Muttkowski—The Fauna of Lake Mendota. 
453 
result of condition number one. This region shows the great¬ 
est specialization among lake animals. 
4. Without surface-breathers among animals. 
5. With a plant zone of variable depth (according to clear¬ 
ness of the water) which is barely indicated in winter, but 
luxuriates during summer. Parallel with plant growth a 
great summer proliferation of phytophagous animals. 
6. With a thermal stratification for a portion of the year. 
In part, these conditions apply to rapids and spring outlets, 
especially those pertaining to the fauna. In part, they may 
also apply to streams. We have, therefore, a gradual merging 
of lake to pond and swamp in one series, and lake to swamp di¬ 
rectly. Kapids, springs, and lakes form the water-breathing as¬ 
sociations, while ponds, swamps, and appendant lake shallows 
form the air-breathing associations, with streams occupying an 
intermediate position. 
From the definition given above it will be seen that the term 
‘ ‘ lake ’ ’ for certain bodies of water is inapplicable. At best, the 
term has been used in a relative sense as applicable to any 
broad and enclosed expanse of water. Thus, the shallows of 
streams when spread over large areas have been called lakes, 
although lacking every true characteristic. Again, the vege¬ 
tation character is not always distinct. Shallowing of lakes 
produces ‘‘swamping,’’ a feature which is invariably character¬ 
ized by emergent growth of plants. Yet there are lakes where 
this shallowing process is so gradual that a breaker line or 
rachion is barely indicated on the bottom, and the transition 
from lake to swamp is practically imperceptible. 
The absence of surface-breathers is a good characteristic, but 
is applicable also to the open water of ponds, and rivers, that 
is, to a large extent. Again, stagnation coupled with the forma¬ 
tion of a thermocline may be characteristic of a great many 
lakes; yet so large a body of water as Lake Vaettern in Sweden 
has only a very gradual thermocline and the bottom waters con¬ 
tain oxygen the year round. 
Hence this definition will be found inadequate for some lakes, 
and inapplicable for certain others. But for the “plains” 
lakes it will probably be generally acceptable. Alpine lakes 
have certain special conditions; but even here the conditions 
present some similarities. 
