188 The Outline of Science 



Colour Permanently Like That of Surroundings 



Many animals living on sandy places have a light-brown 

 colour, as is seen in some lizards and snakes. The green lizard is 

 like the grass and the green tree-snake is inconspicuous among 

 the branches. The spotted leopard is suited to the interrupted 

 light of the forest, and it is sometimes hard to tell where the 

 jungle ends and the striped tiger begins. There is no better case 

 than the hare or the partridge sitting a few yards off on the 

 ploughed field. Even a donkey grazing in the dusk is much more 

 readily heard than seen. 



The experiment has been made of tethering the green variety 

 of Praying Mantis on green herbage, fastening them with silk 

 threads. They escape the notice of birds. The same is true when 

 the brown variety is tethered on withered herbage. But if the 

 green ones are put on brown plants, or the brown ones on green 

 plants, the birds pick them off. Similarly, out of 300 chickens in a 

 field, 240 white or black and therefore conspicuous, 60 spotted and 

 inconspicuous, 24 were soon picked off by crows, but only one of 

 these was spotted. This was not the proportion that there should 

 have been if the mortality had been fortuitous. There is no doubt 

 that it often pays an animal to be like its habitual surroundings, 

 like a little piece of scenery if the animal is not moving. It is safe 

 to say that in process of time wide departures from the safest col- 

 oration will be wiped out in the course of Nature's ceaseless sifting. 



But we must not be credulous, and there are three cautions to 

 be borne in mind. ( 1 ) An animal may be very like its surround- 

 ings without there being any protection implied. The arrow- 

 worms in the sea are as clear as glass, and so are many open-sea 

 animals. But this is because their tissues are so watery, with a 

 specific gravity near that of the salt water. And the invisibility 

 does not save them, always or often, from being swallowed by 

 larger animals that gather the harvest of the sea. (2) Among the 

 cleverer animals it looks as if the creature sometimes sought out 

 a spot where it was most inconspicuous. A spider may place itself 



