reaches the bottom: when it is released. The process is reversed in the 
vase of the beetles. They clasp a bit of sunken plant stem in order to 
keep them at the bettom. A part of this often separates and is carried 
toward the surface. It is by the innumerable repetition of these processes 
that the mass of finely Gomminuted particies at the bottom of the pond is 
formed. 
COMPARISON WITE LAKES. 
The fundamental difference between this pond and a lake is that of 
dimension. Jt has a smaller area and is not so deep. A pond has no 
abysmai region but has some of the characters of the littoral and pelagic 
regions of lakes. Fiom this fundamental difference, secondary differences 
arise. The changes in level affect the relative depth much more in ponds 
than in lakes. The lowering of the level of a lake one-half meter would 
not affect its fauna to any marked degree, while the same difference in 
level occurring in a pond whose depth was a meter or less would pro- 
foundly influence the organisms inhabiting it. 
The temperature in ali parts of the pond is near that of the atmosphere 
above it. In this it resembles closely the shallow littoral region of some 
lakes, e. g., “barren sheals’ of Walaut Lake (Hankinson, 07). So far as 
observed the difference in temperature in different parts of the pond at 
any given time has not exceeded 2° C. It is practically holothermous, the 
thermocline ard associated phenomena are absent. 
Forel (04) has shown in the case of Lake Geneva, that the littoral 
region is one of variety. This fact is perfectly familiar to all students of 
lakes. In the same lake, one part of this region may be covered with 
rushes (Scirpus), another by water lilies (Nywmphea), another by Potamo- 
geton, while another may be barren sand or rock or an equally barren 
marl bed. 
An indivigual pond lacks this variety. It is in this particuiar that this 
pond and others like it differ most frem the littoral region of lakes. (It 
is more nearly comparable to a limited section of a lake shore.) If Typha 
is introduced, it soon spreads over the whole area, limiting the light, ex- 
cluding other phanerogams, and developing very uniform conditions over 
the entire pond. If a pond is developed on the side of a hill so that silt 
is carried into it, a muddy, barren condition exists over the whole area. 
Ponds in the woods rarely develop aquatic seed plants; the leaves from the 
