398 M. P. de Grijs on the Faculty of 



also there ensues a movement of the pigment-cells, which 

 certainly is quite slow, but nevertheless analogous to that 

 which takes place in the creatures which are capable of 

 changing colour quickly. This view is supported by the 

 fact that also in the case of the Lacertilia which change their 

 colour in the course of quite a short space of time the will is 

 not always active, but, on the contrary, external influences 

 compel the animal to change its colour. A chamaileon that 

 has been exposed for a time to great heat always becomes 

 bright yellow. 



The intensity of the colour-change differs extremely in the 

 different species. In the first instance two groups can be 

 established : — 



J. Ground-colour and marking alter equally in tone, but 

 the marking does not disappear. 



II. Ground-colour and marking each alter in tone indepen- 

 dently of the other; spots may entirely disappear. 



To the former group belongs, for instance, Eumeces 

 Schneideri. Under the influence of heat this species always 

 appears quite bright greyish yellow, with bright yellowish- 

 red spots. In an unheated cage the animal assumes a dark 

 greyish-brown colour, and the spots appear brick-red. Of 

 the lizards that I have kept I also assign to this group 

 TarentoJa annularis, which varies from blackish brown to 

 bright sand- colour, with constant marking ; Uromastix, 

 which likewise appears lighter under the influence of heat ; 

 Scehporus undulatus, the upper surface of which assumes a 

 considerably brighter tone under the influence of the sun's 

 rays ; Crotaphytus collaris, a species which, when the tempe- 

 rature is low, appears dusky grey, while in heat the head 

 becomes almost white, the body bluish grey, the feet appear 

 shining bluish green, and the tail becomes bluish white, but 

 all markings composed of spots remain constant ; besides 

 these there are Phrynosoma cornulum and Amphibolous bar- 

 batus, both of which become brighter under the influence of 

 heat, various species of ground AganiEe {Agama mossambica 

 and A. steLlio), as also Cachryx defensor, of which the body- 

 colour passes from blackish grey to light blue, while the 

 marking of spots remains unaltered. 



To the second group belong, among others, Anolis, Agama 

 sanguinolenta and A. inermis, Phrynocephalus, Iguana, 

 dilutes, and Charncehon. Chanutleon really forms a group 

 by itself, since its power of changing colour materially exceeds 

 that of all other species. In the species mentioned there con- 

 sequently takes place a double change of colour in a more or 

 less pronounced degree, in so far as ground-colour and spots 



