SELECTION: ORGANIC AND SOCIAL 197 



stand, then, that if a certain shape were particularly 

 well suited for special conditions, that shape would 

 be selected, i.e. the birds that were constitutionally 

 unable to lay eggs of the fit shape would be 

 eliminated. Now Darwin points out that, in 

 sea-birds like guillemots and razor-bills, which 

 lay their eggs on the narrow ledges of precipitous 

 cliffs, the shape of egg is very markedly top-like. 

 The adaptiveness of the shape is that, if the egg 

 be jostled by the parent or some other bird, or 

 be caught in a swirl of wind, it rotates on its 

 short axis without rolling from its original position. 



Let us take another instance. The ^Esop prawn 

 (Hippolyte varians) may be red, yellow, blue, 

 orange, olive, violet, brown, green, and other 

 colours. It is born without a bias and it takes 

 on the colour of its surroundings, both when 

 young and when adult. Put some in a glass 

 aquarium, and line the sides and floor with paper 

 of almost any colour ; the prawn follows, and 

 from one colour it may be changed to any other. 



As bright yellow, blue, and violet are not 

 common colours among the seaweed, it has been 

 argued that the power of colour-change cannot be 

 the outcome of selection. But that is an absurd 

 conclusion ; it is by no means certain that the 

 bright colours are absent among seaweed, and, 

 besides, adaptiveness is rarely perfect. 



Many colour-adaptations are very striking. Thus 

 Prof. Poulton has shown that certain caterpillars 

 will, within certain limits, take on the colour 

 of their surroundings, and Engelmann has shown 

 that the peculiar algae known as Oscillatoria 

 become green in red light, red in green light 

 physiologically the best possible colours. 



