460 The Bird 
account for this latter condition on the hypothesis that an 
actual change—an increase—is slowly taking place in the 
number of eggs of this species, the abnormal shell reflect- 
ing the as yet only partial readjustment of the pigment- 
gland to meet the extra demand? 
The carbonate of lime, of which the shell is chiefly 
composed, varies in its composition, being sometimes so 
fine that the surface has a high gloss, the eggs of wood- 
peckers being a good example, or again loose and chalky, 
as in cormorants. In tinamous the glossiness is carried 
to an extreme, their eggs resembling ovals of highly 
burnished metal, green and purple in colour. 
The shells of ducks’ eggs are impregnated with an oily 
substance, which must be of great use in resisting the 
dampness and moisture of their surroundings. 
The eggs of some entire Families of birds are easily 
recognized by the resemblance of the grain of the shell; 
while, on the other hand, this microscopic appearance in 
the eggs of individual species may differ considerably, 
as in the case of the eggs of the Mute and Whooping Swans. 
The eggs of the North African Ostrich have a surface 
smooth as ivory, while the eggs laid by the South African 
birds are deeply pitted. The beautiful eggs of the casso- 
wary show an extreme condition, the light green surface 
of the egg being covered with raised irregularities of a 
darker green colour. 
The thickness of the shells of ostrich eggs is remarkable, 
and their strength permits their use as water-bottles 
invaluable boon to the Arabs of the desert. 
With the relative size of the egg and the bird which 
an 
