252 BOTANICAL GAZETTE [ocTOBER 
from the original home after the glacial epoch, which circum- 
scribed the area of the original dense forest of the Miocene period. 
The outer confines of any particular genus is usually occupied, as 
shown in the maps, by a single species. Nearer the center two 
species are found; still nearer, if the genus is a large one, three, 
and still nearer four, etc. The position of the various shades of 
green on the maps suggests the circles of impulse produced 
when a stone is thrown into a basin of water. Theoretically 
these waves spread circumferentially in all directions, unless they 
meet with obstacles, when they are deflected. Similarly, the 
maps suggest a series of distributional impulses, by which the 
various species of oaks, ashes, hickories, and chestnuts were 
forced out from a parent forest of great density into the area 
left bare by the retreat of the great ice-sheet. The force which 
impelled this migration outward from the original forest, a 
relict of the continental forest of Miocene times, was the 
tension produced by the species associated together and in a 
struggle for supremacy, as regards room, light, etc. This 
struggle for existence must have been intense, as evidenced by 
the great size, height, and straightness of bole of the deciduous 
forest trees. Only two alternatives were left the species com- 
posing this original forest, namely, extinction or migration. 
Fortunately for the forest, an area of little or no tension was 
opened up upon the retreat of the glacial ice. The migration 
from an area of great tension has been and always will be toward 
an area of little tension. 
MacMillan*5 remarks on this point, with reference to the 
character of the Minnesota flora, that it has been shown that, 
while the valley of the Minnesota is geographically central, it is 
by no means botanically central, but, on the contrary, strongly 
southern and eastern. Bessey * has shown that the trees and 
shrubs of Nebraska have come up the Missouri bottoms and 
spread from the southeastern corner of the state west and north- 
west. Mason states that the trees of Kansas show the same 
*S MACMILLAN, The Metaspermae of the Minnesota Valley 758, 759. 1892. 
16 BESSEY, =o forests and forest trees of Nebraska. Ann. Rep. State Bd. of 
Agric. 1899 :7 
