> 
40 
birds, point to the disuse of wings in flight as the main condition of increase of size in 
species of birds — the next condition being absence of lethal enemies during the years 
requisite for such course and rate of growth. 
Let foes arise from whom a power of flight is the main condition of escape, and the 
wingless giants of the feathered class soon succumb. Among the genera above-cited, 
Aptornis, Cnemiornis, ASpyornis, Palapteryx, Linornis, Pidus, and Pezophaps, with the 
largest of the Auks, have thus passed away, while Notornis and Apteryx are on the 
verge of extinction through the rapid increase of population in the small island to 
which they are restricted. In sparsely peopled continents, such as Africa, South 
America, and Australia, brevipennate giants may still range the deserts, pampas, and 
unfrequented wilds. The ascertained recent advent of Man in New Zealand, New 
Britain, Ceram, Banda, Salwattie, Mauritius, Bodriguez, significantly points to the 
conditions under which have come to pass, in lapse of time, so strange an anomaly as a 
bird with the specially modified instruments of flight reduced below the power of 
exerting that mode of locomotion, yet, as a bird, retaining the conditions of the 
respiratory and tegumentary systems of the volant class, of which it has become a 
degenerate member. With the cessation of the chief of those conditions, viz. the ' 
absence of enemies, such birds necessarily perish. 
Eefraining, however, from further indulgence in an easy and seductive vein of specu- 
lation, I would recall attention to the notable protuberance in the cranial cavity of the 
Dodo (PI. XI. fig. 1, o) developed towards the upper part of the vertical tentorium, 
contracting at its lower part into the ridgq dividing the prosencephalic from the mesen- 
cephalic chamber. In the latter are the orifices for the issue of the trigeminal nerve, the 
larger and posterior (ib. tr) giving passage to the third and second divisions, and answering 
to the combined foramen ovale and rotundum of mammals, and the smaller and anterior 
foramen dismissing the first or orbital division of the fifth nerve. At the upper part of 
the mesencephalic fossa the narrow groove for the lateral venous sinus impresses and 
defines the back part of the tentorial protuberance, above which it bifurcates, the lower 
branch bounding or defining the wall of the superior semicircular canal and the upper 
part of the primitive acoustic capsule. Below this arch is an oblong cerebellar fossa (ib. n) 
which appears to have received veins from the cranial diploe. Beneath this fossa, and 
just behind the mesencephalic chamber, is the multiperforate internal auditory depres- 
sion. Next behind this is the outlet for the vagal nerve and entojugular vein. Below 
this are the small precondyloid foramina. There is a falcial ridge, low and thick, indi- 
cating the division of the prosencephalic chamber into lateral compartments for hemi- 
spheres ; and this ridge shows a narrow groove as for a small longitudinal sinus. A 
transverse linear groove abruptly defines the fore part of the ridge. 
The vertically expanded anterior part of the premaxillary (ib. fig. 1, 22 ) has a large 
pneumatic cavity communicating by a reticulate wall with the cells of a cancellous struc- 
ture, larger than those of the cranial diploe. The maxillary branch of the premaxillary 
