590 Mr. Hans Gaclow [April 26, 



not voluntary, not even subconscious, but pure reflex actions. It is 

 quite conceivable tbat the sensation of sitting on a rough surface 

 starts a whole train of processes : Boughness means bark, bark is 

 brown, change into brown. But one and the same Tree-frog does not 

 always assume the colour of the bark on which it rests. He will, if 

 it suits him, remain grass-green upon a yellow stone, or on a white 

 window-frame. The sensory impression received through the skin 

 of the belly and through the tactile corpuscles of its fingers and toes 

 is the same, no matter if the deal board be painted white, black or 

 green. How does it come to pass that the frog adjusts his colour to a 

 nicety to the general hue or tone of his surroundings, provided he is 

 so inclined ? 



That is the point. Forty years ago Lord Lister was much nearer 

 the true solution when, by careful observation and ingenious experi- 

 ments he showed that the guiding impressions ai'e those of the eye. 

 The creature sees, studies its surroundings and then adjusts itself to 

 them, if it thinks it necessary and if it is in the proper mood. This 

 does not prevent such colour changes from becoming an unconscious 

 habit, the habit having been repeated so often, and having become so 

 strong, and last, not least, having proved so useful that at last it 

 comes off whilst volition is suspended. In other words, the change 

 has become a reflex action. But I insist upon this, that at any time 

 the changes can be inhibited by the will, and can be produced at will. 

 A consideration of the coloration in its totality, the colour-pattern, 

 is intimately connected with the use of the colours to these creatures. 

 Paramount is concealment ; and it is an almost universal law that 

 colour is restricted to those parts of the body which are exposed, while 

 neutral tints and white are relegated to the hidden parts, chiefly the 

 under surface. 



The production, and still more the distribution, of colour pig- 

 ments, especially of pretty tints, is pre-eminently due to light. 



Concealment and restriction of vivid colours to the exposed visible 

 parts sound at first antagonistic — rather paradoxical — still more so 

 when we remember that many Amphibia have a pattern, or combina- 

 tion of colours which renders them most conspicuous. It is difficult 

 to generalise. 



The simplest case of concealment is exemplified by the common 

 Toad. Earthy-brown above, whitish below, with individual and local 

 variations ; but the effect is in harmony with the ground upon which 

 this Toad hunts in the dusk. 



The same principle applies to the vividly green Tree-frog, whitish 

 below, green above, like the leaves upon which it spends most of 

 its life. And we remember that these creatures can change colour 

 besides, so as to adjust themselves to a nicety to their surroundings. 



Then there are what I propose calling Flash colours — vivid, con- 

 spicuous — confined to such parts of the body or limbs as are abso- 

 lutely hidden when the animal is at rest. These colours are shown 

 off only on emergency, suddenly producing a bright flash. One of 



