43 
lias never observed. M. de Lucy calculates that a bird the size of 
a raven, in order to render itself as light as the air, must increase 
its dimensions to about a cubic metr6, or 35 feet 547 cubic inches. 
It is obvious tlierefore that the air-cells which are so largely dispersed 
over the b(jdy and in the bones of the bird, are not intended to enable 
It to render itself lighter than the air, which would in fact reduce it 
to utter helplessness. Without doubt, however, the air-cells are of 
immense importance in many ways, the greatest of wdiich, perhaps, 
is that of aiding respiration ; they also form an admirable contriv- 
ance for maintaining the temperature of the bo<ly, and by their 
presence in the bones, give lightness to the skeleton without taking 
from its strength Probably there arc many other useful purjioses 
served by the air-cells, some of which have been hinted at but are 
not clearly understood. 
Tn another place Sir Charles Bell says, “ it is remarkable how 
small an addition to their body will prevent them rising on the 
wing,” and further tells us that binls are oviparous, because they 
never could have risen on the wing, Avhen with young, had they 
been viviparous. Tins would certainly have been true had the hot 
air theory been the correct one, but I do not tbink we find it to bo 
the case ; the pelican and heron carry home heavy loads of fish to 
their young, and the larger birds of prey rise with considerable 
weights in their talons, the smaller ones with weights in proportion. 
1 he flying mammals are A’iviparous, and tire bat not only carries its 
young during gestation, but also attached to its bre<ists during their 
babyhood, and that without apparent inconvenience in flight. We 
must therefore, I think, seek some other reasons for birds being 
created oviparous, r<>asons Avhich I have no doubt will be found to 
apply equally to the oviparous reptiles, and to be altogether inde- 
pendent of the faculty of flight. It is a mistake to suppose the 
bird possessed only of sufficient jvowers of flight just to enable it to 
sustain and direct itself in its passage through the air, for we know 
circumstances in its chequered life often require the display of ex- 
ertions immensely superior to those required for the mere ordinary 
purposes of flight— now cradled by the wind, which bears it along 
its way without the slightest exertion — anon, laboriously beating up 
against a gale, which it taxes its utmost powers to succeed in making 
way against — always pursuing or pursued, its safety and sustenance 
depend upon its swifliie-ss and power of enduranca The vulture. 
