NATURAL SYSTEMS. MACLEANS. 209 



of strong analogy. The groups in question have so long 

 engaged our attention, that we shall venture now to ex- 

 press our opinions upon them. The intestinal worms 

 (Intestina) do not appear to enter into the circle of the 

 Acrita, but rather represent that class among the An- 

 nulosa, where they follow, in close affinity, the Annelida 

 or red-blooded worms. The Cirripeda, which enter 

 also among the Annulosa, may possibly represent the 

 ' Branchiopod Mollusca; but we think this very doubtful : 

 while the analogy between the reptiles and Ornithorhyn- 

 chus is neither direct nor natural, seeing that it is by 

 this latter form that the Mammalia are connected in 

 the most satisfactory manner to birds. If any na- 

 turalist should be inclined to doubt this latter theory, 

 he will be fully convinced of its accuracy by analysing 

 the class of Mammalia to which it belongs. If, however, 

 Mr. MacLeay had merely said that the opposite points 

 of a circle always evince a strong analogy to each other, 

 he would only have illustrated one of his own propo- 

 sitions ; for this resemblance follows, as a matter of 

 course, if the contents of one circle show parallel rela- 

 tions of analogy to the contents of another circle. 



(263.) The fifth proposition of our author, and 

 which he considers as one of the characteristics of natural 

 groups, is this, that one of the five larger groups in 

 every circle contains representations of all the others. 

 An instance of this will best explain to the reader his 

 full meaning. Mr. MacLeay considers that the polypes 

 (Acrita) form one of the five great classes or groups of 

 the animal circle ; and he proceeds to explain how this 

 group contains types of all the others, together with one 

 peculiar to itself. ee At first sight," he observes, ' ' there 

 appears to be a want of that symmetry in this circle 

 (the Acrita), so observable in the others which compose 

 the great divisions of the animal kingdom ; for the Ra~ 

 diata (or star -fish) have all a classical type to which 

 their several structures may be referred, as also have 

 the Annulosa, Vertebrata, and Mollusca; whereas here 

 we see nature choosing every possible type of form, and 



