176 
FRANK C. GATES 
thing in succession is the replacement of the dominant species of the 
existing association by those of the invading association. Changes 
in the flora of the ground layer are secondary events. 
With these facts in mind, one can not dodge the issue that, in a 
given local area, invasion takes place under the existing conditions. 
With the development of the invading species the evaporation con- 
ditions of the ground layer are changed, which is usually accompanied 
by a change in the ground flora. In other words, a change of evapora- 
tion conditions of the ground layer is the result and not a fundamental 
cause of succession. 
Summary 
1. Experimentation was carried on in the vicinity of Douglas Lake, 
Michigan, during the summers of 1915 and 1916, with 42 standard 
Livingston atmometers. The usual methods of experimentation and 
calculation of data were employed. 
2. As the investigation was carried on in a small area, the influence 
of edaphic factors was not obscured by the action of broad climatic 
factors. 
3. Invasion, which is the initial stage of succession, must take 
place under the conditions already existing. 
4. The change of conditions coincident with mesophytic succession 
brings about a decrease in the rate of evaporation in the ground or 
chamaephytic layer. 
5. In a given area, the differences in the amount of evaporation 
under which seedlings develop are largely due to the surrounding 
vegetation, which by its size and density controls the evaporation 
beneath it. 
6. The complete range of evaporation conditions present in this 
region, namely, from bare ground to the mature forest, is completely 
within the physiological limits of the seedlings of Acer saccharum, 
Pinus strohus, Pinus resinosa, and Thttja occidentalis. Given suitable 
soil conditions, maple seedlings will develop under evaporation con- 
ditions at least 337 percent more xerophytic than the normal hardwood 
forest, or 400 percent more xerophytic than the very dense forest. 
7. Within their soil requirements and in the presence of light, 
the establishment of the pine, beech-maple and Thuja bog associa- 
tions — three of the most important tree associations in northeastern 
North America — is independent of any particular conditions of eva- 
poration. Consequently a decrease in evaporation is not a prere- 
