1833.] in the cold-blooded Vertebrata. 467 
it may be permitted to entertain a doubt as to the animal to which 
this specimen should be referred. 
The shape of the teeth appear to indicate a carnivorous character 
in the animal to which they belonged, and bear a considerable resem- 
blance to the molares of the seal. 
Supposing this to be the case, the position of this fossil- would not 
be, as Mr. LyE.t imagines, as fatal to the theory of successive deve- 
lopment as if several hundreds had been discovered, since its appear- 
ance is subsequent to the period in which the great Saurian reptiles 
were the most abundant; and should it prove to belong to the genus 
phoca or to some cetaceous animal, it would be an example of the com- 
mencement of the type of mammalia in one of the least perfect tribes 
of the order, and therefore an additional argument in favor of the 
theory it is intended to subvert. 
In endeavoring to show that there actually does exist what has 
been called a stimulus of perfection in the organic world, it will be 
necessary to take a system of organs in its most imperfect form, and to 
investigate the steps by which nature has succeeded in effecting a 
series of gradual improvements. 
Of the various functions conducing to the preservation of the 
individual, none is of more importance than that by means of which 
the oxygenization of the blood is effected, and this fluid rendered fit 
for repairing the waste of the body, and supplying materials for the 
growth and increase of the different organs. The development of the 
respiratory and circulating systems will necessarily be in a certain 
and constant ratio to each other, and, wherever we see a perfect respi- 
ratory apparatus, we have an indication of a proportionally complicated 
set of organs for the circulation of the blood, and consequently an 
increase in the irritability and nervous energy of the animal. 
The respiration of the embryo in warm-blooded animals is at first 
solely cutaneous, and the heart consists of two cavities, both systemic, 
as no respiratory organs are developed. The systemic ventricle is 
then divided by a septum, and the right ventricle thus formed is pro- 
longed into a tube which opens into the aorta subsequently to the 
origin of the branches which supply the upper portion of the trunk. 
This prolongation of the right ventricle is called the ductus arteriosus, 
and from it are given off small branches, which go to supply the lungs. 
The circulation is now that of a reptile, the heart in effect consisting 
of two auricles anda ventricle ; but on the emergence of the animal 
from its foetal state, the lungs become the immediate organs of respi- 
ration ; the blood is more perfectly oxygenized ; the irratibility of the 
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