234 FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 

 Thamnophilinae MyotherinR 



other in many points of strong analogy.* It will be 

 seen that these two circles touch, or blend into each 

 other (a a), by means of the Edolince (a) in one circle, 

 and the Brachypodina (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 Laniance and Thamnophilinte in one, 

 and the Meruladce and Myotherince in another. The 

 internal affinities of the Thamnophilince are two ; one to 

 the Laniance, the other to the Edoliance ; all three 

 groups being parts of the same circle j but then there 

 exists, at the same time, such an intimate resemblance 

 also between the ThamnophUince and the Myotherince, 

 that we believe the relation is one of affinity ; and this 

 affinity, being out 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 groups 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, &c. 



