CONVERGENT EVOLUTION 445 



family, which are known commonly as " Jacanas ". By most 

 systematists placed with the Rails, they are now known to be 

 Plovers which have assumed the fashion of a Rail in response 

 to the demands of a like environment. The Rail-like char- 

 acters are chiefly to be observed in the great length of the toes, 

 which is still further increased by claws of enormous length and 

 great slenderness. Unlike the Plovers and like the Rails these 

 birds pass their lives mainly on the water, and running about 

 over the surface of floating aquatic weeds. 



The Brazilian Jacana {Parra nigra) resembles our Water- 

 hen {Gallinula chloropus) in the development of a bare fleshy 

 shield on the forehead ; but this shield is yet more reminiscent, 

 both in appearance and structure, with the similar skin-folds 

 which occur around the base of the beak in certain Plovers of 

 the Genus Lobivanellus and Hoplopterus, which birds the Jacanas 

 still further resemble in the possession of powerful spurs in the 

 carpal or wrist-joint. The largest and most beautiful, however, 

 of all the ten species of Jacanas is the Indian Jacana or Water- 

 pheasant, Hydrophasianus chirurgus. Certain peculiar char- 

 acters of this and other members of the family are described 

 elsewhere. 



The Cranes {Gruiformes) and Plover tribe {Charadriiformes) 

 are undoubtedly closely allied groups, and each contain within 

 their ranks many aberrant and puzzling types. Each has pro- 

 duced forms resembling members of the opposite group. One 

 instance of this kind we have already examined in the case of 

 the Jacanas, which, though indubitable Plovers, have come to 

 bear a very close resemblance to the Rails. Yet another is 

 furnished by the Norfolk Plover or Thick-knee {CEdienemus 

 crepitans) which bears an unmistakable likeness to the Bustards, 

 so much so that systematists have endeavoured by various com- 

 promises to reconcile this fact. The difficulty has generally 

 been met by placing the Bustards — which belong undoubtedly 

 to the Crane tribe — with the Plover tribe, using this term in 

 a broad sense. 



The case is an interesting one, and shows clearly the results 

 of adaptation to environment. The Plover in question on the 

 one hand, the Bustards on the other, are descendants of fen and 

 marsh-haunting ancestors, and have exchanged this milieu for 

 dry arid plains. What are the factors which brought about 



