PROTECTIVE COLOURING IN ANIMALS 307 



observed under the open canopy of heaven, a deep shadow 

 will exist on its lower surface and make it as obvious as a 

 shaded charcoal drawing on a piece of light-brown paper. 

 But if the back of the animal is of a dark tint and its belly 

 white or whitish, then the effect of light and shade is 

 (Mr. Thayer showed) completely counteracted and the 

 animal becomes totally invisible in its natural sur- 

 roundings. 



Mr. Thayer's model demonstrating this consists of two 

 life-size wooden models of ducks seated on a stick one 

 to the left, the other to the right. The stick, with the 

 two models on it, is fixed horizontally in a box, which is 

 open above (that is, has no lid) and is also open in front. 

 The box is, in fact, a little stage, lit from above by the 

 light of the sky, and its three remaining sides are suffi- 

 ciently high to form a complete background to the model 

 ducks, whose perch runs across the "scene" at some 

 7 in. or 8 in. from the floor of the box. The box itself 

 is lined with a pale purplish-brown flannel, and each 

 bird is tightly covered with the same material. When so 

 prepared the box is placed on a table under a skylight 

 (where it is to stay), the table being high enough to bring 

 the ducks just below the line of sight. Of course, deep 

 shadows are formed by the top-light on the under side of 

 the beak, head, and body of the models, and in spite of 

 their colour being itself identical with that of the walls of 

 the box, they are as obvious as it is possible for anything 

 to be. Now Mr. Thayer takes his paints and very carefully 

 darkens the back of one of the ducks and whitens its 

 belly and the under side of its head and beak. The light 

 and dark regions merge into one another along the side of 

 the bird by skilful gradation. When this shading and 

 whitening is finished (and, of course, the perfection of the 

 result depends on the continuance of the right amount 

 of sunlight, which is not a thing one can always ensure 



