998 FIRST FRINCIPLES OF NATURAL CLASSIFICATION. 
Every one knows that vertebrated animals, above alk 
others, are the most distinctly marked by possessing an 
internal skeleton. ‘They have been, moreover, demon- 
strated to be a circular group. What, then, are the 
divisions? These are no less obvious. Quadrupeds, birds, 
reptiles, amphibia, and fishes, are acknowledged to be so 
many types of the vertebrated circle. There is, however, 
good reason to believe that the last three of these types 
form a circle of their own; in which case, we should 
have, in fact, three primary circles of vertebrated animals: 
the first, or typical, comprising the quadrupeds; the se- 
cond, or sub-typical, consisting of the birds; and the third, 
- or aberrant, including the reptiles, amphibia, and fishes. 
On the other hand, if each of these latter classes of 
animals is found to be of the same rank as quadrupeds or 
birds, then the number of primary divisions will be five. 
In reference, however, to the above exemplification, it 
should here be observed, that the absolute union of the 
reptiles, amphibia, and fishes, into one circle of their 
own, has not yet been demonstrated. That there is, 
nevertheless, a high degree of probability attached to such 
a supposition, will be apparent, when we consider how 
much nearer they are allied to each other, in comparison 
to their affinity with birds and quadrupeds. How 
closely the water serpents and the eels approach each 
other, and how well are they all three characterised by 
their cold blood, while that of birds and quadrupeds is 
warm. There are also similar reasons for believing in this 
union of the aberrant groups in all the other divisions of 
the animal kingdom not yet analysed. In ornithology, 
however, so many analytical details have been gone into*, 
that we consider this proposition to be fully demon- 
strated. If, again, one of these larger divisions is 
analysed, the same results follow, —there will be three 
secondary circles united into one; and thus we go on, 
reducing every group to a smaller one, until we come to 
a genus, where again we find three groups of sub-genera, 
* See Northern Zoology. 
