448 A HISTORY OF BIRDS 



tribe — birds far enough removed from any possible relationship 

 either to the Swifts or the Swallows. 



No less remarkable in this connection are the Gulls and the 

 Petrels, the Auk tribe and the Divers. 



To begin with the two first-named groups. Until com- 

 paratively recently these were regarded as close allies. Even 

 to-day there may be found some who are still of opinion that 

 this is the case. Certainly the close resemblance which certain 

 of the Petrels, e.j^., the Fulmar Petrel (Fu/juan's g-/aaa/is), pre- 

 sent to the Gulls lends colour to this view. Superficially the 

 readiest way of distinguishing the one from the other is by 

 means of the shape and position of the nostrils and the char- 

 acter of the beak-sheath. 



In the Petrels the rhamphotheca is compound, that is to 

 say, the horny sheath of both upper and lower jaws is made up 

 of several pieces ; while the nostrils take the form of tubular 

 apertures opening either on the ridge of the culmen or on each 

 side thereof In the Gulls the nostrils are slit-like, and placed 

 midway down the beak. Furthermore, in the absence of a 

 nasal septum the right and left narial apertures merge into one. 



A brief survey of the morphology of the two groups is 

 however sufficient to show at once that they are absolutely 

 distinct. The Petrels are scions of an ancient stock not very 

 distantly removed from the Stork tribe {Ciconiifornies). The 

 Gulls, on the other hand, are unquestionably a highly specialised 

 branch of the Plover group {Charadriifonnes) and related there- 

 fore to the Cranes {Gruiforines). 



The close superficial resemblance between the groups is to 

 be attributed entirely to homoplasy — the action of a like en- 

 vironment on similar parts of similar organisms. Both types 

 are aquatic, and seek their prey on the wing ; and both feed on 

 fish and carrion. The Petrels are essentially marine forms ; the 

 Gulls are not so uniformly wedded to the sea, many species 

 ascending rivers, and breeding inland on meres and similar 

 situations. The Petrels lay white eggs, those of the Gulls are 

 of the typical Plover type, so much so indeed, that it was on 

 this account that their affinities with the Plovers was first sus- 

 pected. 



The effects of their environment are well illustrated by these 

 two groups. The Gulls, as we have just remarked, are by no 



