DIA 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 
sroup, in this sense of the term, in the whole circle 
of ornithology, is perhaps that of the sub-family 
Piciane, or true woodpeckers, wherein we have 
- ascertained, by the inductive process here expiained, 
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 ( Zrochilide) ; 
a group, moreover, which every one perceives is as 
natural as that of the parrots, the owls, or the birds 
of prey. The Trochilide, 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 
