610 METASPERMAE OF THE MINNESOTA VALLEY. 



Conditional of the present. Clearly all of the phenomena of 

 the distribution of plants in the valley of the Minnesota are 

 now discovered to be phenomena of evolution. Does this evo- 

 lution go on before us? The question scarcely needs an an- 

 swer, so evident is it that such forces as have always been at 

 work in the distribution of plants are at work to-day. Cer- 

 tainly there is not the advancing glacier of 8,000 years ago, 

 but in other ways the struggle is directed so that pressures 

 and tensions are set up throughout the region of our study. 

 The reechoing influences of the past, the constant struggle of 

 the present — these are the two deeper factors of distribution 

 that demand our careful investigation. To-day we find this 

 struggle organised under the different degrees of tension and 

 we observe constant, although slight, changes in the plant 

 population. The influence of man is now more important than 

 the rest of the biological influences. Through his interposi 

 tion a large portion of the prairie and bottomland has been put 

 under cultivation. In 1890 the basin contained 327,852 human 

 beings, or an average of 20.5 to the square mile. The activity 

 of the human population, by importing new plants and estab- 

 lishing them, by decimating the number of originally estab- 

 lished individuals in some of the species, by permitting a 

 group of 130, or more, alien plants to escape during the last 

 forty years and establish themselves in varying degrees, has 

 had a profound influence upon the plant-physiognomy of the 

 valley. Among the biological factors of modern times the 

 activity of man is conspicuous. Not only directly has he influ- 

 enced the distribution, but indirectly through the importation 

 of new animals, such as sheep, cattle, swine, fowls or horses, 

 that, in turn, by their activities, have modified the aspect of 

 the plant-population He has exterminated many of the wild 

 animals, notably the bison, which had a peculiar influence 

 upon the distribution of plants, different from that of the 

 domestic animals. He has planted trees, felled them, burned 

 the underbrush, torn up the prairie with the plow and in a 

 hundred ways altered the adjustments between individuals, 

 species and formations of plants in the valley of the Minnesota. 



Sunimary. The distribution of plants in a natural region 

 presents many problems. These are found to be complex and 

 demand for their solution a wide range of collateral informa- 

 tion. Plants are found everywhere maintaining dynamic rela- 

 tions with each other. These relations have much to do with 

 the facts of their distribution. Ditferences exist between the 



