v.] IN THE THEORY OF NATURAL SELECTION. 



309 



are detrimental, the organism attempts to defend itself against 

 the stimulus : a confluence of nutrient fluid takes place towards 

 the part upon which the stimulus has acted, and new tissues 

 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 scat 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 surrounding 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 constitutes 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 



^ Compare Brucke, ' Farbcnwechsel dcs Chamalcon.' Wicn. Sitzbcr. 

 1851. Also Leydig, ' Die in Deutschland lebenden Sauricr,' 1872. 



