565 



a decided preference for the CO or treated water. (See Chart I) This 

 does not mean that the fishes were overcome in this end and thus 

 showed an apparent preference only, for the graphs show that in many 

 cases they swam ciuite reguhirly back and forth from one end of the 

 tank ti^ the other but spent the greater part of the time in the CO 

 water. In some instances they actually turned back from the tap 

 water. 



This preference for treated water over the tap water was noted 

 in a series of experiments made later with salts, and at that time it 

 was found that the acidity of the tap water, due to the presence in 

 it of 1 8 c.c. per liter of carbon dioxide, was the cause of the nega- 

 tive reaction of the fishes (see Wells, 'i.Sa)- When the salt ex- 

 periments were made in aerated tap water in which the COo was 

 diminished to 5 c.c. per liter, normal results were obtained. In the 

 carbon-monoxide experiments the fishes were evidently negative to 

 the tap water because of its acidity, which, though comparatively low, 

 was more stimulating to them than was the much more highly fatal 

 concentration of CO. Furthermore, it would seem that the CO 

 antagonized to some extent the action of the COo, for otherwise the 

 fishes would not have shown a preference for the CO end of the 

 gradient unless they are actually positive to CO solutions in spite of 

 their fatal effect. That such selection of fatal environments may 

 actually occur is not impossible, but it would not be safe so to con- 

 clude until the acid factor, referred to above, has been eliminated, and 

 this has not yet been done. However, it is safe to say that CO in solu- 

 tion produces no avoiding reaction upon the part of the fishes used, 

 and for this reason its introduction into natural waters would be 

 doubly dangerous to fishes inhabiting them. 



Generai, Resistance of Fishes 



Whether or not a particular species can persist in a given environ- 

 ment depends, so far as the organism is concerned, upon its ability 

 to detect detrimental changes in the environment and to react to them, 

 and upon its power of resisting such hurtful factors as can not be 

 avoided by the proper reaction. These factors of reaction and resis- 

 tance can never be entirely separated, but their relative importance 

 varies widely with different species. Attached or sluggish organisms 

 m_ust depend to a great extent upon their powers of resistance to 

 tide them over a period of unfavorable conditions ; highly active organ- 

 isms, on the other hand, may seldom find it necessary to put their 

 resistance powers to the test, for they can move away from the dis- 

 turbing conditions if these do not cover too large an area. It is 



