TROUT WATERS AND CARP WATERS 



of functional action which follows from it. They are 

 animals of variable temperature. The interior tem- 

 perature produced in them rises only slightly above 

 the temperature outside. The case of the tunny and 

 the large scombers, whose massive bodies become 

 considerably warmer when they are swimming, is an 

 exception. Usually the temperature of the creature 

 depends upon that of the water, is modelled upon it, 

 varies when it varies, and the organic functions are 

 arranged in such a way as to work at different tem- 

 peratures. There appears to be in every case a certain 

 optimum at which the organic well-being is complete. 

 Below and above it, the functions are restricted, deviate 

 and even stop, and the organic machine finds itself 

 incapacitated in its work of living. In the carp, the 

 capacity to move freely begins when the water is at a 

 temperature of about 50 degrees; the capacity to 

 feed and digest at about 59, to breed at about 68; 

 and above 77 the vitality again begins to be affected 

 but in the opposite direction. This is a dependence 

 from which the higher vertebrates are very largely 

 exempt. 



Fishes show us, better than land animals, the 

 essential quality of every living body. The creature, 

 in fact, is really an automaton which has within 

 itself, fashioned and built up by its own efforts from 

 the embryonic stage, all the wheels of its vital mechan- 

 ism. But it is incapable of putting those wheels into 

 action by itself alone. There must always be some- 

 thing to start it, to keep it on the track. These im- 

 pulses come to it from its environment, from the 

 matter and energy of which that environment is made 

 up. It is placed under their control and can only 

 act in accordance with that control. 



None the less, it is constructed in order to receive 

 them, even to solicit them. It is an automaton pos- 

 sessed of senses. It has apparatus, organs of sense, 

 which enable it to perceive most of the vibrations and 

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