78 The Lake as a Microcosm. 
They are probably in all cases either parts of former river channels, which 
have been cut off and abandoned by the current as the river changed its course, 
or else-are tracts of the high-water beds of streams over which, for one reason 
or another, the periodical deposit of sediment has gone on less rapidly than 
over the surrounding area, and which have thus come to form depressions in 
the surface which retain the waters of overflow longer than the higher lands 
adjacent. Most of the numerous “horse-shoe lakes” belong to the first of 
these varieties, and the “‘blufflakes”’ situated along the borders of the bottoms, 
are many of them examples of the second. 
These fluviatile lakes are most important breeding grounds and reservoirs of 
life,— especially as they are protected from the filth and poison of towns and 
manufactories by which the running waters of the state are yearly more deeply 
defiled. . 
The amount and variety of animal life contained in them as well as in the 
streams related to them, is extremely variable, depending chiefly on the fre- 
quency, extent, and duration of the overflows. This is, in fact, the character- 
istic and peculiar feature of life in these waters. There is perhaps no better 
illustration of the methods by which the flexible system of organic life adapts 
itself, without injury, to widely and rapidly fluctuating conditions. When- 
ever the waters of the river remain for a long time far beyond their banks, 
the breeding grounds of fishes and other animals are immensely extended, 
and their food supplies increased to a corresponding degree. 
The slow or stagnant backwaters of such an overflow afford the best situa- 
tions possible for the development of myriads of Entomostraca, which fur- 
nish, in turn, abundant food for young fishes of all descriptions. There thus 
results a sudden outpouring of life,— an extraordinary multiplication of near- 
ly every species,— most prompt and rapid, generally speaking, in such as have 
the highest reproductive rate, -- that is to say, in those which produce the lar- 
gest average number of eggs and young for each adult. 
The first to feel this tremendous impulse are the Protophytes and {Protozoa, 
upon which most of the Entomostraca and certain minute insect larve depend 
for food. This sudden development of their food resources causes, of course, 
a corresponding increase in the numbers of the latter classes, and, through 
them, of all sorts of fishes. The first fishes to feel the force of this tidal wave. 
of life, are the rapidly-breeding, non-predaceous kinds; and the last, the game 
fishes, which derive from the others their principal food supplies. Evidently 
each of these classes must act as a check upon the one preceding it. The 
development of animalcules is arrested, and soon sent back below its highest 
point by the consequent development of Entomostraca; the latter, again, are 
met, checked, and reduced in number by the innumerable shoals of fishes with 
which the water speedily swarms; and the lower fishes, springing up at first in 
excessive ratio, are soon driven back to a lower limit by the following exces- 
sive increase of the higher carnivorous kinds. In this way a general adjust- 
ment of numbers to the new conditions would finally be reached spontaneous- 
ly; but long before any such settled balance can be established, often of course 
before the full effect of this upward influence has been exhibited, a new cause 
of disturbance intervenes in the disappearance of the overflow. As the waters 
retire the lakes are again defined. The teeming life which they contain is 
restricted within daily narrower bounds, and a fearful slaughter follows. The 
lower and more defenceless animals are penned up more and more closely with 
their predaceous enemies, and these thrive for a time to 4n extraordinary de- 
gree. To trace the further consequences of this oscillation would take me too 
far. Enough has been said to illustrate the general idea that the life of waters 
subject to periodical expansions of considerable duration, is peculiarly unsta- 
ble and fluctuating,— that each species swings, pendulum-like, but irregularly, 
between a highest and a lowest point, and that this fluctuation affects the dif- 
