VERTEBRATE ANIMALS. 



nently maintained at a higher temperature than that- of the 

 medium in which they live. Hence the distinction generally made 

 between the Hot-blooded and the Cold-blooded Vertebrates. 



The variations in the temperature of the blood above alluded 

 to are, moreover, the cause of other important differences, ob- 

 servable in the clothing, habits, and instincts of these creatures. 



To retain a high degree of animal heat necessarily requires a 

 warm and thick covering of some non-conducting material, and 

 consequently in the hair, wool, and feathers of the warm-blooded 

 tribes, we at once recognize the provision made by Nature for 

 preventing an undue expenditure of the vital warmth. Such in- 

 vestments would be but ill adapted to the inhabitants of a watery 

 medium ; and consequently the fishes, destined to an aquatic life, 

 or the amphibious reptile, doomed to frequent the mud and slime 

 of the marsh, are deprived of such incumbrances, and clothed in 

 a scaly or slippery covering, more fitted to their habits, and 

 equally in accordance with the diminished temperature of their 

 blood. 



Still more remarkable is the effect of mere exaltation of animal 

 heat upon the instincts and affections of the different races of the 

 Vertebrata. The cold-blooded fishes, absolutely unable to assist 

 in the maturation of their progeny, are content to cast their spawn 

 into. the water, and remain utterly careless of the offspring to be 

 derived from it. The chilly reptile, nearly as incapable of appre- 

 ciating the pleasures connected with maternal care, is instructed 

 to leave her eggs exposed to the genial warmth of the sun, until 

 the included young escape. But no sooner does the vital heat of 

 the parent become sufficient for the purposes designed by Nature, 

 than all the sympathies of parental fondness become developed, 

 all the delights connected with paternity and maternity are super- 

 added to other enjoyments ; and the bird, as she patiently per- 

 forms the business of incubation, or tenderly watches over her 

 newly-hatched brood, derives a pleasure from the performance of 

 the duties imposed upon her, second only to that enjoyed by the 

 mammiferous mother, who from her own breast supplies the 

 nutriment prepared for the support of her infant progeny. 



