46 Bulletin Wisconsin Natural History Society [Vol. 12, Nos. 1 & 2 
many places the ice beneath the dense forest is not less than a 
thousand feet thick” (Russel, 1893, p. 76). In view of these 
observations of Russel upon the heavy spruce-alder forest cov- 
ering parts of Malaspina Glacier, it seems not improbable that 
during the glaciation of the Wisconsin ice sheet a dense plant 
growth might have existed on top of the border of the ice lobe 
itself as well as a short distance below the edge of the ice. The 
cold, damp substratum of such an habitat would lead one to 
suspect that the plant growth occupying it would be mostly 
xerophytic, and possibly not essentially dissimilar to the flora of 
the Cassandra-Tamarack-Spruce Associations of Ridgeway Bog; 
Russel found, in part, exactly such a flora on Malaspina Glacier. 
The probabilities seem to be, therefore, that the biota of these 
two associations existed on the moraines over the Wisconsin ice 
sheet and just beyond its borders during maximum glaciation, 
and that later, upon recession of the ice, it adjusted itself to the 
bog habitat — also a habitat with a cold, damp substratum adapted 
to xerophytes. However that may be, the biota of the Cassandra- 
Tamarack-Spruce Associations shows northern affinities and has, 
undoubtedly, had a northern source of ingression ; it may have been 
driven south to the southern edge of the ice sheet, but there is no 
conclusive evidence that a major part of it ever occupied the area 
south of the Wisconsin drift. 
The biota of the Cedar-Balsam-Hemlock and Hillside Associa- 
tions consists in large part of plants and animals of eastern affini- 
ties. Many of the species in these associations are from a north- 
eastern center of dispersal; among some of those which appear 
to have had a northeastern source of ingression are Pinus resinosa, 
Pinus sPohus, Tsuga canadensis, Abies balsamea, Thuja occiden- 
ialis, Acer spicatuni, Sorbus americana, Clintonia borealis, Epigaea 
repens, GauUheria procumbens, Mitchella repens, Nemopanthus 
mucronata, Poptdus grandidentata, Prunus pennsylvanica, Bomhy- 
cilla cedrorwn, Dendroica magnolia, and Erethizon dorsatuni. 
Many of these forms probably migrated north along the Appa- 
lachian Mountains from a southeastern center of dispersal (Adams, 
1905, p. 59), thence west and southwest from this secondary 
(northeastern) center of dispersal; probably among such are 
Pinus strobus, Tsuga canadensis, Epigaea repens, GauUheria pro- 
cumbens, and possibly others. Other species in these associations 
