VERTEBKATA. 313 



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 expenditure of the vital warmth. 

 Such investments would be but ill-tdapted to the 

 inhabitants of a watery medium ; and consequently 

 the fishes, destined to an aquatic life, or the amphi- 

 bious 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 appreciating 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 superadded 

 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 nutri- 

 ment prepared for the support of her infant progeny. 



