XII COLORATION OF EGGS 325 
they arc? Guillemots seem never to lay two eggs 
that are similarly coloured. The ground colour 
varies from blue through many stages to whitc, and 
the flail-like marks vary much or are omitted 
altogether. Out of thirty Guillemots’ eggs it is often 
impossible to pick out two that are really alike. 
How is it that we do not find in other eggs a similar 
tendency to variation? To answer this question is 
difficult, but perhaps not so difficult as it might at 
first appear. Eggs have either a plain wash of colour, 
or else a ground colour marked with spots or dashes. 
All the markings, it is believed, are originally circular, 
but as the egg moves down the oviduct, they become 
smeared or lengthened out. Sometimes they take 
a spiral form—for instance, in some birds of prey—and 
this would seem to show that the egg rotates as it 
moves forward. Thus there is nothing that can 
properly be called a pattern to account for, and, as 
the pigment is simply a waste product used up, its 
constancy does not perhaps require a very profound 
explanation. There is really more variation in the 
eggs of many species than one is apt to think. But 
in a small egg the change in the shape of spots and 
blotches does not attract attention as it does on a 
larger egg, such as the Guillemot’s, where they too are 
ona large scale. In that, the most striking example 
of variation, the remarkable phenomenon is the range 
of the ground colour from what is almost a deep bluc 
to white, and this can be paralleled. With birds that 
lay only two eggs it often happens that nearly all the 
colouring matter is deposited on one only, sometimes 
on the first, and somctimes on the second of the two, 
