208 THE ZOOLOGIST. 
beautifully white, often shining as if enamelled. The birds con- 
struct slight nests of twigs, placed crosswise on horizontal 
branches of trees. Much light can pass through the interstices 
between the twigs, and it is a difficult matter, even for the 
trained human eye, to detect from below whether there are eggs 
in the nest or not. Here the white, light-reflecting eggs are at a 
positive advantage. 
The Australian finches conceal their eggs in the depths of 
relatively huge covered baggy nests, provided with side spout- 
like entrances. The eggs are in no way visible from without, 
are securely stowed away, and are pure white. All of the 
English finches, on the contrary, lay in open nests, and the eggs 
are spotted, usually, too, on a neutral-tinted ground. In this 
case we may presume that we have preserved the ancestral type 
in Australia. 
Since a glaring uniform white must be a dangerous colour for 
exposed eggs, we are not surprised to find that variations, 
favourable to preservation, have been originated and preserved, 
and that colour is now a protection to the great majority of 
eggs. In all cases we have to consider two questions :—(1) How . 
could the colour have been acquired ? and (2) How is the 
colour now protective or otherwise beneficial ? That natural 
selection would be called into play to preserve favourable 
markings or tints we may allow, but we believe, with Mr. 
Seebohm, that ‘‘ natural selection is not the cause of evolution ” 
in this case, “ but only its guide.” 
The first question then is, How could the colour have been 
acquired ? and I do not know that anyone has attempted 
hitherto to give any answer to it. The following has occurred to 
me as a probable explanation of the process; at least the 
phenomena are referred back to principles already recognised :— 
In the first place, it is important to note that the shell of 
the ovum is formed in the third portion of the oviduct (“the 
uterus”), and entirely during the 12—18 hours which immedi- 
ately precede the expulsion or laying of the egg. ‘This is the 
length of the period in the case of the common fowl; we may 
assume, generally, a similar number of hours, probably shorter, 
in the case of the smaller species. That the formation of the 
shell is a process distinct from the formation of the yolk, is 
further brought before us strikingly, by an experiment of 
