THE PRIMARY DIVISIONS OF 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, however, of exhibiting 
collectively the first truths of the natural system in a 
connected series, a popular explanation may not be mis- 
placed. The progression of affinity 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 
acircle. 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 ail 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 linkin the chain of continuity 
appears wanting, it is then termed a perfect group. 
* Preliminary Discourse on Nat. Hist. 
Q 
