INTRODUCTION. XI 



of consequence to this view of their relations ; and here we 

 have, on the other hand, a close approximation between the 

 Reptilia and the birds themselves. 



These considerations appear to me to exhibit insurmount- 

 able objections to the consistency and unity of the quinary 

 arrangement, as representing an uniform and perfect plan or 

 system upon which the animal kingdom was created ; and I 

 cannot believe that the occasional occurrence of even striking 

 and important coincidences, which appear on a partial view to 

 prove its truth, are sufficient to counterbalance the evidence 

 of its inconsistency which I have just adduced. 



I shall now enter into a more particular description of the 

 structure of these animals, commencing with the organs of cir- 

 culation and respiration. The heart, which is formed of three 

 cavities, namely, of two distinct auricles opening into one 

 common ventricle, sends to the lungs, on each contraction, a 

 portion only of the blood which it has received from the 

 different parts of the body by the veins, so that the - blood 

 which, by the heart's contraction, is distributed to the body 

 through the arteries, is of a mixed nature, consisting of a por- 

 tion of the impure blood just returned from thence, with that 

 which has been aerated in the lungs. It is clear that the blood 

 is by this mechanism but partially changed by the action of 

 oxygen ; in other words, that the quantity of respiration, 

 speaking with reference to the physiological meaning of the 

 term, is comparatively small. Hence arises the circumstance 

 that these animals have what is called cold blood ; for as it is 

 from respiration that the blood derives its heat, and the tem- 

 perature of the body is thereby sustained in animals which 

 have more perfect respiration, it follows that where this func- 

 tion is but imperfectly performed, the animal heat, muscular 

 force, and all the other functions dependent upon respiration, 

 will be diminished. In the fishes the same effect is produced 

 by different means. These are likewise cold-blooded ani- 



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