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Jackson: Land Vertebrates of Ridgeway Bog 
49 
in the next few decades. We are not yet warranted in drawing 
definite conclusions as to upon what physical and ecological factors 
this northward extension of biota depends but there are strong 
indications that light, humidity, and evaporation, as well as tem- 
perature, play important roles. That a northward extension of 
distribution of certain plants and animals of the prairie and the 
deciduous forest border types is gradually progressing is an in- 
disputable fact. The decision of the causes of this distribution 
can come only after detailed, intensive, comparative field studies 
upon all the forms of plants and animals of this general region, 
supplemented by laboratory experiments, and correlated with 
similar studies in other regions. 
Two very interesting hypotheses have been suggested by the 
investigation which have not been discussed in this paper. One of 
these is the possible order of the different ingressions. The soil, 
and other factors, are so different in various parts of the region, 
and the time has been so long since the last ice sheet receded, 
that it has been impossible to decide the order, in time, of the 
different ingressions for the region as a whole. Were the entire 
region one great bog area, however, the order of ingression fol- 
lowing the retreat of the ice sheet would probably be thus: first, 
the combined biota of the Cassandra-Tamarack-Spruce Asso- 
ciations, which, as we have noted, probably existed on the mo- 
raines over the ice sheet and along its edge during maximum gla- 
ciation; second, the biota of the Aquatic-Sedge Associations; 
and, third, the biota of the Cedar-Balsam-Hemlock Association. 
A second, most interesting hypothesis suggested, and one which it 
may be possible to prove definitely, is the possible occurrence of 
two subspecies (of one species) together in the same region. I 
mean, by a subspecies, a variant of a given species showing dis- 
tinctive morphological characters, usually over considerable 
geographic range, sufficient to warrant its appellation with a 
trinomial. A species may thus be composed of one form, or it 
may be composed of several forms (subspecies). It has generally 
been conceded by most taxonomists that only one subspecies (of a 
given species) can occur at a given locality; for this reason it 
has not infrequently happened that two types of variants of a 
species, found together at a locality, have received the same sub- 
specific identification. Might it not be possible, however, for 
