THE FACTORS OF EVOLUTION 169 



from the hedgehog imparts to it temporary immunity. What is 

 true of the hedgehog is true of all other snake-hunters, so that 

 mimicry fails so far as these enemies of snakes are concerned. 



The only remaining enemy, man, makes no difference what- 

 ever in the destruction of venomous and harmless snakes, except 

 perhaps that the destruction of the former is carried on more 

 energetically. Probably snakes possessed their coloration long 

 before the species man appeared on earth. 



As a last interesting instance let us examine the little trees 

 frog, which is said to adapt itself voluntarily and consciously to 

 its surroundings. Like many other amphibians, the tree frog 

 possesses to a high degree the faculty of changing its colour. In 

 its skin there are found two kinds of contractile pigment-cells, or 

 chromatophores, of which one contains granules of a yellowish- 

 green, the other a bluish-black pigment. In proportion as the 

 light or dark chromatophores contract or expand, the colour of 

 the frog changes from a light green to a deep bluish-grey. It 

 was at one time generally believed that the frog was able to effect 

 this striking change as the result of an eye impression, and adapt 

 itself accordingly to its surroundings. But this statement if 

 incorrect, quite apart from the fact that it ascribes to the animal 

 an exalted degree of intelligence. This change of colour is nothing 

 but an unconscious reflex-action resulting not from an eye im- 

 pression but from the reaction to the stimulus of touch. For, as 

 Biedermann has shown, if we place the frog on a smooth surface, 

 whether light or dark, yellow or grey, it will invariably assume a 

 greenish hue, whilst on a rough surface it assumes a dark colour. 

 Whenever the animal, therefore, harmonizes with its surroundings, 

 it is merely due to an accident which must very frequently happen 

 in Nature, because numerous leaves have a smooth surface, 

 whilst the earth's surface is rough. In many cases, however, 

 the tree frogs are completely deserted by this characteristic. 

 After Biedermann's experiments we can therefore no longer speak 

 of conscious adaptation. 



What, after these observations, is now our position towards 

 the hypothesis of protective colouring and mimicry? Though 

 we do not agree with Eimer and other investigators, who flatly 

 deny the usefulness of such characteristics, there is no doubt that 

 the biological importance of colour-adaptation and mimicry has 



