Changing Colour in Reptiles. 297 



might well ask why it is that, in the struggle for existence, no 

 species of snake has been produced with a skin similar in 

 constitution to that of the Lacertilians which change their 

 hue. It may be that the constitution of the integumentary 

 coverings and of the skin itself is of such a kind that 

 a different arrangement of the pigment-cells cannot take 

 place. In order to decide the question it would be necessary 

 in the first place to determine what are the qualities in 

 respect of which the skin of reptiles which change their colour 

 differs fundamentally as regards anatomy from that of 

 those not endowed with this faculty. After that we might 

 perhaps hope to acquire information as to the reason why 

 a large number of lizards and all snakes are without the 

 power of changing colour. 



In the Lacertilia the external constitution and covering of 

 the skin does not allow us to infer with certainty the posses- 

 sion of the power of changing colour. It is true that, gene- 

 rally speaking, it may be slated that lizards with a porous 

 integumentary covering (those that become wet if placed in 

 water) frequently possess the power of changing colour, and 

 that those with non-porous scales (from these water drips off 

 as from a greasy object) have none ; but there are exceptions 

 to both categories. Most of the Lacertilia that change their 

 colour are distinguished by having small non-imbricated 

 scales ; but here also there are exceptions, and, on the con- 

 trary, many fine-scaled Lacertilia cannot change colour. It 

 is consequently impossible to set up any definite rules, and to 

 undertake a description of the coloration of Lacertilia based 

 upon spirit-specimens has therefore only a conditional value. 

 It is true that the Lacertilians which change colour generally 

 assume the same coloration in death, so that it is possible to 

 determine spirit-specimens. An attempt, however, to deter- 

 mine species of Anolis, for example, from descriptions of 

 colour alone would scarcely lead to a definite conclusion as to 

 the classification of a species. 



The power of changing colour in Lacertilia differs greatly — 

 on the one hand as regards intensity and frequency, on the 

 other in respect of the purposes which it serves. 



Whether, after all, in the case of Lacerta agilis, viridis, 

 and muralis we would regard as the faculty of changing 

 colour the regular alteration of hue in spring or at the 

 pairing-season of ihe species inhabiting temperate zones is a 

 question that may remain undecided ; it is true that it does 

 not depend upon the will of the creatures themselves, but 

 neither can it be compared with the alteration of colour in 

 mammals and birds. At any rate I am of opinion that here 



