608 METASPERMAE OF THE MINNESOTA VALLEY. 
for those that prefer the muddy or sandy shore, or for those 
that dispose themselves in the running water of the outlet or . 
inletstreams. Andas the topography has had its influence upon 
the distribution of the plant-immigrants so they in turn have 
had their reciprocal influence upon topography. By choking 
the ponds with generation after generation of individuals they 
have hastened the disappearance of the water and have then 
themselves either generally disappeared to make room for 
plants better fitted. for the drier condition or have adopted 
more terrestrial habits. And. by clothing the hillsides or 
shading the sides of ravines they have, both directly by their 
interposition, and indirectly through their influence upon rela- 
tive humidity, modified the erosive activities of the water or 
the desiccating activities of the wind. As a foundation for all 
these complex, interdependent phenomena it is clear that we 
must assume the original surface of the till when the valley 
was abandoned by the ice-sheet in its retreat towards the pole. 
Both the general features of the topography and many of the 
special ones are therefore glacial in their proximate analysis. 
It must not be forgotten, however, that preglacial forces and 
conditions, by hollowing out the ancient gorge of the Minne- 
sota and by determining its sea-level at different points are of 
similar importance in the final comprehénsion of the general 
and special topography. But, so far as concerns the more 
modern times it is clear that a base-line for historic discussion 
is very properly derived from the period when the glacier 
left its mass of undulating till to be worked upon by the rains, 
sunshine, winds, plants, animals, rivers, temperature of the 
succeeding years. 
Under the second division of the subject—the action of the 
glacial period and its results as shown in the modifications of 
plants—there is little that need be added in so general a discus- 
sion. It has already been shown how distribution, under con- 
ditions variably favorable, will induce pressures and tensions; 
how these tensions will themselves move from one position to 
another; how the weaker plants are ejected to the periphery of 
formations where they enjoy less favorable conditions of nutri- 
ment, perhaps, but more favorable conditions of competition; 
how the tensions and competition are modified by various direct 
and indirect forces, chemical, physical or biological; how in the 
southward and northward oscillations of a plant-population, 
modifications of tensions, types, localities, habitats, physiology 
would ensue, and how the recurrence of glacial epochs accentu- 
