31 6 VICTOR E. SHELFORD AND EDWIN B. POWERS. 



by the lowering of the oxygen content and covering the surface 

 with materials which bury and tend to smother the eggs during 

 development. The number of individuals of a species is never 

 any greater than the breeding grounds can support. Finley 

 ('13) has shown that the number of prairie chickens in certain 

 counties of Illinois is directly proportional to the area of breeding 

 grounds. Likewise the senior author (Shelf ord, '11) has shown 

 that in a series of ponds at the head of Lake Michigan, food 

 fishes are absent where their food is greatest in quantity because 

 the breeding conditions are absent, due to the covering of the 

 bottom with the decaying food of fishes. It is especially note- 

 worthy that the food of the youngest fishes is especially abundant 

 in ponds where the best food fishes cannot breed. This is not 

 due to the failure of young fishes to destroy the small Crustacea, 

 because the same principle holds for ponds in which there are 

 as many crustacea-eating fishes in stages suitable for food fishes 

 as in stages suitable for only non-food fishes. 



The economic justification for the study of the movements of 

 fishes is two fold. First experimental studies are concerned with 

 the question of the conditions which the fishes select or reject when 

 presented with two or three kinds of water to which they have 

 free access under experimental conditions. Their importance 

 in this connection is based upon the fact that so long as we are 

 concerned with conditions which the fishes habitually encounter 

 in nature, the selections or rejections represent in a general way 

 the physiological character of the fishes and as a rule conditions 

 which fishes reject are detrimental if continued for a long time. 

 Thus, as we shall see later, fishes turn away from water contain- 

 ing hydrogen sulfide and we will show further that they die very 

 quickly when exposed to only a small excess of this gas in the 

 sea water. Here then the fish is so constituted that its behavior 

 and safety are intimately linked. Of course there are exceptions 

 to this rule and it does not hold when we are concerned with 

 changes in conditions which are not commonly encountered in 

 nature. Thus we learn something of the conditions that are 

 probably deleterious to the animals without either killing them 

 or breeding them continuously under the modified conditions. 

 The second justification lies in the fact that we can learn by 



