74 OEGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBJLL. 



former must in such animals be largely dependent on that of the 

 latter, falling and rising with it. Hence, most of the lower animals 

 are poikilothermic,* or, as they have less appropriately been called, 

 cold-blooded. 



The higher animals, on the contrary, in which, on account of their 

 highly developed respiratory organs and energetic metabolism, the 

 thermogenic activity is great, and which are protected from a rapid 

 loss of heat by radiation by the size of their bodies and by the 

 possession of a covering of hairs or feathers, possess the power of 

 maintaining a constant temperature, which is independent of the 

 rising and falling of the temperature of the surrounding medium. 

 Such animals are designated homothermic, or warm-blooded. Since 

 they require a high internal temperature, varying only within small 

 limits, as a necessary condition for the normal course of the vital 

 processes, or one may say for the maintenance of life itself, they 

 must possess within themselves a series of regulators whose function 

 is to keep the body temperature within its proper limits, when the 

 temperature of the surrounding medium is high. This may be 

 effected either by diminishing the production of internal heat 

 (diminishing the metabolism) or by increasing the loss of heat from 

 the surfaces of the body (by radiation, evaporation of secretions, 

 cooling in water) ; and, on the contrary, when the temperature of the 

 outer medium is too low, by increasing the production of internal 

 heat (increasing the metabolic activity by more plentiful food supply, 

 more vigorous movements), or by diminishing the loss of heat by 

 the development of better protective coverings. 



When the conditions necessary for the action of these regulators 

 are absent (want of food, small and unprotected bodies), we find either 

 the phenomenon of winter sleep, in which life is preserved with 

 a temporary lowering of the metabolic processes ; or, when the 

 metabolic processes of the organism do not enter into abeyance, the 

 remarkable phenomena of migration (migration of birds). 



Organs of Secretion. The respiratory organs stand to a certain 

 extent intermediate between the organs of nutrition and those of 

 excretion, in that they take in oxygen and excrete carbonic acid. 

 In addition to this gas a number of excrementitious substances, 

 mostly in a fluid form, which have entered the blood from the 

 tissues, pass out by the lungs. The function, ho\vever, of excretion 



* Comp. Bergmann, " Ueber die Verhaltnisse der Warmeokonomie der Thiere 

 zu ihrer GrBsse," Gottingcr Studicn, 1847; also Bergmann und Leuckart, 

 " Anatomisch-physiologische Uebersicht des Thicrreichs," Stuttgart, 1852. 



