PRINCIPLES OF PROTECTIVE COLOURATION 219 



together suggest a definite fixed reflection-like picture of a fringe 

 of bushes along the shore, with the bright sky beyond cutting 

 in among their crowns and showing here and there between 

 them lower down. The white in the head and neck and cheeks 

 shows duly bright, while that on the throat from which the 

 higher spots are off-shoots is, in the bird's normal life 

 postures, dull with shadow and belongs mainly to obliterative 

 shading. In the resultant water picture, it renders a duller 

 sky reflection mixed perhaps with underwater effect." 



Supplemented as are these wonderful contrivances by ripple 

 sand and water pictures on its flanks, the water duck has 

 become a miracle of elusiveness. " The ripple marks he leaves 

 behind in his wake and those that roll out from his further side 

 are continued and repeated on his obliteratively coloured body 

 and this gives the final touch of perfection to his vanishment 

 and invisibility, so that one may scan a wood-duck-haunted 

 pool at close range and fail to see the ducks that are floating 

 upon it, just as one often looks in vain for the ruffled grouse 

 that is perching motionless in the apple-tree." 



Enough perhaps has now been written to show both the 

 originality of Thayer's work and the qualities of style which 

 distinguish the book produced by himself and his son. But a 

 glance at its illustrations is alone enough to demonstrate the 

 reality of this artist's contribution to the advance of science. 

 It is the underlying principle of Darwinism that adaptability 

 to environment is the primary condition of survival. He and 

 his immediate followers have elaborated evidential support for 

 this theory in regard to the organs and structure of animals, 

 till it may be regarded as proved. What all, with very few 

 exceptions, such as Prof. Poulton of Oxford, have hitherto 

 failed to do with any thoroughness is to explore that great 

 field of evidence which consists of the approximation to their 

 environment of the surface colouration of animals. Indeed, 

 by the two hypotheses of sexual selection and protective 

 mimicry, many of them deliberately assumed that such approxi- 

 mation to environment was not the governing law of animal 

 colouration. Thayer's contention is that such approximation 

 is, so to speak, actually more literally true of colouration 

 than it is of organic development. Indirectly the prairie 

 over which he roamed is responsible for the development of 

 the horse's hoof. It is directly responsible for the colour 



