150 THE STORY OF FISH LIFE. 



retirement we know as "hybernation." Ex- 

 tended observation has shown that extremes of 

 heat are followed by a similar retirement on the 

 part of many animals, so that we may discuss 

 the facts herein concerned under two heads : (1) 

 winter sleep and (2) summer sleep. 



The winter sleep seems to be gradually in- 

 duced by the reduction of the temperature, and 

 to be sustained so long as the low temperature 

 continues. The desire to sleep felt by ourselves 

 on exposure to extreme cold is well known, as 

 are the fatal effects which follow any yielding to 

 this desire. 



The carp amongst the fishes is one of the most 

 familiar instances of winter-sleepers. In winter 

 great numbers bury themselves in the mud 

 amongst the roots of plants, where they remain 

 torpid for many months. So, too, does the 

 tench. 



The facts concerning summer sleep are much 

 less familiar. The drowsiness that overcomes us 

 on a hot summer's day will naturally be recalled 

 in this connection, and w^e may even proceed to 

 connect this with the similar inclination to sleep 

 under the influence of extreme cold. To suppose 

 that in either case the temperature alone is the 

 cause of this deep sleep — chill-coma and heat- 

 coma — would be to fall into an error. This deep 

 sleep is rather a way provided by Nature as an 

 escape from famine. Excessive cold and exces- 

 sive drought alike cut off the food supplies, and 

 drought, in the case of many fishes even the 

 element in which they live. The African mud-fish 

 {Protopterus) will afl'ord a case in point, illustrating 



