52 



from other fishes. I am inclined to suspect that the observa- 

 tion of Davy on the Bonito, and of the Maltese fishermen 

 on the Tunny, were made at a period of greater activity of 

 these animals when they assemble together on the surface 

 for the purpose of copulation. The activity of the respira- 

 tory function is, of course, proportioned to the heat-produ- 

 cing power, and has generally been used as an index of it. 

 I believe that although the Salmon is guided by a peculiar 

 instinct to seek the river in its period of greatest activity, 

 yet that one of the objects for which it is so endowed is to 

 obtain for the greater necessity for respiration, at this period, 

 a medium in which oxygen gas exists in greater quantity 

 than in the sea. The observations of Newport on the 

 temperature of insects, and more especially upon the 

 hymenoptera, as Bees and Ants, will illustrate what I have 

 conjectured upon the subject of reptiles and fishes. He 

 finds, as might be expected, that in insects the heat-produ- 

 cing power exists in different degrees in the different orders 

 and in the different conditions of the same insect. The 

 Nymph, the insect in its most perfect form, generates heat 

 in the highest degree. The Pupa, which is the transition 

 state between the Nymph and Caterpillar, in which the 

 organic functions are tolerably active, but the animal 

 functions are generally in repose, generates a very small 

 quantity of heat. In the Caterpillar, in which both vital 

 and animal functions are active, but in less degree than in 

 the Nymph, the temperature is higher than in the Pupa, 

 but lower than in the Nymph. The heat generated is 

 greatest in amount in those insects, in all conditions, whose 

 habits are the most active, and is, of course, the greatest in 

 the volant species. It is also the greatest in amount when 

 the animal is in action, and then the temperature is in 

 proportion to the activity of the animal. The degree of 

 respiration, measured by the production of carbonic acid, 

 is found to be in the same proportion ; and it is at this time, 

 also, that the digestion is most actively carried on. No doubt. 



