175 
The temperature conditions here described are those as- 
signed by Whipple (’98) to lakes of the temperate type and 
third order, those whose bottom temperatures are seldom very 
far from their surface temperatures, and in which there is con- 
siderable vertical circulation at all seasons when the surface is 
not frozen. At no place in the region examined by us has a 
depth been found sufficient to permit the occurrence of a 
stratum of cold water at the bottom unaffected by the vertical 
circulation and warming process in the surface regions, such, 
for example, as has been found by Birge (’97) in Wisconsin 
lakes. This absence in the river environment of the “thermo- 
cline” and of summer and winter periods of stagnation in 
lower levels, marks another point of contrast between the river 
and some lakes as units of environment. 
The temperature conditions in the bodies of water adja- 
cent to the river do not differ to any considerable degree from 
those here diseussed. The limited extent, greater amount of 
vegetation, shallower waters, or greater access of spring water 
in some of these will cause slight variations from the condi- 
tions found in the river. 
The ice conditions attending the winter minimum are of 
profound biological significance, since they produce important 
alterations in the winter routine. As a result of the presence 
of an ice sheet on a body of water, the temperatures become 
more constant, the mingling of waters due to winds ceases, the 
usual processes of aeration are interrupted, and the ‘propor- 
tions and amounts of the gases dissolved in the water may be 
very much altered, the degree of the change depending upon 
conditions such as the completeness with which the surface is 
sealed by the ice, the amount of sewage, the relative abundance 
of plant and animal life, the duration of the ice, and the exist- 
ence of currents. So far as our observations goat Havana, the 
stage of stagnation attended by the destruction of the animal 
life which is sometimes found in small lakes is rarely realized 
in this environment. Several reasons may be assigned, the 
principal one being being the instability of river levels in the 
