234 FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 
Yyrannine 
—————" 
Gateropodiaz 
other in many points of strong analogy.* It will be 
seen that these two circles touch, or blend into each 
other (aa), by means of the Edoline (a) in one circle, 
and the Brachypodine (a) in the other. This is the 
usual and most obvious point where circular groups 
respectively meet: but the sort of affinity of which we 
are now speaking also regards the union of the typical 
groups in two distinct circles. In the present instance 
these are the Laniane and Thamnophiline in one, 
and the Merulade and Myotherine in another. The 
internal affinities of the Thamnophilineé are two ; one to 
the Laniane, the other to the Kdoliane ; all three 
groups being parts of the same circle ; but then there 
exists, at the same time, such an intimate resemblance 
also between the Thamnophiline and the Myotherine, 
that we believe the relation is one of affinity ; and this 
affinity, being owt of the circle, we term external. In 
ordinary cases, where groups so situated are not very 
rich in species, the resemblance between them is not so 
strong, and is then termed an analogy ; but sometimes, 
as in the present instance, we believe that analogies 
blend into affinities, and that the two circles are actually 
united at more points than one. The subject, however, 
of the external affinities of typical grcups demands 
much abstruse investigation, before the theory can be 
placed beyond doubt ; and the student will do well to 
pass it over, until he is fully master of such principles 
* See the demonstration of this group, and of all its divisions, in North. 
Zool. vol. ii. p. 164, &. 
