71] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 71 



then, that the fish are 23.44% more sensitive to light in the morning than 

 they were in the evening. From this one must conclude that the sensitivity 

 of the fish to light increases in the absence of light, which is perhaps not 

 surprising when one recalls that the fish lives at the very surface of the 

 water where it receives at all times the maximum amount of light. This 

 sensitivity to light following a period of darkness was checked in the 

 laboratory by confining a number of fish in the total darkness of a photo- 

 graphic dark room for varying periods of time. Each fish was kept in a 

 separate dish, and the following data were taken (table 12): 



Table 12 



LIGHT INTENSITY IN TERMS OF SECONDS OF EXPOSURE OF THE 



EXPOSURE METER, AT WHICH FISH REACT AFTER CONFINEMENT 



IN DARKNESS FOR VARYING PERIODS OF TIME 



In this experiment the light from a 100 watt Mazda light was permitted 

 to fall upon the fish, and the length of time required for their reaction was 

 noted. Thus one finds that up to a certain point, namely 72 hours, the 

 sensitivity to light increases, and that beyond this point there is practically 

 no change. Confinement in total darkness for 48 hours has increased their 

 sensitivity 11% over what it was when confined for 24 hours; that this 

 sensitivity is increased 21.25% when the fish are confined in darkness for 

 72 hours over that recorded for the 48 hour period, and that the total 

 increase in sensitivity is 30.11% when kept in the dark for 72 hours over 

 that recorded for the 24 hour period. This bears out one of the fundamental 

 principles of physiological ecology: that a factor becomes of increasing 

 importance to an organism when there is the least of it. 



GROWTH OF THE YOUNG 



During the period immediately following the hatching, the young 

 silversides grow rapidly. This growth approximates very closely the 

 surprising total of a millimeter a day for a period of nearly two weeks, this 



