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 thewhole: "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 MOLLUSCA. Upon this 

 group our author says, <f I have by no means determined 

 the circular disposition to hold good among the MOL- 

 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. 



