450 II. BOSCHMA. 



Ptychozoon, when the external conditions are quite the same 

 except the color of its surroundings, assumes the color which 

 simulates in the highest degree that which it sees with its eyes. 

 In other reptiles no instances are known that the changes of 

 color are influenced by the vision of the animals, and also experi- 

 ments on this question only gave negative results (cp. van 

 Rynberk, 1906; Fuchs, 1914). 



An important fact is moreover that the color of the skin of 

 Ptychozoon, when the temperature is constant, becomes lighter 

 on a white background and darkens on a black one. After 

 Parker (1906) the pigment-cells of all reptiles expand in the 

 light, whilst they contract in the darkness. In those cases, 

 where by other authors different conclusions have been made, 

 these would be due to changes of temperature which had not 

 been taken into account by these authors. In those experiments 

 in which the color changes of reptiles alternately in the shade 

 and in the sunlight were studied, heat reactions may have in- 

 fluenced the movements of the chromatophores, in my experi- 

 ments, however, the animals remained always on the same spot 

 in diffuse daylight. The temperature was always very uniform 

 and the lizards reacted directly on stimuli of white and black 

 surroundings in the above described manner. 



A contradiction to Parker's theory is found in the statements 

 given by Thilenius (1897) for Varanus. According to this author 

 Varanus assumes a dark color in the shade in 45-50, whilst in 

 the sunlight at a temperature of less than 30 the color becomes 

 light. Parker has expressed his doubts as to the correctness of 

 these temperatures, which might have been read from an ordinary 

 mercury-bulb thermometer. With a more precise instrument the 

 result would have been a much higher temperature. The differ- 

 ence between the true temperature and the one recorded in this 

 case, however, would then have been more than 20, and this 

 difference is too large to be put on account of the inaccuracy of 

 the instruments. In the case of Ptychozoon the color changes 

 take place in the same way as described by Thilenius for Varanus 

 and this proves that the chromatophores of at least some lizards 

 contract in light and expand in dark. 



In another way Fuchs (1914) has tried to explain the fact that 

 light in some reptiles causes an expansion and in others a con- 



