272 PROCEEDINGS OF THE AMERICAN ACADEMY. 



must be regarded as inhibiting slightly the change to the green state. 

 The facts that ordinarily the brown condition is retained only in the 

 light and that the change to the green state is somewhat inhibited when 

 the nerves are intact, suggest that the sympathetic centres exert a tonus 

 influence over the melauophore cells as long as the animal is exposed to 

 daylight, and that the tonus only gradually subsides when the animal is 

 placed in the dark. 



The green state is not only produced by darkness, the withdrawal of 

 the circulation, and possibly the cutting of nerves, but also in other ways. 

 Most animals in narcosis from ether are green, and nicotiue, as already 

 mentioned, calls forth the green state. All animals change to green when 

 they die. Thus the green state is in all cases the result of influences 

 which either greatly reduce the stimuli or absolutely prevent them from 

 reaching the melanophores. I therefore believe that the green state repre- 

 sents the resting condition of the melanophores — the state to which the 

 cell returns on ceasing to receive stimuli. The brown state may be pro- 

 duced by direct mechanical stimulation of the melauophore cell, but I 

 believe that usually it is the result of a continuous stimulation received 

 from the sympathetic centres, which in turn are indirectly stimulated by 

 light falling on some part of the animal's skin. 



If this last conclusion is true, it follows that a brown animal, when 

 placed with its head or its trunk in the dark-box, ought to show no 

 change in color, for in each case enough of the body is exposed to light 

 to keep up the sympathetic tonus for the whole iutegumeut. As a 

 matter of fact such is the case ; thus when each of six animals in the 

 brown state were placed in the box, first with the head in, then with 

 the trunk in, all remained brown. 



From these various observations I conclude that the green state in 

 Anolis represents the non-stimulated or resting state of its melanophores, 

 and that this state is brought on by any means that wholly interrupts or 

 greatly reduces the stimuli which naturally come to the melanophores. 

 The change to this state is somewhat retarded by the partially inhibitory 

 action of the sympathetic centres. 



4. Comparison with other Lizards. — Enough has already been pub- 

 lished on the color changes in lizards to show that these changes are not 

 carried out in any uniform way in this group of animals. Thus in 

 Chameleon, according to Briicke ('52), Keller ('95), and in fact all who in 

 the last fifty years have examined the skin of this reptile carefully, the 

 melanophores show an outward migration of pigment in the light and an 

 inward migration in the dark. The same is true of Anolis. But in 



