THE CLASSIFICATION OF BIRDS 23 



:. Aves with normal plantar tendons, and with 

 the fifth secondary present. 



DROM^EOMORPH^E. Aves with the palatines articulating with 

 the pterygoids, and not with the rostrum of the basisphenoid. 

 Keel of the sternum obsolete. Coracoids fused with the 

 clavicles. 



The order in which these five subclasses may be placed is a 

 matter of no importance whatever, except so far as any evidence 

 may exist to- show that any two of them are more nearly related 

 to each other than they are to the rest. Thus it is possible that 

 the Dromaeomorphas, which are supposed to be quincubital, are 

 more nearly allied to the ^EgithomorphEe than they are to the 

 Pelargomorphge. The Coraciomorphse contain both quincubital 

 and aquincubital species, and may consequently be placed between 

 the aquincubital Pelargomorphse and the quincubital ^githo- 

 morphse. On the other hand, the diagnosis of the three central 

 groups may be regarded as weak, whilst those of the two external 

 ones are exceptionally strong. 



It is very unlikely that the ancestors of the five subclasses 

 of birds were isolated from each other contemporaneously. At 

 what particular date the isolation of the Penguins took place it 

 is impossible to determine. There is no evidence to suggest 

 specially near relationship to any other group, though isomor- 

 phisms can be traced in abundance. It must be assumed that 

 the Penguins are descended from birds which were able to fly, 

 but whether they were isolated after the aquincubital birds were 

 developed, and if so, to which party they belonged, it were idle 

 to guess, inasmuch as all traces of primaries or secondaries have 

 disappeared from the wing of the Penguin. The Penguins occupy 

 a position amongst Birds analogous to that of the Whales amongst 

 Mammals. They are the most highly specialised amongst birds, 

 the aim of their specialisation being to adapt them to an aquatic 

 life, and to enable them to get their food by diving. To give them 

 time to metamorphose their wing into paddles so completely as 

 they have done, it must be assumed that their isolation occurred 

 at a very early date, sufficiently early to warrant us in regarding 

 the Penguins as the survivors of a group of birds whose isola- 

 tion dates back far enough to entitle them to hold rank as a 

 subclass. 



