ORIGIN AND CHARACTERISTICS 37 



extent in the Plantain-Eaters and certain forms 

 of Rollers. In other groups this temporary con- 

 dition has become permanent, and we have what 

 is termed a ^' zygodactylous " foot, as presented 

 by the Woodpeckers, Cuckoos, and Parrots. 

 In the Mouse Birds (Colius) the hallux can be 

 brought forward whilst the fourth digit can be 

 turned backwards at will, whilst in some of the 

 Swifts and Goatsuckers all the digits constantly 

 turn forwards, this class of foot being techni- 

 cally called a ^^ pamprodactylous " one. In 

 many water birds the anterior digits are joined 

 together by a membrane, thus forming what we 

 call a webbed foot, so admirably adapted for 

 swimming, whilst in the Pelicans the hallux is 

 also joined to the other digits by a web. The 

 hallux is by far the most variable of the digits, 

 and may either exist on the same plane as the 

 other digits, when it is usually large and of 

 service, or be raised above the others, and is 

 then smaller and without function. This is 

 especially the case in the Wading Birds, in 

 others it is reduced to a mere stump as in the 

 Petrels and certain three-toed Woodpeckers, and 

 in others it is entirely wanting, as in the Auks, 

 Bustards, Ostriches, and allied birds (Ratitae), 

 and one or two Waders (Sanderling, Grey 



