VITAL FORCES OF INSECTS. 425 



which is the peculiar characteristic of the animal organism ; 

 for on the one hand the demand for food, on the other the 

 amount of metamorphosis indicated by the quantity of car- 

 bonic acid exhaled, bear a very close relation to the quantity 

 of that power which is put forth. This relation is peculiarly 

 manifest in insects, since their conditions of activity and re- 

 pose present a greater contrast in their respective rates of 

 metamorphosis, than do those of any other animals. Of the 

 exercise of generative force we have no similar measure ; but 

 that it is only a special modification of ordinary vital activity 

 appears from this circumstance, that the life of those insects 

 which ordinarily die very soon after sexual congress and the 

 deposition of the ova, may be considerably prolonged if the 

 sexes be kept apart so that congress cannot take place. 

 Moreover, it has been shown by recent inquiries into the 

 agamic reproduction of insects and other animals, that the 

 process of generation differs far less from those reproductive 

 acts which must be referred to the category of the ordinary 

 nutritive processes, than had been previously supposed. 



Thus, then, we find that in the animal organism the de- 

 mand for food has reference not merely to its use as a mate- 

 rial for the construction of the fabric ; food serves also as a 

 generator of force ; and this force may be of various kinds 

 heat and motor-power being the principal but by no means 

 the only modes under which it manifests itself. We shall 

 now inquire what there is peculiar in the sources of the vital 

 force which animates the organisms of the higher animals at 

 different stages of life. 



That the developmental force which occasions the evolu- 

 tion of the germ in the higher vertebrata is really supplied 

 by the heat to which the ovum is subjected, may be regarded 

 as a fact established beyond all question. In frogs and 

 other amphibia, which have no special means of imparting a 

 high temperature to their eggs, the rate of development (which 

 in the early stages can be readily determined with great exact- 



