VERTEBRATA. 487 



the systemic and pulmonary circulation, the effect of respiration is 

 increased to the utmost ; and, pure arterial blood being thus abun- 

 dantly distributed through all parts, heat is more rapidly generated, 

 the warmth of the body becomes considerably increased, and such 

 animals are permanently maintained at an invariable temperature, 

 considerably higher than that of the medium in which they live. 

 Hence the distinction generally made between the hot-blooded 

 and cold-blooded Vertebrata. 



The variations in the temperature of the blood, above alluded 

 to, are, moreover, the cause of other important differences observable 

 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 

 recognise the provision made by Nature for preventing an undue ex- 

 penditure of the caloric generated in the body. Such investments, 

 however, would be but ill adapted to the inhabitants of a watery 

 medium ; and consequently the fish destined to an aquatic life, or 

 the amphibious reptile doomed to frequent the mud and slime upon 

 the shores, 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 a mere exaltation of animal 

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

 Vertebrata. The fishes, absolutely unable to assist in the matura- 

 tion of their offspring, are content to cast their spawn into the 

 water, and remain utterly careless of the progeny to be derived from 

 it. The reptile, equally incapable of appreciating the pleasures 

 connected with maternal care, is content 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 superadded to other enjoyments ; 

 and the bird, as she patiently performs 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 sup- 

 port of her infant progeny. 



