THE PHYSIOLOGICAL EVOLUTION, ETC. 383 



so lowly an animal as the earthworm possesses the power of 

 perfectly controlling its carbonic acid discharge, it is ob- 

 vious that in many other members of the animal kingdom, 

 thus far unexamined, the same condition of things may be 

 present. It is noticeable that the intervals of constant 

 metabolism more or less cover such variations of tempera- 

 ture as the animals would be ordinarily exposed to under 

 normal conditions, except in the extreme cold of winter and 

 extreme heat of summer. The teleological explanation of 

 such an arrangement is obvious. If the metabolism de- 

 pended absolutely on the temperature, the animal would 

 lose much of its control over its own powers. It would of 

 necessity be always lively in hot weather, and torpid in cold. 

 All its movements would be regulated by the temperature 

 of its surroundings, and not by its own wants and desires. 

 In cold weather it could not move fast enough to escape its 

 enemies, should they be warm-blooded ones, and in hot 

 weather it would have difficulty in obtaining sufficient food 

 to replace the increased tissue loss. 



In the last column of the above table are given the tem- 

 perature increments, or relation of the carbonic acid output 

 at 24° to that at io c . It will be seen that with one or two 

 exceptions, which are probably in part accidental, these 

 values remain at about 2*0, or about the same as those 

 obtained in the former experiments on teleost fish. In the 

 case of the cockroach, however, the very high value of 4-3 

 was obtained, or as great an effect as was noticed in the 

 transparent pelagic ccelenterates and tunicates. 



We have thus seen that in certain amphibia and other 

 animals the nervous system has been sufficiently evolved to 

 exert a controlling power over the tissue metabolism. 

 The only other conditions necessary to convert these animals 

 into warm-blooded, or homoiothermic, animals of even tem- 

 perature, would therefore appear to be an increased heat 

 production of the tissues, coupled with a modification of the 

 external covering of the body so as to diminish the heat 

 loss. Thus the temperature of fish and amphibia is, as a 

 rule, only a fraction of a degree above that of their sur- 

 roundings. Now the actual carbonic-acid discharge of a 



