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 

 inlet streams. And as 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 comprehension 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- 



