204 ON SYSTEMATIC ZOOLOGY. 
and our opinion on this point is confirmed by the author 
himself, in the following passage, when alluding to his 
remarks upon the whole: —‘‘ The foregoing observations, 
I am well aware, are far from accurate, but they are 
sufficient to prove that there are five great circular groups 
in the animal kingdom, which possess each a peculiar 
structure: and that these, when connected by means of 
five smaller osculant groups, compose the whole province 
of zoology.” * Now these smaller osculant groups are 
to be viewed as circles, for, as it is elsewhere stated, 
“* every natural group is a circle, more or less complete.” 
This, in fact, is the third general principle of Mr. Mac- 
Leay’s system, and he has exemplified his meaning of a 
natural group in the above diagram, where all animals 
are arranged under five large groups or circles, and five 
smaller ones. Let us take one of these groups, the 
VERTEBRATA: does that form a circle of itself? Yes; 
because it is intimated that the reptiles (Reptilia) pass 
into the birds (Aves), these again into the quadrupeds 
(Mammalia), quadrupeds unite with the fishes (Pisces), 
these latter with the amphibious reptiles, and the frogs 
bring us back again to the reptiles, the point from whence 
we started. Thus the series of the vertebrated group 
is marked out and shown to be circular ; therefore it is 
a natural group. This is an instance where the circular 
series can be traced. We now turn to one where the 
series is imperfect, but where there is a decided tend- 
ency to a circle: this is the Moxtnusca. Upon this 
group our author says, ‘‘ I have by no means determined 
the circular disposition to hold good among the Mot- 
LuscA ; still, as it is equally certain that this group of 
animals is as yet the least known, it may be improper, 
at present, to conclude that it forms any exception to 
the rule: it would even seem unquestionable that the 
Gasteropoda of Cuvier return into themselves, so as to 
form a circular group; but whether the Acephala form 
* Hor. Ent.*p. 318. 
