PROPERTIES OF GROUPS. $29 
three lesser ones, and so on, until we arrive at the lowest 
groups in nature, which are called sub-genera. 
(402.) Some other properties of natural groups need 
not here be alluded to, since they belong to a more 
intimate acquaintance with the science than is usually 
aimed at by beginners, and they have already been dis- 
cussed in the body of this work. So soon as the 
student understands so much of the nature of groups as 
we have now endeavoured familiarly to explain, he will 
be fully competent to pursue the subject as discussed in 
the former chapters. There are, nevertheless, certain 
other properties in natural groups, which the young 
naturalist should be acquainted with ; because they serve 
as tests by which all groups should be tried ; as these 
enter into the laws of verification, they will not now 
be considered. 
(403.) After perusing thus far, the student may pro- 
bably say, ‘‘ I understand that all natural groups form 
their own circle; andthateach circle contains three smaller 
ones, two of which are typical, and one aberrant: but 
in what manner am I to prove my circle, whether it be 
natural, since I have been told that false circles can be 
made? If, for instance, I am desirous of discovering, 
without the aid of books, the manner in which the 
family of thrushes (Merulide Sw.*) describe their cir- 
cle of affinity, what check have I upon my own arrange- 
ment, after I have placed these birds in such a way as 
to exhibit a circle? Must I be guided only by what 
appears to be the circle of affinity? or are there other 
circumstances by which my circle is to be verified, and 
my fancy kept in check ?” 
(404.) These are questions which may naturally be 
asked, and which we shall now proceed to answer. There 
are three modes, or processes, then, by which natural 
groups are to be verified. 1. By their circularity. 2. 
By the parallel analogy of their contents to other groups; 
and, 3. By the order in which their types or subordinate 
divisions occur. This latter may be termed the theory 
* See their natural arrangement in Northern Zoology, vol. ii. p. 149. 
