28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ']2 



larger vessels from becoming overheated. The importance of the 

 enlarged cervical sacs in preventing excessive heating of the carotid 

 artery carrying blood to the head may readily be seen. 



SIGNIFICANCE OF TEMPERATURE CONTROL 



In common parlance animals are divided in two groups distin- 

 guished as those with " warm blood " or " cold blood," according to 

 their condition as regards body heat. Though two classes may be 

 recognized without difficulty, the criterion implied in these two terms 

 is not exact as a " cold-blooded " animal temporarily may have its 

 body temperature raised to a high degree. The distinction between the 

 two, in fact, is not one of actual degree of heat, but rather one of main- 

 tenance of a more or less uniform temperature in the group defined 

 as possessing " warm blood," and of fluctuation in bodily temperature 

 in those distinguished as " cold-blooded." To express this idea with 

 exactness, the first group of animals is said to be homoiothermal, and 

 the second poikilothermal, terms proposed by Bergman* in 1847. 



It will be admitted without question that the possession and main- 

 tenance of warm blood is of advantage to any animal. We may 

 suppose, therefore, with what amounts to some certainty that this 

 faculty when once gained, would not be lost. On the basis of this 

 assumption it may be concluded further that the first vertebrates were 

 cold-blooded, an hypothesis in line with facts of evolution as they 

 are understood and accepted at the present time. Whether these types 

 developed in regions of equable temperatures or in areas with moder- 

 ate seasonable changes is a matter of no moment in the present discus- 

 sion. In either case these early vertebrates as they extended their 

 ranges, encountered barriers erected by cold during a part of the year. 

 Groups successful in coping with this condition developed an ability 

 to undergo certain periods, longer or shorter in length, in the state 

 of suspended animation that we term hibernation, and then to revive 

 and carry on their activities as before with return of a period of 

 increased warmth. In meeting these conditions of cold it was of 

 advantage to develop increased resistance to the torpor induced. In 

 other words, it was an advantage to the organism to maintain its 

 activity at lower and lower temperatures. In order to accomplish 

 this it was necessary to evolve a mechanism for temperature control 

 in the body, and for regulation of the rate of production of heat from 

 ingested food elements. When once begun, such control would prove 



*Gottinger Studien, Vol. I. 1847, p. 593. 



