140 The Outline of Science 



Professor Poulton's experiments with the caterpillars of the 

 small toitoise-shell butterfly showed that in black surroundings 

 the pupa? tend to be darker, in white surroundings lighter, in 

 gilded boxes golden ; and the same is true in other cases. It ap- 

 pears that the surrounding colour affects the caterpillars through 

 the skin during a sensitive period the twenty hours immediately 

 preceding the last twelve hours of the larval state. The result will 

 tend to make the quiescent pupae less conspicuous during the criti- 

 cal time of metamorphosis. The physiology of this sympathetic 

 colouring remains obscure. 



Seasonal Change of Colouring 



The ptarmigan moults three times in the year. Its summer 

 plumage is rather grouselike above, with a good deal of rufous 

 brown ; the back becomes much more grey in autumn ; almost all 

 the feathers of the winter plumage are white. That is to say, they 

 develop without any pigment and with numerous gas-bubbles in 

 their cells. Now there can be no doubt that this white winter plu- 

 mage makes the ptarmigan very inconspicuous amidst the snow. 

 Sometimes one comes within a few feet of the crouching bird 

 without seeing it, and this garment of invisibility may save it 

 from the hungry eyes of golden eagles. 



Similarly the brown stoat becomes the white ermine, mainly 

 by the growth of a new suit of white fur, and the same is true of 

 the mountain hare. The ermine is all white except the black tip 

 of its tail ; the mountain hare in its winter dress is all white save 

 the black tips of its ears. In some cases, especially in the moun- 

 tain hare, it seems that individual hairs may turn white, by a loss 

 of pigment, as may occur in man. According to Metchnikoff, the 

 wandering amoeboid cells of the body, called phagocytes, may 

 creep up into the hairs and come back again with microscopic 

 burdens of pigment. The place of the pigment is taken by gas- 

 bubbles, and that is what causes the whiteness. In no animals is 

 there any white pigment; the white colour is like that of snow or 



