63 



49. Equally essential to this action is the pre- 

 sence and operation of heat. The ova of myriads of 

 insects are evolved by its immediate influence; and its 

 power is not less necessary to the maintenance, than 

 to the beginning of living action. When abstract- 

 ed to a certain degree, numerous tribes of animals 

 pass into a torpid state, and again recover action as 

 the heat is restored. Caterpillars, spiders, and ants, 

 were many times in succession rendered torpid, and 

 again restored to action, in the experiments of Dr 

 Michelotti, by the alternate abstraction and commu- 

 nication of heat *. In a temperature, 1 Reaum. 

 Spallanzani found living action in snails to cease ; 

 and the same result was obtained by exposing the 

 marmot to a similar abstraction of heat t. The heart 

 moves quicker in hot than in cold animals, says Dr 

 Irvine ; and in many animals, during the severe cold 

 of winter, it does not move at all. The heart of snails, 

 which beats manifestly in summer, was found to be 

 perfectly at rest in winter ; and the same thing is ob- 

 served in many of the fly tribe. Of the same descrip- 

 tion too, are the serpent and viper tribe, frogs, 

 toads, and tortoises : even the bat, which is naturally 

 a hot animal, becomes, during the winter, as cold 

 as the surrounding medium, and its heart is perfect- 

 ly at rest J. If an egg be opened some days after 

 incubation, so that the fiunctum saliens may come in- 

 to view, according as it is exposed to heat or cold, 



* Phil. Mag. June 1804. 



f Memoirs on Respiration, p. 154. 334. 



t Irvine's Chemical Essays, p. 201. 



