338 MORRIS M. WELLS. 



detect and react to carbon dioxide even in quite low concentra- 

 tions quicker than they do to low oxygen. It has already been 

 stated that higher concentrations of carbon dioxide (e. g., 100 c.c. 

 per 1.) are more fatal than o.i c.c. per liter of oxygen. 



VI. GENERAL DISCUSSION. 



There are a number of questions relating to the physiology of 

 fishes that are touched upon by the results of the foregoing 

 experiments, but no attempt will be made to discuss them at 

 this time, for the data are far too incomplete. There are, how- 

 ever, certain ecological and economic bearings, which may be 

 taken up briefly, with some profitable results. 



From the experiments described in this paper, and from those 

 of other workers (Shelford and Allee, '13; Ransom, '66) we may 

 conclude that, in general, the distribution and at times the 

 existence of fishes depend upon two things, namely, (i) the 

 resistance of the fishes to any condition or set of conditions in 

 the environment, that may vary so as to become harmful; and 



(2) the reaction of the fishes to any such varying condition or 

 set of conditions. 



Of these two factors, namely, resistance and reaction, neither 

 can be said to be all-important in any environment, and in most 

 environments the two are inextricably woven together. In the 

 following discussion no attempt will be made to separate the 

 two, but the factor of reaction and behavior will be emphasized 

 over that of resistance, for it seems to me that future investi- 

 gation must show fully, what previous investigation has already 

 indicated (Shelford, 'n; Shelford and Allee, '130), that the 

 reactions of fishes to the conditions of the environment are more 

 vital in determining their distribution and persistence than is 

 their power of resisting adverse conditions. 



With regard to behavior and resistance, the fish life cycle can 

 be broken up into four rather distinct periods: (i) the breeding 

 behavior of the adults; (2) the resistance of the eggs and fry; 



(3) the behavior and resistance of the young fishes; and (4) the 

 resistance and general behavior of the adults. 



i. The Breeding Behavior of the Adults. It is generally known 

 that the eggs and fry are stages of relatively low resistance. It 



