332 OBSERVATIONS ON A SPECIMEN OF " SULA BASSANA." 



that are observable in nature ; or, at least, that any others that can 

 be pointed out are only occasional accompaniments of some of these. 



One other observation, entirely overlooked, I believe, by Mr. Swain- 

 son and his followers, is in my estimation of very high importance for 

 attaining to a good and natural classification. It is that occasionally, 

 perhaps frequently, under one general type of structure we may find 

 not only the five tendencies of development as above explained, but 

 these exhibited in connection with several different well-marked de- 

 grees of development. Thus in the highest class, that which peculi- 

 arly expresses power and elevation of structure, of the sub -kingdom 

 or branch Vertebrata — the class Mammalia — we have four circles ex- 

 pressing degrees of development, in each of which — excepting the 

 highest, which is occupied by man alone — we find manifestations of 

 all the five tendencies of development in natural groups, which are 

 here called Orders ; and in the class Birds, which in the same sub- 

 kingdom expresses the tendency to activity and grace, with prevailing 

 adaptation to aerial or arboreal life, we obtain the clearest idea of the 

 affinities, by placing in the centre the most especially typical groups 

 of small birds forming the great families of the warblers and finches ; 

 surrounding these by a circle of five sub-orders of Insessorial birds ; 

 and again placing outside of them five more deviative forms exhibiting 

 greater extremes of the tendencies of development already displayed 

 in the inner circles. The great mass of birds forming the two inner 

 circles, constitute the order Insessores, or perching birds, in technical 

 systems ; whilst the outer circle embraces Eap tores, birds of prey ; 

 Scansores, climbing birds ; Rasores, poultry and game birds, with 

 which are connected the ostrich tribe, sometimes regarded, very need- 

 lessly, as a separate order ; Grallatores, stilted or wading birds ; and 

 Natatores, swimming birds, -^the latter being the lowest group of 

 birds, and analogous with the Cetacea among mammalia. 



With due attention to these concentric circles expressing varying 

 degrees of development of the same ty{)e, I am disposed to maintain 

 that the several tendencies pointed out will bring before us all really 

 distinct families in every part of the animal kingdom excepting that 

 in the lowest divisions, as might have been anticipated from reason- 

 ing, the lower forms can have no place, so that we recognise only three 

 tendencies instead of five ; and that in various parts of the general 

 system there will be cases in which, from our ignorance of certain ex- 

 isting forms, or from the incompatibility of a certain tendency of 



