CONSTRUCTIVE FORCES OF ANIMALS. 429 



metamorphosis of tissue ; and it can hardly be questioned (if 

 our general doctrines be well founded) that the constructive 

 force that operates in the completion of the fabric will be de- 

 rived in part from the heat so largely generated by chemical 

 change, and in part from the descent which a portion of the 

 fabric itself is continually making from the higher plane of 

 organized tissue to the lower plane of dead matter. This 

 high measure of vital activity can only be sustained by an 

 ample supply of food ; which thus supplies both material for 

 the construction of the organism, and the force by whose 

 agency that construction is accomplished. 



How completely dependent the constructive process still is 

 upon heat, is shown by the phenomena of reparation in cold- 

 blooded animals ; since not only can the rate at which they 

 take place be experimentally shown to bear a direct relation 

 to the temperature to which these animals are subjected, but 

 it has been ascertained that any extraordinary act of repara- 

 tion (such as the reproduction of a limb in the salamander) 

 will only be performed under the influence of a temperature 

 much higher than that required for the maintenance of the 

 ordinary vital activity. After the maturity of the organism 

 has been attained, there is no longer any call for a larger 

 measure of constructive force than is required for the mainten- 

 jince of its integrity ; but there seems evidence that even then 

 the required force has to be supplied by a retrograde meta- 

 morphosis of a portion of the constituents of the food, over and 

 above that which serves to generate animal heat. For it has 

 been experimentally found that, in the ordinary life of an adult 

 mammal, the quantity of food necessary to keep the body in 

 its normal condition is nearly twice that which would be re- 

 quired to supply the " waste " of the organism, as measured 

 by the total amount of excreta when food is withheld ; and 

 hence it seems almost certain that the descent of a portion of 

 the organic constituents of this food to the lower level of sim- 

 ple binary compounds is a necessary condition of the ele- 



