2/0 COLOURATION OF REPTILES AND AMPHIBIANS. 



containino- cyanide of i)Otassiuni they become a uniform yellow, 

 all markings being completely obliterated, within a few minutes. 

 A male specimen taken on the veld at midday on December i6th 

 was this colour. After spending a short time in a box with 

 some lichen it had become a very dull l)rown with darker and 

 lighter marks, and a light vertebral streak. In trying to make it 

 produce its original yellow, I applied an electric current to various 

 parts of the bod}-, and, on the following day, put it into a trough 

 where the water was fairly deep. These experiments produced the 

 same results : in both cases all marks became obliterated on the 

 top of the head, which became pale green or yellow, the body 

 assumed a reddish brown hue Avith various lighter vermiculations ; 

 while the most striking change of all was a broad vivid bluish- 

 green line down the centre of the back and half-way down 

 the tail. The colour of this line was most pro- 

 nounced in places where there is a dark bar on the tail and a 

 dark patch on the back ordinarily. This green vertebral line was 

 also produced by allowing the water from a tap to flow down the 

 centre of the back. A male A. atra, when subjected to the same 

 treatment, became a dark pink, while the head and half-way down 

 the back became a ])ur]jle colour. 



The effect of the foregoing stimulus is more or less general, 

 for instance, an electric current applied at the extremity of the 

 tail may produce colour effect in the head, etc. Allowing water 

 to run from a tap on the head of the specimen seems, however, 

 to produce a more or less local effect. 



Two (|uite black specimens of Xeiioptts hcvis, one of which 

 was blind, were put into a white-enamelled trough in which there 

 was about six inches of water. The surface of the water was 

 covered with thin, white paper; in less than 12 hours the non- 

 lilind speciment was a light yellow, while the other remained a 

 uniform black. 



The physiological mechanism by means of which such a 

 change takes place is described by Prof. Poulton''' in the follow- 

 ing way : — 



Certain kinds of reflected liglit act as specific stimuli to tlie eye of 

 the animal, and dilTering nervous impulses pass from this organ along the 

 optic nerve to the brain. The brain, being thus indirectly stimulated in a 

 peculiar manner by various kinds of reflected light, originates different im- 

 pulses, which pass from it along the nerves distributed to the skin, and 

 cause varying states of concentration of the pigment cells. 



In page 8 of his book the same author remarks : 



In some cases of colour in animals the chinks between the layers of 

 tissue are kept open by films of less powerfully refractive liquids. When 

 the tissue becomes dry the films evaporate and the colour disappears. We 

 must suppose that the denser layers come together, obliterating the chinks 

 and excluding the air; otherwise the colours would be more brilliant than 

 ever, because the refractive power of air is even more than that of liquids. 



After reading the foregoing I removed the ejjidermis of a 

 female J. eculcata on which scarlet blotches were j^resent. This 

 I dried and i)ut awav in a book for a few weeks; at the end of 



*The Colours of Animals. 



