1 INTRODUCTORY OBSERVATIONS 



admit a group to be natural which does not form a circle more or less 

 complete. How far the central group of M. Fries will agree with 

 this definition, I am not botanist enough to determine ; but the normal 

 group of Mr. Macleay unquestionably does not, since he himself tells 

 us that it forms not one circle but two #. The first division, therefore, 

 of every group into two, although useful perhaps in some respects, is 

 clearly artificial. 



The aberrant group, as being that which more immediately connects 

 one circle to another, comprehends, from necessity, a greater variety 

 of forms than are generally to be met with in the two others. The 

 union of all these aberrant forms into a circle of their own, is a 

 principle of natural arrangement which has hitherto been undisco- 

 vered, and which has therefore claimed, in the following pages, most 

 particular attention. This union, however, must be kept perfectly 

 distinct from that property which was long ago suspected to exist in 

 opposite points of a circle, and of which instances have been given f. 

 These examples, illustrating Mr. Macleay's meaning, appear to me, as 

 they do to him, mere relations of analogy ; since, if the suspicion of 

 their affinity had been just, we should have had a union of one typical 

 and two aberrant groups ; a mode of combination which, to me at 

 least, Nature has in no instance exhibited. 



As to the relative rank of the three primary divisions, some differ- 

 ence of opinion may arise ; for the truth is, that the principles by 

 which the value of zoological groups are regulated have not been 

 sufficiently investigated. Two groups may be of equal rank, and still 

 be vastly disproportionate in their contents. No one, for instance, 

 would think of pronouncing the class of birds a superior group 

 to that of quadrupeds, merely because the contents of the first, in 

 comparison to the second, are as six to one. Number cannot decide 

 the question ; and therefore, as natural groups like these may be 

 equivalent in rank without being so in extent, we are left no other 



* " M. Fries lays it down as a rule, that he admits no groups whatever to be natural unless they 

 form circles more or less complete. Let us then apply this rule to what he terms his central group, 

 and which he makes always to consist of two. Does this form a circle? If not, the group cannot be 

 natural according to his own definition." — Macleay on certain general Laws, &c. Linn. Trans., xiv. 58. 



t Horae Ent, ii., p. 349. 



