610 METASPERMAE OF THE MINNESOTA VALLEY. 
Conditions 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 svarcely 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 reéchoing 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 180, 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 infiu- 
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 ina 
hundred ways altered the adjustments between individuals, 
species and formations of plants in the valley of the Minnesota. 
Summary. 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. Differences exist between the 
