THE PLEISTOCENE CLISERES AND COSERES. 377 



by the climaxes before the ice invasion were reached (fig. 37). Theoretically, 

 the number of seres would be indicated by the number of climaxes, though 

 in the long period occupied by an advance or retreat a hydrosere or lithosere 

 may have occurred more than once in a particular climax. The cosere would 

 be represented by at least one sere for each climax. Thus the zone at the 

 southern edge of the maximtun ice-mass would have been occupied successively 

 by hardwood, conifer, scrub, timdra, scrub, conifer, and hardwood. Thus, 

 the cosere consists of seres whose climaxes are determined by the cUsere. If 

 a complete record of these climax changes had been preserved in the peat, it 

 would constitute a clistase. Such perfection of the record is not to be 

 expected, for even the excellent sections of Scandivanian and Scottish bogs 

 give us no such complete series (c/. Steenstrup, p. 14; Blytt, p. 22; and 

 Semander, Andersson, and Lewis, in the abstract section that foUows). In 

 American bogs the known record is much more imperfect. It is largely com- 

 prised in the work of Penhallow (1896, 1898, 1900; cf. Harshberger, 1911 : 184) 

 on the floras of the Don and Scarborough formations near Toronto, Canada. 

 The first of these represents the maximimi of an interglacial phase, during 

 which many species extended northward beyond their present range, and hence 

 is assigned to a warm temperate climate. The Scarborough beds He above 

 the Don beds, and their species are such as to indicate the cold-temperate 

 conditions which would follow the interglacial maximum as the ice began 

 again to advance. While the evidence is not altogether unequivocal from the 

 standpoint of succession, the current interpretation will doubtless stand until 

 the American record of the Pleistocene has been much more systematically 

 studied. 



Postglacial succession. — ^With the retreat of the Late Wisconsin ice the 

 climax zones moved northward, as already indicated (fig. 37), and finally 

 reached the position which they occupy to-day. In America the record of 

 this movement i^ foimd chiefly in relict boreal species persisting several 

 hundred miles behind the zone to which they belong. A striking example of 

 this is seen in certain canons of the Niobrara in northern Nebraska, where 

 Betvla papyrifera is the dominant tree (Poimd and Clements, 1900 : 69). In 

 northern Europe the record of this movement has been more or less completely 

 preserved in the peat clistase, which has furnished such a fertile field of study 

 from Steenstrup's investigations in 1837-1842 to the present. Blytt, Nathorst, 

 Sernander, Andersson, Holmboe, Fries, Wille, Samuelson, Keilhack, Weber, 

 Reid, Lewis, and many others have considered this evidence from various 

 points of view. An adequate account of their results and conclusions is 

 impossible within the scope of the present treatment. A concise summary of 

 most of the articles bearing directly upon this subject is given in the following 

 section. 



With respect to its cUmate and vegetation, the present, which corresponds 

 approximately to the Hmnan period, is essentially interglacial in character 

 that is, the climax zones probably occupy much the position they did during 

 the majority of the interglacial phases, as well as toward the close of the PUo- 

 cene. The glacio-lacustrine stage which followed the final retreat of the ice 

 was characterized by the "making and unmaking of lakes" (Chamberlin and 

 Salisbury, 1906 : 3 : 395), and hence by the striking production of bare areas for 

 the development of hydroseres. This was felt particularly in and about the 

 region of the Great Lakes, and also in the Great Basin, and its effects can still 



