Pa 
grounds; it imperatively demands that in calculating values a standard 
shall be set which applies to the country as a whole rather than to a mere 
group of individuals (Austin, 17). 
I. Is the animal used in testing the toxicity of a polluting substance 
one of representative sensitiveness? 
The resistance of different useful aquatic animals to polluting oie 
_ stances varies greatly, as does also that of their living food. In the 
case of fish, for example, it is not sufficient to secure for making tests 
any fish that may be convenient. The fish used should be one of the more 
sensitive ones. The tests of toxicity must of course be of a character to 
determine means of affording protection to fish, but not to fish alone; the 
organisms on which they feed are commonly more sensitive than the 
fishes themselves. 
The following table gives an estimate of the relative resistance of 
several widely distributed species of North American fishes. While it 
needs careful verification by new methods, it will serve as a rough pro- 
visional guide. It is based largely on death in waters containing little 
oxygen and much carbon dioxide. Since fishes rank differently in 
resistance according to the poison in which they are killed (Shelford, ’17 
p. 408), the immediate need for further investigation is obvious. 
TABLE I 
Indicating the relative resistance of a very sensitive minnow and of some 
common game-fishes of the eastern and central United States and of the goldfish. 
The resistance of the least resistant species is arbitrarily taken to be unity. 
(After wos 18, beste SB the ¢ or ange- spotted sunfish and the goldfish.) 
Relative J : ; Relative 
Bur ies of fish resistance | Species of fish mesistnce 
; | | 
Labidesthes sicculus | Ambloplites rupestris 
(Brook silverside ) alts | (Rock bass) 10. 
Mozxostoma aureolum | | Perca flavescens 
(Red-horse) 2.3 (Yellow or American perch) 10. 
Catostomus commersonii | | Lepomis humilis 
(Common sucker ) 2.4 (Orange-spotted sunfish ) 12. 
Micropterus dolomieu levi Carassius carassius 
(Small-mouthed black bass )) 5. (Goldfish or Crucian carp) 12. 
Micropterus salmoides Lepomis cyanellus 
(Large-mouthed black bass )) 6. (Blue-spotted sunfish ) ibs 
Pomozis annularis »,Ameiurus melas ’ 
(White crappie) 8. (Black bullhead) 45. 
Pomocis sparoides 
(Black crappie, Calico bass) | 8. 
Choice of Animals used for Tests——The common suckers, Catosto- 
mus commersonii Lac. or Moxostoma aureolum Le S. are recommended 
