IN THE THEORY OF NATURAL SELECTION. 301 



are formed which restore the integrity of the organism by replacing 

 the lost structures as far as' possible. Thus in plants the healthy 

 tissues begin to grow actively around the seat of an injury, tending 

 to close it up, and to afford protection by impenetrable layers of 

 cork.' Purposeful reactions of this kind are certainly common in 

 the organic world, occurring in animals as well as in plants. Thus 

 in the human body an injury causes a rapid growth of the surround- 

 ing tissues, which leads to the closing-up of the wound ; while in the 

 Salamander even the amputated leg or tail is replaced by growth. 

 An extreme example of these purposeful reactions is afforded by 

 the tree-frog (Hyla\ which is of a light-green colour when seated 

 upon a light-green leaf, but becomes dark brown when transferred 

 to dark surroundings. Hence this animal adapts itself to the colour 

 of its environment, and thus gains protection from its enemies. 



Admitting this capability on the part of organisms to react under 

 certain stimuli in a purposeful manner, the question remains 

 whether such a power is a primitive original quality belonging to 

 the essential nature of each organism. The power of changing the 

 colour of the skin in correspondence with that of the surroundings 

 is not very common in the animal kingdom. In the frog this 

 power depends upon a highly complex reflex mechanism. Certain 

 chromatophores in the skin are connected with nerves * which pass 

 to the brain and are there brought into relation, by means of nerve- 

 cells, with the nervous centres of the organ of vision. The relation 

 is of such a kind that strong light falling upon the retina consti- 

 tutes a stimulus for the production of an impulse, which is conducted, 

 along the previously mentioned motor nerves, from the brain to 

 the chromatophores, thus determining the contraction of these 

 latter and the consequent appearance of a light-coloured skin. 

 When the strong stimulus (of light) ceases, the chromatophores 

 expand again, and the skin becomes dark. That the chromato- 

 phores do not themselves react upon the direct stimulus of light 

 was proved by Lister 2 , who showed that blind frogs do not possess 

 the power of altering their colour in correspondence with that of 

 their environment. It is quite obvious that in this case we are not 

 dealing with a primary, but with a secondarily produced character ; 



1 Compare Briicke, ' Farbenwechsel des Chamaleon.' Wien. Sitzber. 1851. Also 

 Leydig, 'Die in Deutschland lebenden Saurier,' 1872. 



* 'Philosophical Transactions,' vol. cxlviii. 1858, pp. 627-644. 



