328 PRACTICAL AND SCIENTIFIC ZOOLOGY. 
other hand, so numberless are the forms of nature, that 
false circles can be made, and are frequently made, by 
putting in, to fill up our gaps, animals which have no 
real connection with that circle which we wish to ren- 
der perfect. Hence, although we must first look to the 
circularity of a group as a primary requisite, still the’ 
accuracy of this circle must be proved by other tests, 
which will be shortly explained. 
(400.) The second property possessed by natural 
groups regards those only which we call aberrant, and 
consists in the three aberrant groups or divisions of a 
circle being united among themselyes into one circle, 
independent of their union also with the two typical 
groups. This theory, although it virtually makes the 
primary division of every circle to be three, does not, 
in fact, affect the accuracy of a group which is first 
divided into five, any more than this, that it shows 
these aberrant divisions to have other properties than 
were formally suspected ; so that, besides being united 
to the typical groups, they also blend in a circle of their 
own, as if they were independent of the two others. 
(401.) As we have hitherto looked to the vertebrated 
animals as furnishing one of the most familiar illustra- 
tions of natural arrangement, we will again use them to 
exemplify the union of which we are now speaking. 
Quadrupeds and birds, then, are the two typical groups 
of vertebrated animals; while reptiles, amphibia, and 
fishes are the three aberrant. Now, if these latter are 
found, upon investigation, to form a circle by themselves, 
it naturally follows that the primary circles in every 
group are three, and not five ; the three aberrant divi- 
sions being merged into one. This union, however, 
cannot always be traced, from the causes elsewhere 
assigned ; and therefore, in dubious cases, it is more ad- 
visable to adhere to the usual method of distinguishing 
each of the aberrant groups separately by themselves. It 
follows, nevertheless, that, wherever it can be demons 
strated, we must consider that the circle is first divided 
into three others, each of which is again resolved into 
