584 METASPERMAE OF THE MINNESOTA VALLEY. 
By the assistance of this fact of organised and stratified com- 
petition in the realm of plant society the dynamic relations of 
plants to one another are, in general, to be explained. We no 
longer permit ourselves to look at a grove standing in the midst 
of the prairie as stable or even as quiescent, but we picture to 
ourselves the complex condition of strain which exists in vary- 
ing degree and under different degrees of organisation, between 
the different plants, species and groups of species. Such a dy- 
namic condition should perhaps be recognised in terminalogy 
more than it is and instead of speaking of the northern group 
of plants one should name such a floral element the south-bound 
group and instead of calling by the name of ‘‘southern” those 
plants which occupy a southern range one should refer to them 
as north-bound. For certainly the exigency of existence is 
such for every creature that it welcomes an expansion of op- 
portunity for development. Room for growth is an important 
factor of such an opportunity, and for the plant already estab- 
lished in high northern latitudes this room for growth is to be 
found only by a southward extension. 
General features of plant-distribution. The total number 
of flowering-plant species at present occupying the crust of the 
earth is estimated by De Candolle to be in the vicinity of 
250,000. The mean area of each species is about ;4, of the 
surface of the globe or 45,500 square leagues. Of this number 
the valley of the Minnesota with its 16,600 square miles of 
country contains 1,174 species or about 34; of the whole rum- 
ber. The relationships of this flora can be understood only 
after a general survey of the distribution of plants over the 
whole earth. 
If one should follow any parallel of latitude that might be 
selected until he returns to his starting point he would pass 
through regions characterised by diversities of plant-popula- 
tion. As he crossed rivers, mountain ranges and oceans, the 
familiar plants of one region would become fewer in the 
adjacent region and very likely disappear. When half way 
around the earth from the point of departure our hypothetical 
traveler would find himself in a floral region distinctly different 
from the one of his starting point. This difference would in 
general increase in amount and distinctness inversely with the ~ 
height of the latitude. At the equator or near it the difference 
would be great, while on the parallel of 70° N. lat. or 70° S 
lat. the differences both in amount and distinctness would be 
less. The ‘increase in differences as the latitude decreased 
