THE RAZORBILL AND GUILLEMOT 23 



by the supposition that it helps each bird to find its own among the 

 many that may be crowded upon a ledge or stack. In support of 

 this, Yarrell states the fact that birds marked with splashes of paint 

 were found in their accustomed places day after day. The fact, 

 however, does not necessarily prove his contention, for there can be 

 little doubt that the birds return again and again to their accustomed 

 places before their eggs are laid. No doubt each bird does recognise 

 its own egg as long as it remains fairly clean, but, as Saxby pointed 

 out long ago, the eggs tend in a short time to become so coated with 

 filth, especially in wet weather, that they cease to show any notice- 

 able variation. When they are in this condition it is probable that 

 each bird would recognise the accustomed place much more readily 

 than the egg. In any case, there is no clear evidence that the varia- 

 tion has been evolved to facilitate recognition. It may more plausibly 

 be explained by the supposition that here natural selection has ceased 

 to act. The ancestral plover, from which the Auks are descended, no 

 doubt laid its eggs on the ground, and those survived whose eggs 

 assimilated most closely in coloration to the site habitually chosen. 

 Hence, as in the case of the Waders generally, the emergence of 

 eggs of a relatively uniform coloration. But from the moment the 

 ancestor of the Auks started to lay on open rock-ledges, chalk, red 

 sandstone, limestone, and others, the coloration of its eggs would, 

 owing to the change in the character of the site, cease to have pro- 

 tective value. The guillemot appears to have met the difficulty by 

 systematic sharing by the sexes in incubation, so that when one of 

 a pair was off the other was on, or usually so. This made the colora- 

 tion of the egg a matter of little importance, and variation, previously 

 checked by natural selection, had free play. 



The pear shape of the guillemot's eggs, and the somewhat more 

 conical shape of the razorbill's, serves, it has been suggested, to pre- 

 vent them from rolling off the ledge into the sea by causing them to 

 spin round when disturbed. In fact, the shape of the eggs does not 

 prevent them falling off the ledges in cascades when a colony is 



