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 

 Vertkbrata : does that form a circle of itself? Yesj 

 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, " 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 retiirn into themselves, so as to 

 form a circular group ; but whether the Acephala form 



* Hor. Ent. p. 318. 



