xii COLORATION OF EGGS 325 



they arc ? Guillemots seem never to lay two eggs 

 that arc similarly coloured. The ground colour 

 varies from blue through man)- stages to white, and 

 the flail-like marks vary much or arc omitted 

 altogether. Out of thirty Guillemots' eggs it is often 

 impossible to pick out two that arc really alike. 

 I low 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 docs on a 

 larger egg, such as the Guillemot's, where they too arc 

 on a 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 blue 

 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 sometimes on the second of the two, 



