574 HISTORY OF SYSTEMS. 



circle, Mr. Swainson considered one to be typical, one sub- 

 typical, and one to be aberrant, — the latter carrying with it 

 two satellitious circles, and wandering away from the per- 

 fect character of that to which their organization as a whole 

 proved them to belong. The class Mollusca, then, must, 

 theoretically, have three coequal and two lesser circles, all 

 of an ordinal rank. In 1835, Mr. Swainson said that these 

 were the Gasteropoda, the Acephala, the Nudibranchia, the 

 Pteropoda, and the Cephalopoda ; but in 1840, his views 

 were greatly altered. He then says, — "The great natural 

 divisions of the testaceous Mollusca appear to us to be these : 

 — The Jirst, or pre-eminently typical, are unquestionably the 

 Gasteropoda, or spiral univalves, whether we consider the 

 comparative perfection of their internal or their external 

 structure. The second, or sub-typical class, is composed of 

 the Dithyra of Aristotle, or the bivalves, whose structure 

 is less perfect, but which are in like manner protected by 

 a regularly formed, and often richly coloured, bivalve shell. 

 The third, or aberrant group, as usual, comprehends three : — 

 1. The Nudibranchia of M. Cuvier, or the naked Gastero- 

 poda ; 2. The Parenchymata, or intestinal Testacea ; and 

 3. The Cephalopoda, or cuttlefish." The following dia- 

 gram may illustrate the circular connection of these orders: — 



That these orders form a 



circular group is not to be 



GAsrERorojm. \ proved unless we admit the 



host of extinct races into the 

 category ; and unless we even 

 grant that of some orders, as 

 of the Nudibranchia, there 

 have been many which have 

 not only disappeared from 

 amongst the living, but have 

 not " left even a wreck be- 

 hind" of their ancient exist- 

 ence.* We must also admit as certain, that Nature begins 

 her creations " from a small rudimentary group, — a point, 



* I have never been able to understand why extinct species of a past 

 creation should be admitted into the circular or quinary theory of the exist- 

 ing creation, for it seems natural to believe that, as every successive creation 

 was called into existence, the Creator would make that complete in itself, 

 and without reference to forms that he had willed to annihilate, or predeter- 

 mined to live at a long distant era. For example, I believe that the zoology 

 of the Old Red Sandstone was a perfect system in itself ; just as I believe 

 the existing creation to be so. To permit gaps or empty spaces where the 

 old and now extinct circles were occupants seems to me a defect ; but we 

 know that all is perfect and very good. 



