1894- THE SIGNIFICANCE OF THE BIRD'S FOOT. 455 



Screamers, combine with their slightly-webbed feet a normal hallux, 

 so that in this group there is a fairly clear indication of a transition 

 from semi-arboreal land-birds to waterfowl, which may help to explain 

 the tree-nesting habits of many of the ducks, such as Dendrocygna, 

 Aix, Clangula, etc. 



Among birds of the family Limicolae, the hallux is on the 

 point of disappearance ; in many plovers it is absent, and where 

 present very small and useless. Among the gulls, which are usually 

 considered as offshoots of the terrestrial Limicohne stock, though 

 usually present in a functionless condition, this toe is on the very 

 point of disappearance in the common Kittiwake (Rissa tridactyla), 

 some individuals of which species have it, while others have 

 only a mere tubercle. Here, again, it is noteworthy that Dvotnas, 

 probably a connecting Hnk between Gulls and Plovers, and there- 

 fore presumably more archaic than either, exhibits semi-palmate feet 

 with a fair-sized posterior toe. 



The arboreal pigeons have a typical grasping foot, while in their 

 terrestrial relatives the Sandgrouse it is disappearing, being quite 

 absent in the highly-specialised Syrvhaptes. 



Few important groups exist in which the hallux is entirely 

 wanting, the most noteworthy among Carinates being the Bustards ; 

 but the affinities of these to hallux -possessing forms are obvious. In 

 many birds usually regarded as tridactylous, subcutaneous vestiges 

 of the missing toe are to be found, notably in some Woodpeckers and 

 Albatrosses, thus actually proving what is easily inferred, that the 

 toe has disappeared in these forms. It is noteworthy that in almost 

 every case where a hallux appears at all, it retains to the last its 

 posterior direction, thus showing its original function. This may be 

 seen even in some Ratitae, e.g., in Apteryx. 



The exceptions to this rule may usually be easily explained. The 

 forwardly-directed first toe of some Swifts is obviously a special 

 adaption, as also are the versatile halluxes of the Coliidse and 

 Goatsuckers ; while the hallux of the Steganopodes, often stated to 

 be turned forward, really has an inward or backward direction, and 

 functions in perching in the ordinary way, as anybody may observe 

 in living birds. These birds are, still, less attached to the water than 

 most aquatic birds, and their semi-arboreal habits have no doubt 

 descended to them directly from an ancestry which they share with 

 the Herons. 



It may be argued that many arboreal birds may have been 

 originally terrestrial, just as certain geese {e.g., Cereopsis) have left 

 the water for the land. But we know that terrestrial life leads 

 sooner or later to a reduction of the toes, and that the hallux 

 is the first to disappear in almost every case, so that it would 

 have had to be re-acquired to fit a ground-dweller for a Hfe 

 in trees — and there is, as Garrod has already pointed out, no 

 reason to believe that an organ originally lost is ever reproduced. 



