146 ACQUIRED CHARACTERS sec. 



or simple general stimulation of the nerves, determines the 

 change of colour, or whether, lastly, the gradual action of 

 selection simultaneously plays a part in the matter, it will of 

 course be often difficult to decide for any given case. 



The following really astonishing case of " adaptation " of 

 the colouring of an animal to the ground on which it lives 

 forces upon me the idea that selection does not play a part 

 in it, but that it is due to the direct external action of the 

 colour-stimulus ; the effect, however, probably not showing it- 

 self at once, but appearing more gradually than in many cases. 



The grasshopper with red hinder-wings banded with black, 

 which is so common with us (in Germany) in summer, 

 Acridium germanicum (CEdipoda germanica), when it occurs 

 on the reddish-brown Triassic clay of Tubingen, resembles 

 this ground so closely with its wings folded, that it cannot 

 be distinguished from it. A little above the clay on the hills 

 of this neighbourhood there occurs a whitish sandstone, 

 sometimes only for the breadth of a path or in somewhat 

 larger surfaces, frequently surrounded by the former. On 

 these small patches of lighter ground I find regularly only 

 grasshoppers with quite light upper wings, so that they can 

 scarcely be distinguished from the soil. And I have else- 

 where observed the same remarkable adaptation. One of my 

 friends, who is not usually accustomed to pay special attention 

 to such animals, told me that he had been much surprised to 

 notice that on the two banks of a brook on which the soil was 

 of different colours, the grasshoppers were in each case exactly 

 like the ground in colour. Without doubt these were Acri- 

 dium germanicum or Acridium ccerulescens — the latter species 

 appears to show the same adaptation. 



Experiments must decide whether this is a case of stimula- 

 tion-colouriu!^. That such stimulation-colouring occurs in the 

 tree-frog I have mentioned above, and it evidently occurs 

 also in other Amphibia. 



