NATURAL SYSTEMS. MACLEAY. 207 



sketch/' observes our author, " we are at first struck 

 with the analogy which opposite points of the same 

 circle hear to one another, an analogy sometimes so 

 strong that it has been mistaken for a relation of affinity; 

 and, indeed, I am still unable to state whether this be 

 not the fact, and that the opposite points of the curve, 

 if I may so express myself, do not meet each other. 

 Thus the resemblance which the intestinal Acrita (In- 

 testind) or Monogena of Latreille bear to the Nematoidea 

 of Rudolphi, and the Annelides, need not be descanted 

 on, nor the affinity which the Cirripeda, according to 

 some naturalists, appear to have with the branchiopod 

 Mollusca. It will be sufficient to state, that as this 

 peculiarity of natural distribution was detected by ana- 

 lysis in the former part of this work, and the use to be 

 made of it was visible among the Petalocera, so the dis- 

 covery of it served to prevent my falling into several 

 mistakes, which I could not otherwise have avoided, in 

 deciding between relations of analogy and affinity, as 

 they exist in the more general groups. The quadruped 

 reptiles may, in this way, be separated from the Mam- 

 malia by the intervention of birds on one hand, and of 

 fishes on the other ; and yet Dumeril may, possibly, 

 not be far wrong in urging that the paradoxical orni- 

 thorhynchus bears a nearer relation to reptiles than to 

 birds. But my province more particularly is entomo- 

 logy ; and this property of a distribution, which, for 

 convenience only, we have considered as circular, will 

 serve to make the hex apod Acaridce approach to the 

 Anoplura of Leach, as appears to be the case in nature."* 

 That the meaning of this passage may be rendered more 

 clear to the student, we must beg his attention to the 

 following diagram, which, in its outlines, is pre- 

 cisely the same as the former, but those groups, not 

 now alluded to, are omitted, while those which are sup- 

 posed to " meet each other," that is, to unite, are in- 

 dicated by dotted lines. 



* Hor. Ent. p. 319. 



