716 AVES. 



are similar in all the oviparous Yertebrata ; it will therefore be more 

 convenient, and prevent unnecessary repetition, if we defer the consi- 

 deration of this important subject to the next chapter, the reader bear- 

 ing in mind that in all essential particulars the details which will be 

 given there, when we come to consider the growth of the Bird in ovo, 

 are equally applicable to the Chelonian, Ophidian, and Saurian Reptiles. 



CHAPTER XXVIII. 



AVES BIEDS. 



(2027.) THE class of Vertebrate animals which now offers itself to 

 our notice contrasts remarkably with the cold-blooded and apathetic 

 inhabitants of the water, and even with the slow-moving Reptile, that 

 languidly crawls upon the surface of the ground, or drags on an amphi- 

 bious existence in the marsh or on the shore. The Bird, ordained to 

 soar into the regions of the air, and not only to sustain itself in that 

 thin medium, but to skim from place to place with astonishing rapidity, 

 needs a strength of muscle and activity of limb even greater than that 

 conferred upon the mammiferous quadruped. Senses of the utmost 

 acuteness are now requisite, combined with instinct and intelligence 

 of a high order ; and accordingly, both as regards their faculties and 

 enjoyments, the feathered tribes far surpass the other oviparous Verte- 

 brata. 



(2028.) Next to that improvement in the condition of the nervous 

 system, which we have all along been able to trace advancing paripassu 

 with the increase of sagacity and the expansion of the bodily faculties, 

 the most remarkable circumstance observable in the economy of Birds 

 is the elevated temperature of their bodies and the heat of their circu- 

 lating fluids. In the Reptile an impure and semioxygenized blood was 

 slowly propelled through the system from the undivided ventricle of 

 their trilocular heart, and we found their energies, their instincts, and 

 their affections proportionately feeble and obtuse ; but now, not only 

 does the heart become divided into four cavities (one ventricle being 

 appropriated to transmit venous blood to the lungs, while the other 

 drives a pure and highly- arterialized fluid in copious gushes to the 

 remotest regions of the body), but, as though even this was not suffi- 

 cient to meet the necessities of the case, the whole interior of the Bird 

 is permeated by the atmospheric air, which penetrates even into the 

 bones ; and the respiratory function being thus rendered as complete as 

 possible, all parts of the muscular system are abundantly supplied with 

 blood arterialized to the utmost, and every fibre, quivering with life 



