from the 

 interglacial phases indicates temperate climates, either warm or cool. As a 

 consequence, the clisere of one cycle is essentially like that of another in its 

 main features, or, more broadly, the clisere of the glacial period exhibited the 

 same general course during each of the glaeial-interglacial cycles. Hence it 

 will suffice to trace the clisere for the Jerseyan-Aftonian cycle, and to show the 

 essential harmony for the intermediate and final cycles. 



It has already been stated that the zonation of North American vegetation 

 before the first advance of the ice was very similar to that of to-day, if not 

 identical with it. Hence the polar ice-mass of the PKocene must have been 

 bordered by a tundra cHmax, followed successively to the southward by bog- 

 scrub, coniferous forest, and deciduous forest climaxes, except in the region 

 of the Great Plains, where coniferous forest was in contact with a grassland 

 climax. The Rocky Mountain system must have shown a similar series of 

 alpine tundra, bog-scrub, and coniferous forest at least, and probably with 

 scrub and grassland at the base. Moreover, it seems not improbable that 



