ON EXTERNAL AFFINITIES. 235 



of the natural system as have been extensively demon- 

 strated. 



(289.) The nature of external affinities was first in- 

 timated in the following passage : " I must now advert 

 to another, and, in my estimation, an unquestionable 

 principle of natural arrangement : this is, the direct 

 union of typical groups, without the intervention of 

 those which are aberrant. A vague suspicion of some- 

 thing like this first occurred to me when studying the 

 affinities of the Laniada, in the year 1824.* This 

 property, however, belongs to very few groups, since it 

 has only been detected in such as are pre-eminently 

 abundant in species, and are not of a higher rank than 

 families. Beyond such groups, the higher we ascend 

 the more dissimilar are the typical groups in approxi- 

 mating circles, until, in looking to a diagram of the ver- 

 tebrated animals, not only does all appearance of affinity 

 between the external or typical divisions vanish, but it 

 becomes even difficult, in some instances, to trace their 

 analogy. The theory of external affinities, however, 

 belongs to a question so abstruse, and requires such 

 nicety of investigation, that, for the present, I should 

 rather, perhaps, put it as a query, than consider it as a 

 demonstrated fact. At the same time, I must confess 

 my utter inability to reconcile, by any other theory, the 

 evident and universally acknowledged affinity between 

 the Thamnophilinae and the Myotherince ; between the 

 MerulincB and the Philomelince, and more especially 

 between the typical Setophaga and the typical Sylvicolce. 

 Unless these affinities, which I have fully detailed, can 

 be disproved or explained by some other mode of rea- 

 soning, it seems to be impossible to arrive at any other 

 conclusion."f 



(290.) From the above theory on external affinities 

 would result another principle of natural arrangement, 

 superadded to those we have already explained as belong- 

 ing to natural groups. On this principle we shall not, 



* See Zool. Jo urn. vol. i. p. 302. 



f Swainson, in North. Zool. vol. ii. pref. li. 



