INTRODUCTION. XI 



called, two ordinal groups exhibit as great a discrepancy in 

 their relative plan of organization as any two classes do, 

 then the relation of the former to either of the latter is not, 

 and cannot be, the same as that which exists between the 

 latter two. Yet in this predicament stand the three first 

 classes of the Vertebrata ; the relations of the Mammalia 

 and birds being much stronger and more obvious than those 

 of the Reptilia to either, and the two groups of the latter, 

 which I have just sketched, the Tortoises and the Serpents, 

 being nearly or quite as far removed by their structure 

 from each other as the birds are from the Mammalia. The 

 mode of reproduction is the sole exception 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 insur- 

 mountable 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 inconsis- 

 tency which I have just adduced. 



I shall now enter into a more particular description of 

 the structure of these animals, commencing with the or- 

 gans of circulation 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 dis- 

 tributed to the body through the arteries, is of a mixed 



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