382 Wisconsin Academy of Sciences, Arts, and Letters. 
the seasonal representation are not identical. I^ccording to 
the stage of life history a species may attain a periodic abund¬ 
ance,—generally during the warmer months of the year. 
Again, owing to the peculiar conditions of winter,—that is, 
ice on the shore line, and a more equal distribution of gas in 
the aphytal area,—the horizontal distribution of the species 
frequenting these regions will vary from that of other pe¬ 
riods of the year. 
Seasonal representation or seasonal succession of dominant 
faunal types is apparent to the collector. More will be said 
of this later, but it is desirable to call attention to the fact that 
in a quantitative study the seasonal element plays a very im¬ 
portant part. To cite an instance, the collections made during 
late June and July show a remarkable scarcity of the case¬ 
building Trichoptera; yet collections made a month later will 
show them as the dominant faunal type-of the plant zone. 
The same is true of Platyhelmia, of Annelida in general, of 
Hydra, Ephemerida, etc. This feature has had to receive at¬ 
tention in averaging the quantitative results. 
2. Lake Physiology 
A. EXTRINSIC FACTORS 
Molar Agents .—Under this term are grouped eroding agents, 
such as winds, waves, currents. Waves are the result of sur¬ 
face disturbances caused by winds. The size of the wave is 
proportional to the wind pressure on the surface, and to the 
distance traveled, less so to the velocity of the wind. On Lake 
Mendota the largest waves do not exceed 1.5 meters in height 
and may reach a length of 15 m. in storms. 
The daily winds are an important feature of lake physiology. 
These light winds are frequent, occurring at various times dur¬ 
ing the day, even during summer calms, and may persist for 
brief periods. They may be of varying intensity, frequently 
covering the lake with choppy, crested waves of 7-8 meters 
length and half a meter’s height. 
In their effect on the lake physiognomy the waves are of pri¬ 
mary importance. On the sandy shores there is a constantly 
moving him of sand. Rock and gravel shores are clean-swept, 
and the floating rubbish pulverized. On gravel shores pebbles 
and stones are torn loose and form a movable substratum with 
