THE PRIMARY DIVISIONS OP GROUPS. 225 



and are therefore to be regarded as the primary 



TYPES OF NATURE, 



V. That the different ranks or degrees of circular 

 groups exhibited in the animal kingdom are nine 

 in number, each being involved within the other. 



(275.) We shall now proceed, without further com- 

 ment, to adduce, in detail, the reasons upon which these 

 opinions are grounded, and state these reasons as simply 

 and as concisely as their nature will admit of. . 



(276.) I. In regard to the first proposition on the 

 circularity of natural groups, it seems needless to repeat 

 what has already been said both in this and in a pre- 

 ceding volume.* For the sake, liowever, of exhibiting 

 collectively the first truths of the natural system in a 

 connected series, a popular explanation may not be mis- 

 placed. The progi-ession of aiRnity in any assemblage 

 of animals is known to be natural, if it is circular. 

 This is shown when, by beginning at some one point of 

 the series, and following closely the line of affinity, we 

 are imperceptibly conducted to that point again. The 

 two extremities of the series will thus obviously be 

 united ; and this union, of course, gives us the figure of 

 a circle. Between the two points, thus blending into each 

 other, a greater or lesser number of modifications of 

 form, in the intervening animals, will occur, depend- 

 ing entirely on the greater or lesser extent of the circle 

 we are tracing. These deviations, however (as will be 

 hereafter shown), are ail upon a uniform plan ; and, 

 besides, in all cases, are secondary, or inferior, to the 

 leading characters of the whole assemblage, which — in 

 one way or other — they all retain. Such a circle is 

 called a natural group : the word group being em- 

 ployed, on this occasion, to designate, indiscriminately, 

 every series or assemblage of beings, whose affinities 

 have been so made out. When such a series is so gra- 

 dually developed that no link in the chain of continuity 

 appears wanting, it is then termed a perfect group. 



• Preliminary Discourse on Nat. Hist. 



