222 MORRIS M. WELLS. 



the discussion of the data shows that the phenomena outlined 

 receive much support from the work of other investigators and 

 that the environmental factors which are important to fresh 

 water fishes are probably of importance to many, if not all, 

 other organisms as well. 



The investigation has been carried on at the University of 

 Illinois, in Professor V. E. Shelford's laboratory. The work 

 has been done in connection with another line of inquiry regarding 

 the reactions and resistance of fishes to salts. The results of 

 this second investigation will appear as the second paper of 

 the series. 



II. APPARATUS AND METHODS. 



Two general types of experiments have been run, namely, 

 reaction experiments, and resistance experiments. 



A. REACTION EXPERIMENTS. 



This method of experimentation was devised by Shelford 

 and Allee ('13) and may be designated as the "gradient method." 

 In brief the procedure is as follows: A solution gradient is es- 

 tablished in an observation tank, the fish introduced, and its 

 movements graphed. The graph, together with notes taken at 

 the time, makes up the experimental record. Fig. i shows 

 the type of tank used. A similar tank was used by Shelford 

 and Powers ('15) in their experiments with marine fishes. A 

 black hood screens the tank, the movements of the fishes being 

 viewed through slits in the front of the hood. 



The tank has a plate glass front and is lighted by symmetrical 

 lights placed above. A plate-glass cover fits into the top and 

 rests against the surface of the water. This cover is useful in 

 experiments with gaseous gradients as it lessens the vertical 

 gradient due to escape of gas at the surface. 



The water flows into the tank through the openings (inlets) 

 in the ends, then toward the middle; at the middle the water from 

 the two ends mixes, the water from each end drifting somewhat 

 past the middle, thus forming the gradient. The water flows 

 out through the exits (outlets) in the bottom and at the top of 

 the tank. An experiment consists of first establishing the grad- 

 ient, and then introducing the fish and graphing its movements. 



