218 GENERAL PHYSIOLOGY 



they live is high, these animals, such as insects and reptiles, are 

 extremely lively, move about much, and show in general an intense 

 activity. With decreasing temperature the liveliness of their 

 movements decreases, and at in many cases vital activity is 

 hardly to be observed in them, the transformation of energy has 

 almost ceased. " Wherever we look into the realm of living 

 organisms," says Pfliiger (75, 1), "we see how the intensity of 

 vital processes, and hence decomposition, varies proportionately 

 with the temperature. When I observe the lively, moving, nimble 

 lizard in summer, and then see how the same animal, exposed to a 

 temperature below 0, becomes gradually quiet and sinks into a 

 death-like torpor, and inquire what is the reason why the animal 

 becomes again so active in warmth, appearance tells me that it is 

 because heat has been introduced into the organs ; heat puts the 

 atoms into vibration and promotes dissociation." The heat that 

 is introduced serves in this way directly as a source of energy for 

 the work of the organism. 



This completes the enumeration of the sources from which the 

 organism receives energy. The other forms of energy have 

 almost no importance in this respect. 



C. THE PRODUCTION OF ENERGY BY THE ORGANISM 



At present it is wholly impossible to follow the tortuous paths 

 taken by the energy that is introduced in its changes through 

 the living body. Scarcely a beginning has been made in investi- 

 gating the transformations that this energy undergoes under the 

 various conditions found by it in living substance. There is here 

 needed a long series of exhaustive special researches and especially a 

 detailed knowledge of metabolic processes, before an intelligible 

 conception can be formed of the mechanism of these transforma- 

 tions. The field of physiological energetics offers rich problems 

 full of reward for the future, which thus far have been scarcely 

 noticed. Only the final links of the chain of metamorphoses, the 

 outward achievements of the living organism, are now known with 

 certainty. 



The evolution of energy outward, especially that of mechanical 

 energy, which expresses itself in the movements of the living 

 body, is undoubtedly the most evident of all vital phenomena ; it 

 is more or less the first criterion of life for the untrained observer, 

 and perhaps this is the reason why physiology from early times 

 has made the phenomena of movement a favourite object of 

 research. Less evident, because either uncommon or difficult to 

 observe, is the production, on the part of living substance, of other 

 forms of energy, such as light, heat and electricity. 



