1901.] on Colour in Amphibia. 589 



enough to injure or disarrange the yellow layer, appears at once deep 

 blue. If the lesion penetrates the crystal layer, it looks black. 



When, by the action of the chromatophores, the black pigment is 

 shoved towards the surface, the green becomes more saturated. 

 When the pigment is pushed still further, between the interference- 

 cells, or even enveloping them partly, then the skin assumes a dull 

 dark-brown to black colour. Tree-frogs sometimes do this when 

 they feel dull and cold. 



Again, when the black pigment is withdrawn from the surface 

 into the deepest strata, and when the chromatophores themselves are 

 much contracted, the skin becomes yellow. Lastly, when the 

 chromatophores are more or less contracted, and when the yellow 

 colour, instead of being diffuse, concentrates into small drops, our 

 Tree-frog assumes a leaden grey to silky whitish hue. 



It is obvious that those Frogs and Toads which have only black 

 and yellow pigments, but no interference layer, have command over 

 but a limited change of colour, ranging from darker to lighter tints 

 of black, brown and yellow. For instance, in the common Toad and 

 in the common brown or grass-frog. 



As a rule the changes are slow. They may take hours or days. 

 One of the most striking by its rapidity of changes is Hyla aurea, 

 one of the Australian Tree-frogs. 



Whilst the mechanism is clear, the answer to the question, what 

 these changes depend upon, is rather difficult. The play of the 

 chromatophores themselves depends upon various causes. Stoppage 

 of the circulation of the blood in the skin causes contraction of 

 the black chromatophores. A slight overdose of carbon dioxide 

 paralyses them, and they dilate. Low temperature causes also 

 dilation ; high temperature, contraction. Hence, hibernating frogs 

 are much darker than they are in the summer. Frogs kept in dry 

 moss, or such as are drying up, turn pale, regardless of light or 

 darkness surrounding them. The Grass-frog and the continental or 

 edible Water-frog seem to depend to a great extent upon temperature, 

 or the amount of moisture in the air, so far as their changes are 

 concerned. 



The chromatic functions of Tree-frogs, on the other hand, depend 

 greatly upon the sensory impressions received by the skin. For 

 instance, a dark Tree-frog will turn green when put into an absolutely 

 dark vessel in which there are fresh leaves. Eough surfaces cause a 

 sensation which makes the frog turn dark. 



The modern physiologist prefers looking upon all these changes 

 as reflex actions, as not under the control of the will of the creature. 

 Many even assume that the animal neither has control over its 

 colour, nor does it know what it is doing. All this sounds very well 

 in the laboratory, but the frogs, when observed in their native haunts, 

 or even when kept under proper conditions, do not always behave as 

 the physiologist thinks they should. 



There is no doubt that in many cases the changes of colour are 



