452 Wisconsin Academy of Sciences, Arts, and Letters. 
differing in its larger representation of certain Dytiscidae and 
Anisoptera. 
The character of the ponds therefore appear to be chiefly 
stagnancy, shallowness, and the tendency toward swamping. 
Only the fact that there is open water (generally) in the mid¬ 
dle prevents acidiflcation. Circulation is minimal in the vege¬ 
tation areas, with the tendency toward local centers; that is, 
little isolated patches of warmer and cooler water. In a swamp 
such warm patches would soon form decomposition centers, 
while in ponds the periodical disturbance of the open water in the 
middle is sufficiently strong to suppress such centers. Yet 
even in ponds, especially in dry summers, the vegetation may 
gain the upper hand and cover the entire pond area. In such 
instances acidiflcation is strong, decomposition advanced, and 
the remaining water becomes untenable for the major part of 
the fauna. 
Summary of Lake Characters. 
It is evident from the brief survey of types of aquatic habitats 
that the interrelations are comparatively easy to outline and so 
are the larger differences; but it is extremely difficult to fix the 
precise points at which differences begin. No habitat can be 
set forth as a unit which is well-definable in every respect; I 
have rather attempted to show what might be used as criteria 
by calling attention to the interrelations. Physical and phy¬ 
siological conditions intergrade everywhere; for the environ¬ 
ment is labile. Correspondingly, there is an intergradation of 
biota; for the organism, too, is labile. 
Therefore, in attempting to define a habitat such as the lake 
the intergradation with surrounding habitats must be kept in 
mind and not too strict an emphasis laid on any single charac¬ 
teristic. The following is an attempt to define the lake as an 
aquatic unit, distinct from other aquatic bodies: 
The lake is body of water 
1. Of sufficient depth and area to permit strong wave action. 
2. Without emergent or floating vegetation. This is due to 
condition number one. 
3. With a shore line destitute of plants, except Cladophora, 
and harboring only dingers and burrowers among animals. A 
