32 RUDOLF SÖDERBERG, STUDIES OF THE BIRDS IN NORTH WEST AUSTRALIA. 
here the simpled and the more advanced form of nest-building side by side, and 
consequently trace the development of the latter out of the former. On the left 
photograph, taken in Mowla Downs, both kinds will be discerned. This colony was 
erected on the ceiling of a cave. 
It is evident, that natural selection has in the case of the great majority of 
Passeriformes forced forward the characteristic shape of their nests and realized 
the highly developed art of building which these creatures possess. Here it is ac- 
cordingly the nest, and not the eggs, which has been affected by selection. The 
origin of the colours of the latter defies every effort at explanation. No connection 
of causes can be traced, the eggs do not satisfy the claims of systematization. 
It seems to me convenient in this connection to remark briefly that the attention 
I have given to the subject has induced me more and more to disregard the rather 
commonly accepted theory, that the original colour of the birds” eggs was white. 
The white colour may at least have been eliminated so soon, that probably already 
in the case of the first tree birds it had been exchanged a spotted colour. The 
base for the eggs, being the trunks or branches of ferns or trees, not the bottom of the 
hollows in them, was thus exposed to visits of reptiles. 
Sphenodon itself has rust-coloured stains on the eggs, and also among other 
lizards coloured eggs are known. 
If this characteristic, which is so common and recurs so often (as atavism!) 
on later developed unspotted eggs, has its origin during so early a period of the 
birds” long era of development, one has in this fact a cause for the mysterious power, 
with which the eggs of small birds keep up their capacity for variation and spot- 
tedness, although the importance of natural selection must in so many of these cases 
be regarded as slight or even nil. 
Casuariiformes. 
Fam. Dromeaeidee. 
Drom2&us Nov&-Hollandig LATA. subsp. woodwardi. 
Math. handlist n:r 1 (Emu, vol. VII, part 3, jan. 1908). J juv. Nooncanbah, Fitzroy river ”/i 1911; 
J ad. Mount Andersson, Fitzroy r. !9/4 1911. 
Ecological. — Was seen breeding in the neighbourhood of Mowla Downs in Nov. 
On the ?/12, shortly after the first fall of rain, two ostriches were observed at Mowla 
Downs. One of them was considerably darker than the other, so they were most 
probably male and female. They were running forward with a winding motion, but 
stopped at times in order to snatch up something from the ground. Some days 
before the 18th of Nov. two eggs had been found in the same neighbourhood, A 
nestling was obtained shortly afterwards. The breeding-season seems to fall here in 
