OBSERVATIONS ON A SPECIMEN OF " SULA BASSANA." 333 



development with a particular type, breaks will be found interfering 

 with the perfect symmetry of our plan, 



"What Mr. Swainson intends by naming the groups which make up 

 each circle, typical, suh-typical, and aberrant, is that one of these 

 groups will be found to be especially characterized by those peculiari- 

 ties of structure or mode of life which belong to the whole circle, so 

 as to display its distinctions in their most striking form, and this one 

 is generally found to be the most numerous in species : another group 

 will approach this both in its relations to the type and in number of 

 species, but will exhibit special features in analogy with some other 

 circle, so as to show the characters of the type differently modified. 

 The remaining groups, with a preponderance of the characters of the 

 type, unite such deviations expressing their own peculiar plan of de- 

 velopment, as make them appear like transitional forms leading towards 

 other circles. Mr. Swainson placed what he took to be the typical 

 group first, then the sub-typical, and after these the aberrant ; the 

 consequence of which is, that he places analogous forms in different 

 positions in the different circles. I by no means deny that in each 

 circle there is one group whose plan of development is specially 

 adapted to the type, and which thence displays most abundance and 

 variety of species ; this, indeed, logically follows from my general 

 theory ; but as I always place the representatives of the same tenden- 

 cies of development in the same position in each circle, it must follow 

 that the typical groups will have various situations in the different 

 circles, according to the characteristic tendency in each, whilst the 

 analogical groups will always be found in the same position, and this 

 I regard as a very important advantage of my plan. 



The preceding remarks will be sufficient on the present occasion, as 

 illustrating my idea of a natural system in the animal kingdom, and I 

 have had occasion to enumerate the great divisions of the class Birds, 

 with their mutual relations. It has been stated that the family Pele- 

 canidae, with which we are now more immediately concerned, belongs 

 to the order Natatores (swimming-birds), which may upon the whole 

 be regarded as containing the lowest forms of bird life. "We will first 

 inquire how many really distinct primary families are found in this 

 order, and further, how far their structure and habits correspond to 

 the notion of their representing in this circle, by modifications of its 

 general ty[)e, tendencies of development which have been already in- 

 dicated in the orders themselves, and which, according to our theory, 



