72 
any use need be made of such characters as these, when the groups 
can be so well established upon characters more obvious and import- 
ant in their nature. 
We frequently find groups which, though very extended as to the 
number of species they contain, are much more limited in respect to. 
the varieties of structure they present than other groups appa- 
rently of equal rank containing a much smaller number of species. 
Such groups are of course always the most easy to isolate, but the 
most difficult to subdivide: it is in these that we find the most con- 
fusion existing, and the greatest variety of opinion among naturalists 
as to the manner in which their subdivision should be effected. Ex- 
cepting in the highest divisions, it is but of late years that naturalists 
have at all appreciated the distinction between what are usually 
termed ‘‘ essential” and “adaptive” characters, of the former of 
which, as we descend to the lower groups, not only is the existence, 
but also the importance, much less easily recognized. 
The base of the cranium, as I before observed, is, from its having 
less connexion than most parts of the bony framework with the pe- 
culiar wants of the species, by far the most rich in such characters ; 
among those which the foramina may afford, I must here dwell 
rather particularly on the evidences of affinity afforded by the pre- 
sence or absence of the ali-sphenoid canal, and also explain my rea- 
sons for assigning ita new name. As will appear from the observa- 
tions I have brought forward, it exists throughout the Rodentia, ex- 
cepting the aberrant family of Hares ; it is wanting in the Marsupials 
and Edentata; and among the Ungulate division, including the Ru- 
minants and Pachydermata, the Artiodactyle division, including the 
Ruminants and those Pachyderms which have the toes in even num- 
ber, is constantly characterized by its absence ; while in the Perisso- 
dactyla it is as constantly present., In the first edition of the ‘ Lecons 
d’ Anatomie Comparée,’ the illustrious author only alludes to this canal 
in a very vague manner; and in the more recently published edition, 
in which the osteology of the cranium is much more fully elaborated, 
it is spoken of everywhere as being the vidian canal,—the existence 
of a vidian canal being denied in those animals which do not happen 
to possess it. From the time when I commenced the series of obser- 
vations of which the present is an attempt to sum up the results, I 
always felt inclined to the belief that the canal in question did not 
correspond in situation to the vidian canal as known in Human 
Anatomy, since this canal commences just at the root of the internal 
pterygoid process, while that pointed out as such in the work alluded 
to is quite on the outside of the homologue of the outer one. Among 
the rest, the Monkeys are spoken of as wanting the vidian canal; but 
on removing from the skull of a small monkey in my collection the 
whole of the posterior portion, and the temporal bones with auditory 
bulle, the posterior apertures of the vidian canals became very ap- 
parent, and fine bristles passed readily through them into the orbits ; 
and in other skulls belonging to the Quadrumanous order, provided 
that those portions of the upper maxillary bone which originally con- 
stitute the alveoli of the hinder molars do not rise high enough to 
