39 
that cavity in Didus : the diploe gradually decreases in thickness as it approaches the 
foramen magnum. The disposition of the osseous lamellse forming the cells or cavities 
of the diploe is very different in the Elephant and Dodo ; they extend for the most part 
vertically between the outer and inner tables of the skull in the proboscidian mammal, 
leaving long and narrow interspaces ; in the heavy ground-bird they form a congeries 
of small subequal and subspherical air-cells, and this structure obtains in the basal and 
lateral walls as well as in the superior or “ roofing ” wall of the cranial cavity. The 
extent of this cancellous structure at the sides of the cranial cavity may be known by 
the ratio of the breadth of that cavity to the breadth of the cranium, which is 3 inches 
and 8 lines at the broadest part of the brain, viz. the prosencephalon. It would seem, 
at first sight, as if the poorly developed brain of the Dodo had needed, on some account, 
unusual protection ; but the true explanation rests on the size, weight, and power of the 
bill, and the concomitant necessity for adequate extent of attachment of the facial to the 
cranial part of the skull, and of the muscles from the trunk destined to sustain and wield 
the long and heavy-beaked head. The cerebrum of the Dodo does not greatly, and by no 
means prop-ortionally, exceed the size of that part of the brain in the Crown-pigeons 
(Goura). If the great Ground-dove of the Mauritius gradually gained bulk in the 
long course of successive generations in that uninhabited thickly-wooded island, and, 
exempt from the attacks of any enemy, with food enough scattered over the ground, 
ceased to exert the wings to raise the heavy trunk, then, on Lamarck’s principle, the 
disused members would atrophy, while the hind limbs, through the increased exercise 
by habitual motion on land, with increasing weight to support, would hypertrophy. 
In the long course of generations subject to this slow rate of change, there would be 
nothing in the contemporaneous condition of the Mauritian fauna to alarm or in any 
way to put the Dodo to its wits ; being, like other Pigeons, monogamous, the excite- 
ment, even, of a seasonal or prenuptial combat, might, as in them, be wanting ; we may 
well suppose the bird to go on feeding and breeding in a lazy, stupid fashion, without 
call or stimulus to any growth of cerebrum proportionate to the gradually accruing in- 
crement of the bulk of the body. W^hatever part of the brain was concerned in regu- 
lating or controlling muscular actions, might, indeed, be expected to show some concur- 
rent rate of increase with the growing mass of the voluntary contractile fibres ; and the 
size of the cerebellar division (PI. XI. fig. I, n o) of the cranial cavity accords with 
the generally accepted physiology of the superincumbent mass of the epencephalon. 
The lateral depression at the fore and under part of the side of the postcerebral division 
of the cranial cavity indicates that the optic lobes, like the eyes, remained almost 
stationary during the progressive acquisition of the bulk that distinguishes the Dodo 
from the largest existing Doves. 
The proportions of Didus, Pezophaps, Casuarius, Rhea, Dromaius, Struthio, Aptornis, 
Cnemiornis, Palapteryx, Mpyornis, Dinornis, &c. among terrestrial birds, of Notornis 
among the lake-haunting Coots, and of Aptenodytes and Alca impennis among sea- 
