424: CORRELATION OF PHYSICAL AND VITAL FORCES. 



the germinating 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 larvse 

 have been put forth, may be made, by artificial heat, to pro- 

 duce 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 it remarkable that 

 during the latter part of that state, in which the developmental 

 process goes on with extraordinary rapidity, there is in cer- 

 tain insects a special provision for an elevation of the tem- 

 perature of the embryo by a process resembling incubation. 

 Whether, in addition to the heat imparted from without, there 

 is any addition of force developed within (as in the germinat- 

 ing 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, how- 

 ever, that such a retrograde metamorphosis 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 independ- 

 ent of external temperature. 



Further, a like retrograde metamorphosis of certain con- 

 stituents of the food is the source of that sensori-motor power 



