76 CHILDHOOD OF ANIMALS 



birds, in snakes and lizards, in frogs and toads, in very niany fish, 

 whilst most of the creeping insects and their larvse exhibit it. 

 There is no obvious difference in the structure of the body to account 

 for it ; the skin, fur, feathers or scales are formed in the same way 

 from similar materials aU over the body, and the difference cannot 

 be explained as the visible expression of anatomical facts. Nor 

 can it be explained as being due to the direct action of sunlight, 

 for the most probable effect of intenser sunlight on tissues is to 

 bleach rather than to stain, and if there be a difference according 

 to habitat, the contrast is greater in many of the swarthy inhabi- 

 tants of forests than in natives of the open plains. 



Although we do not know the physical cause of this common 

 pattern, Mr. Thayer has shown its advantage to the many animals 

 which possess it. Professor E. B. Poulton, of the University of 

 Oxford, had already shown how in some caterpillars the distribution 

 of light and dark shades destroyed the rounded appearance, and 

 made the plump bodies appear flat and not sharply marked off 

 from the food-plant on which they were resting, but Mr. Thayer 

 worked out the idea independently and showed how it applied on 

 a much larger scale. If a white billiard-ball be placed on a table 

 where it is lighted from above, or carried into the open air and 

 similarly exposed to the sky, it wiU be seen that it looks round and 

 solid chiefly because it is brilliantly white above where it is fully 

 lighted, and almost black near the surface on which it is resting, 

 because there it is in the shade, whilst between the two poles the 

 light and shade gradually pass into each other. Now the natural 

 disposition of Hght and dark colour on an animal is so arranged as 

 to counteract this result of natural illumination, for the dark shades 

 are found on the upper parts where the illumination is greatest, 

 and the light shades on the under surface where the illumination is 

 least. The natural pattern, in fact, is a counter -shading of the 

 natural illumination. It is not easy to get any pigment to adhere 

 to the surface of a bUliard-baU, but if two rubber balls be taken 

 and one painted white all over, the other painted white on one end, 

 black on the other, and gradually shaded off to the equator, and 

 then the two balls be placed alongside where they are illuminated 

 from above, the effect will be seen at once. The white ball will 

 look round and solid, white above and dark below ; the other ball, if 

 it be placed with the dark pole uppermost, will look flat, and almost 

 of an even grey tint all over. 



In this way the plump solidity of the natural contours of an 



