60 Transactions of the Academy of Science of St. Louis 
— fishes, amphibians, reptiles, birds and manimals which feed 
on the smaller forms. 
When the importance of interrelation of aquatic forms as 
forage for other animals is realized, one can look briefly into 
another relation in the aquatic association. Among the larger 
forms there is much competition for food, the smaller members 
of a species serving as food for the larger ones. There is no 
regulated provision as to what each shall eat, but certain natural 
laws apply so that there is a survival of the fittest and a retained 
L 
balance of nature. 
Water alone is not enough to produce fish; it is only the 
medium in which fish exist. It also serves as the vehicle, as con- 
sidered before, for transporting necessary chemicals, oxygen, and 
food to the fish, but these are not produced by the water — they 
are supplied to the water through a chemical linkage with the 
watershed or atmosphere. Besides these primary relations of 
water to fish, there are other important habitat necessities for the 
well-being and growth of fishes. 
A fish, like other animals, must have living room; that is, 
a certain necessary volume of water is required for each fish in 
which It can have freedom of movement without too close con- 
tact with other fishes. Some fish prefer close association with 
other individuals, but in that case the school requires living room 
comparable to that required by a larger fish. The volume of 
water for each fish must have certain dimensions within limits, 
for depth is important in that some species require much greater 
depth than others. 
It IS not the nature of fishes to expose themselves to plain 
view, though at times garfish and black bass spend certain times 
noating qmetly at the surface. When not feeding or swimming 
leisurely about, fishes demand a place of retreat in which they 
can he unobserved, unmolested and in proximity to physical shel- 
tn ; I u '''^^' "'"'^^ ^^ the individual residence of a fish, 
to sei /^ '"'' '^''^^^ ""^"^ ^^^'"^^'^ ^' f^<^^ ^hich ^t can dart 
<;en«:.^t ^7-^' Sediments in water furnish cover, in a certain 
f Ir ' . "' r^^ ' ^'^ ^"^^^^^^^ ^'^^ without ; depth of water, 
,JJ r"l ' '''° '""^^^ ^^ ^°^^^' t)ut the most satisfactory 
afforH ! f ' '^' ^°"^d^^s or general debris. These not only 
but if den '' '"'!^ ^'^''^ '^'' ^«-^ -^d colors of fishes blend, 
physical c:rcratit™^^^^^^ ^°""^^ ^^^ ^^^' ^"^ ''''''''' 
r 
