CHAPTER III. 
BIRDS A DISTINCT CLASS. 
It may be presumed, from the tenor of the preceding 
chapters, that I lay great stress on unity of design, and 
therefore I should not exempt any exterior division from 
the rules or principles applied in ascertaining the natural 
arrangement of the normal division. I must, however, 
admit that there appears to me sufficient ground for 
supposing the existence of some slight difference; for 
whereas the contents of normal groups are, by virtue of 
their central situation, strikingly heterogeneous, those of 
abnormal groups — such for instance as the marsupials 
and birds — are also, by virtue of their situation, much 
more homogeneous and accordant inter se; consequently 
their most abnormal forms scarcely recede so far from 
their respective centres as do those of the placental verte¬ 
brates : the birds, therefore, as a group, possibly form a 
more compact mass than the placentals; a suggestion 
which, if correct, will necessarily admit of a more ex¬ 
tended application ; for winged insects appear to occupy 
a station, as regards the vertebrates, precisely analogous to 
that of birds, as regards the placentals. 
