INTRODUCTION. x1 
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 te the body through the arteries, is of a mixed 
b 2 
