iv CAUSES OF COLOUR-ADAPTATION 147 



Years ago I was much struck to find in a pine-wood in 

 the Tauber valley (at Bronnbach near Wertheim) that the 

 numerous toads living there, without exception, had the 

 yellowish red colour of the pine-needles covering the ground. 

 The common frog also, Rana temporaria, changes colour accord- 

 ing to the colour of the ground, not rapidly, but gradually. 



It seems to me, we may safely assume that such stimu- 

 lation-colouring has been in many cases the ultimate cause, 

 the origin of the formation of a permanent and hereditary 

 adapted colouring : for when any species or variety of an 

 animal has lived throughout a long time on a particular 

 ground, of which the colour has been constant, and has by 

 nerve-stimulation taken on this colour from the beginning, 

 this colour may at last fix itself in the animal unchangeably. 



In this way a colour -adaptation, in a high degree un- 

 changeable, may be produced without any selection, without 

 the struggle for existence; and probably many colour- 

 adaptations of animals of restricted habitat are to be ex- 

 plained in this way. 



Among such cases, probably, belongs a remarkable colour- 

 adaptation in frogs observed by Wiedersheim : the common 

 frog in the Engadine has always the speckled colour of the 

 granite on which it lives. 



It goes without saying, that selection may favour such 

 direct adaptation, and this has probably been the case with the 

 wonderful degree of adaptation to the surroundings shown by 

 the colour of wall -lizards, as I have proved by extensive 

 researches, colour-adaptations which, moreover, have doubt- 

 less been determined by the most various causes. 1 



My chief assistant, Dr. Fickert, has at my suggestion made 

 the following experiment in order to test the action of 



1 Cf. the section on the bearing of adaptation upon the origin of varieties in 

 lizards. 



