TREE HORNBILLS AND GROUND HORNBILLS 
like that of a hare or an antelope, and air spaces traverse 
the body throughout. The bodies of all birds are 
aerated by expansions of the lung; but the hornbills 
are particularly so, and the absence of fat conduces to 
this spareness of appearance. Their flight appears 
heavy in spite of this, and they make-a great deal of 
noise, especially at getting off, which has been compared 
by Dr. Russell Wallace to the puffing of a steam 
engine. It is not all hornbills that lead an arboreal 
life. In Africa are at least two different species of 
ground hornbills, which are rightly placed in a distinct 
genus and termed Bucovax and Bucorvus. Their beak 
is not quite so huge as in the more exaggerated form in 
which it is found in Buceros and its allied tree dwellers. 
But it is still of respectable dimensions. Bucorvus 
sidles and prances with some stateliness, and, instead 
of living upon fruits, catches reptiles. It is not unraven- 
like in aspect, and is distinctly intelligent, to judge 
from the numerous specimens which have been on view 
at the Zoo in late years. The gait of this bird has pro- 
duced some alteration in the structure of the foot. It 
is still of the pattern of its tree-frequenting relatives ; 
but the inner toe of the three forwardly-directed ones 
has a much more massive formation than that of 
Buceros, and is of greater length. It is noteworthy that 
the same effect has been produced here as in man com- 
pared with the arboreal apes. Our great toe is the 
strongest of the toes, and bears most of the weight of 
the foot. So in the Bucorvus it is the physiological 
equivalent of the great toe which is correspondingly 
increased, for the real big toe is turned back. Now in 
ungulate animals which are also digitigrade, pressure 
upon the foot is associated with the disappearance of 
the outer toes and the predominance of the middle toe 
or toes. It is in some of its breeding habits that the 
hornbill is so remarkable and different from other birds. 
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