4 STRUCTURE AND CLASSIFICATION OF BIRDS 



A comparison of the varied forms of feet among birds 

 has an important bearing upon the origin of the foot. From 

 this follows some insight into the nature of the life led by 

 the possessors of the most primitive form of foot. The 

 matter has been put forward in a clear fashion by Finn,' 

 and in a correspondence which his paper elicited. As all the 



-evidence at our disposal seems to show that the four-toed 

 foot is the more primitive, we have to decide whether the 

 palmate foot of the pelicans or the grasping foot of the 

 passerine is the earlier, or whether some modification of 

 these, such as the zygodactyle, or four-toed foot with a 

 rudimentary hallux, is the more primitive. The four-toed 

 foot of the Steganopodes is often figured as if all the toes were 

 directed forwards, but this is really not the case ; they are, 

 as in terrestrial birds with a more or less rudimentary 

 hallux, directed at least sideways. This seems to argue that 

 the original form of the foot was as it is now in the Passeres, 

 a fact which is still further enforced by the foot of Archceo- 

 pteryx (see below). The more purely terrestrial the birds are 

 the more rudimentary is the hallux, until in the purely 

 terrestrial bustards the hallux has disappeared altogether, 

 and in the ostrich, most terrestrial of birds, the second toe 

 has vanished also. Among the gallinaceous birds, moreover, 

 the more arboreal forms, the Cracidse and Megapodidse, the 

 hallux is better developed than in those that do not roost in 

 trees. That the zygodactyle foot is a further modification of 



^the anisodactyle seems to be shown by the transitional state 

 of the owls, which ' always perch in the zygodactyle position,' 

 the fourth toe being capable of reversion.^ 



Beak 



In all existing birds the upper and lower mandibles are 

 invested with a horny sheath, the beak. The form of this 



' ' The Significance of the Bird's Foot,' Natural Science, June 1894 ; see 

 also July and September Nob. of the same periodical for further notes anrl 

 correspondence on the matter. 



'■^ A. Eeichenow-, ' Die Fussbildungen der Vogel,' J. f. 0. 1871, p. 401. 



