344 MORRIS M. WELLS. 



are to be found only in the muddy-bottomed pools. Moreover 

 the two groups of fishes seldom encroach upon one another's 

 environments, even though they be directly adjoining as in 

 streams where ripples and pools alternate. 



If fishes are kept to their own environments by rather definite 

 reactions to conditions, then should the conditions begin to 

 change, as for instance, the pool begin to fill with resulting 

 changes in dissolved gases, temperature, light, current, etc., 

 the fishes will move away and will not settle down until they 

 reach, more or less accidentally perhaps, but also by definite 

 reactions at times, another set of conditions, that is similar to 

 that which they formerly inhabited. Furthermore, the con- 

 ditions might change slowly, or but a little, so that the young 

 fishes and the adults of the more sensitive species would be the 

 first and perhaps the only ones to leave. In this case the result 

 would be the slow, partial and perhaps complete depopulation 

 of the area, provided the adverse conditions prevented the 

 entrance of other fishes from the outside. 



Shelford and Allee ('130) found that certain species of fishes 

 will turn back quite definitely from concentrations of carbon 

 dioxide as low as 5-7 c.c. per liter, and from oxygen as high as 

 .7-1 c.c. per liter. This being the case, the above illustration 

 need not be considered as at all hypothetical, for concentrations 

 such as those indicated may be found in certain parts of nearly 

 any system of rivers and lakes. Often the adverse conditions 

 are seasonal in occurrence, or they may appear only when certain 

 factory and sewage wastes are introduced into the waters (Marsh, 

 '07), but whatever the time or cause, the result must be the 

 partial or complete depopulation of the area so long as the 

 adverse conditions continue. 



It should be noted in this connection, that small variations 

 (e. g., 5-10 c.c. CO 2 per liter) from the normal, probably in many 

 instances, produce in the long run effects similar to those pro- 

 duced by greater variations (e. g., 25 c.c. CO 2 per liter) in rela- 

 tively short periods. From the standpoint of ultimate persistence 

 of the fishes, it makes little difference whether they die within 

 an hour, a week, a month, or do not die at all, but merely stop 

 reproducing successfully, the final result must be the same, 



