34 OF LIFE, AND ITS CONDITIONS. 



but a special expenditure offeree, appears from a fact to be presently stated, 

 corresponding to that already noticed ( 11) in regard to Plants. 



13. Now if we look for the source of the various kinds of Force, which 

 may be distinguished as constructive, seusori-motor, and generative, that 

 are manifested in the different stages of the life of an Insect, we find them 

 to lie, ou the one hand, in the Heat with which the organism is supplied 

 from external sources, and, on the other, in the Food provided for it. The 

 agency of heat, as the moving power of the constructive operations, is even 

 more distinctly shown in the development of the Larva within the egg, and 

 in the development of the Imago within its pupa-case, than it is in the ger- 

 minating seed ; the rate of each of these processes being strictly regulated 

 by the temperature to which the organism is subjected. Thus ova which 

 are ordinarily not hatched until the leaves suitable for the food of their 

 larva have been put forth, may be made, by artificial heat, to produce a 

 brood in the winter ; whilst, on the other hand, if they be kept at a low 

 temperature, their hatching may be retarded almost indefinitely without the 

 destruction of their vitality. The same is true of the pupa-state ; and it is 

 remarkable that during the latter part of that state, in which the develop- 

 mental process goes on with extraordinary rapidity, there is in certain In- 

 sects a special provision for an elevation of the temperature of the embryo 

 by a process resembling incubation. Whether, in addition to the heat im- 

 parted from without, there is any addition of force developed within (as in 

 the germinating seed) by the return of a part of the organic constituents of 

 the food to the condition of binary compounds, cannot at present be stated 

 with confidence; the probability is, however, that such a retrograde meta- 

 morphosis does take place, adequate evidence of its occurrence during the 

 incubation of the Bird's egg being afforded by the liberation of carbonic 

 acid which is there found to be an essential condition of the developmental 

 process. During the larva-state there is very little power of maintaining an 

 independent temperature, so that the sustenance of Vital Activity is still 

 mainly due to the heat supplied from without. But in the active state of 

 the perfect Insect there is a production of heat quite comparable to that of 

 warm-blooded animals ; and this is effected by the retrograde metamorphosis 

 of certain organic constituents of the food, of which we find the expression 

 in the exhalation of carbonic acid and water. Thus the food of Animals 

 becomes an internal source of heat, which may render them independent of 

 external temperature. Further, a like retrograde metamorphosis of certain 

 constituents of the food is the source of that sensor i-motor power 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 carbonic acid exhaled, bear a very close relation to the 

 quantity of that power which is put forth. This relation is peculiarly mani- 

 fest in Insects, since their conditions of activity and repose 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 ordina- 

 rily 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 can- 

 not 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. 



14. Thus, then, we find that in the Animal organism the demand for food 



