224? STUDY OF NATURAL HISTORY. 



ditions which authorise such groups to be called 

 natural. 



(156.) It must, nevertheless, be admitted, that 

 groups so highly perfect as those we have just con- 

 templated, are by no means of common occurrence ; 

 or, at least, our limited knowledge of nature has not 

 yet enabled us to discover them. The most perfect 

 group, in this sense of the term, in the whole circle 

 of ornithology, is perhaps that of the sub-family 

 Piciance, or true woodpeckers, wherein we have 

 ascertained, by the inductive process here explained, 

 the circular succession of affinity in each genus, and 

 consequently the characters of each sub-genus ; all 

 of which have actually been discovered, and are 

 now in the European museums. Another natural 

 group, even still more varied into different modifi- 

 cations, is that of the humming-birds ( Trochilidce) ; 

 a group, moreover, which every one perceives is as 

 natural as that of the parrots, the owls, or the birds 

 of prey. The Trochilidce, however, have not yet 

 been analysed and grouped with that high degree of 

 precision necessary to constitute a demonstration. 

 The parrots, likewise, when we look to the di- 

 versity of their forms, may be included among the 

 more perfect groups ; and the ornithologist, really 

 anxious to investigate truth, cannot have more 

 favourable materials to work upon than these. 

 There are so many considerations to be taken into 

 the account, so many diversities of the same general 

 structure not only to be reconciled but explained, 

 so many degrees of relationship to be unravelled, 

 and so many apparent anomalies to be illustrated 

 by analogous examples in other groups, that a 



