ANALOGICAL TESTS OF CIRCLES. 335 



naturalists who think that other proofs are unnecessary. 

 But the verification of such an extensive group as that 

 we have now instanced, namely, the Merulida, is by 

 no means yet complete. 



(411.) The third test by which a circular group is to 

 be verified, consists in its being in unison with the theory 

 of variation : that is to say, each of its principal modi- 

 fications of form are to follow each other according to a 

 definite rule. This rule has been so fully and so clearly 

 explained in the body of the work, that it seems hardly 

 necessary to repeat it here : familiar examples, however, 

 will render it more apparent to the student ; and these 

 examples, to prevent the recital of those exceptions 

 which must be noted if we attempted to speak generally 

 of the whole animal kingdom will be drawn from some 

 of the groups of ornithology. We have shown that groups 

 can be made to appear natural and circular., when in 

 reality they are not so ; and we have now to show that 

 their erroneous composition cannot always be detected, 

 even by comparing their contents with those of another 

 established group, and thus testing them by the theory 

 of parallel analogies. An ornithologist of this country, 

 now retired from science, who has done much towards 

 the determination of the leading families of birds, and 

 who in many instances has shown great judgment in the 

 location of the groups, has nevertheless been led into an 

 arrangement of the shrike family (Laniadce Sw.) which 

 exemplifies the error we are now speaking of: he disposes 

 the groups of these birds in the following manner : 



Genera. 



Typical. Lanius. True Shrikes. 



Sub-typical Edolius. Drongo Shrikes. 



TTyrannus. Tyrant Shrikes. 



Aberrant. < Ceblepyris. Caterpillar-Catchers. 



(. Thamnophilus. Bush Shrikes. 



(412.) These divisions, it has been said, form a circu- 

 lar group, and each division follows in the order of succes- 

 sion here stated : and as the bush shrikes, as every one 

 knows, blend into the true shrikes, the circle is closed, and 

 the whole has a verisimilitude of being truly natural. 



