REMARKS ON THE CLASSIFICATION OF MAMMALIA. 515 
E cannot but hope that improved knowledge of fishes and reptiles 
might enable us to range them also in corresponding groups. 
The speculations of McLeay, Swainson, and their followers, 
respecting an absolute number of divisions naturally belonging to 
every department of organized nature, and repeated in each sub- 
division; though probably pushed to an extreme, and doubtless often 
erroneous in the details, may express their perception of an impor- 
tant fact, which, properly considered, may bring us at last to the 
most natural classification. sit not true that each grand division 
of the animal kingdom expresses a certain idea of structure; a 
certain general character a8 compared with the other divisions? Is 
it not likewise true that the classes into which each branch or grand 
division resolves itself, wiren really natural and judiciously limited, con- 
sist of one eminently typical, and others making up the number of the 
other branches and exhibiting tendencies towards each of them ? 
Bach class has its orders, and do not these again give us either a 
central typical group with a set of deviations expressing the ideas 
of the primary branches, or else the latter without the former ? 
The tribes again contained in each order often do, and, if we were 
better acquainted with their limits and positions, would more 
frequently, convey tous the same ideas, which are often repeated 
even in inferior groups. The plan of a central group peculiarly 
expressing the characteristic idea with deviations, each prominently 
exhibiting the character of one of the primary branches, has not 
perhaps, been sufficiently attended to, and as soon as we determine 
that we roust find everywhere a peculiar number, we begin to run 
into forced combinations or separations; yet the fact of general 
correspondencies in the natural distribution of different classes, is 
too important to be overlooked, and may ultimately conduct us to 
generai conclusions of the highest interest, which at present we can 
but dimly imagine. In the arrangements of birds and quadrupeds 
which I have brought before you, 1 have surrounded each central 
body with five deviative forms. 
It is true that Cuvier recognizes only four great sub-kingdoms or 
branches of the animal kingdom, and, if it is now acknowledged that 
his Radiata possessed too miscelianeous a character, stili when the 
groups really belonging to the higher divisions are removed, the pro- 
posed sub-kingdom Protozoa can only contain Infusoria, properly 
VoL. V. 20 
