THEORIES OF LAMARCK AND DARWIN 393 



background. They could be detected chiefly by their 

 shadows when the sun was shining. As he walked along 

 the coast he came to a wide band of lava which had flowed 

 from a crater across the intervening country and plunged 

 into the sea, leaving a broad dark band some miles in breadth 

 across the white sandy beach. As he passed from the white 

 sand to the dark lava, his attention was attracted to a tiger- 

 beetle almost identical with the white one except as to color. 

 Instead of being white, it was black. He found this broad, 

 black band of lava inhabited by the black tiger beetle, and 

 found very few, if any, of the white kind. This is a striking 

 illustration of w^hat has occurred in nature. These two 

 beetles are of the same species, and in examining the condi- 

 tions under which they grow, it is discovered that out of the 

 eggs laid by the original white forms, there now and then 

 appears one of a dusky or black color. Consider how con- 

 spicuous this dark object would be against the white back- 

 ground of sand. It would be an easy mark for the birds 

 of prey that fly about, and therefore on the white surface 

 the black beetles would be destroyed, while the white ones 

 would be left. But on the black background of lava the 

 conditions are reversed. There the w^hite forms would be the 

 conspicuous ones; as they wandered upon the black surface, 

 they would be picked up by birds of prey and the black ones 

 would be left. Thus we see another instance of the operation 

 of natural selection. 



Mimicry. — We have, likewise^ in nature a great number 

 of cases that are designated mimicry. For illustration, cer- 

 tain caterpillars assume a stiff position, resembling a twig 

 from a branch. We have also leaf-like butterflies. The Kal- 

 lima of India is a conspicuous illustration of a butterfly 

 having the upper surface of its wings bright-colored, and the 

 lower surface dull. When it settles upon a twig the wings 

 are closed and the under-sides have a mark across them 



