THE PRIMARY DIVISIONS OF GROUPS. 



227 



latter is so much more diversified in its contents (for 

 reasons hereafter to be stated) than the other two, that 

 many naturalists reckon five groups in all ; the number 

 five being made out by dividing the aberrant group 

 into three, instead of considering it as only one. We 

 have seen,, however, that the first test of a natural 

 group is its circular chain of affinities. If, therefore, 

 the three divisions of Mr. MacLeay's aberrant group can 

 be shown to form a circle of their own, independent of 

 the other two, then we must reckon them as one only, 

 thus making the primary divisions of every circle three. 

 We, consequently, have, in 

 every natural group, three 

 primary circles, one of which 

 (the aberrant) is divided into 

 three secondary circles. A 

 good idea of this disposition 

 may be gained by the an- 

 nexed diagram. If, on the 

 other hand, we adopt Mr. 

 MacLeay's theory, that every 

 group is first divided into five circles (the three aberrant 

 not being united into one), then we may express them 

 in this manner : 



3 Aberrant 



(279.) Let us illustrate this first division of a -natural 

 group by an instance drawn from the animal kingdom. 

 Q 2 



