COLOUR VARIATION IN LIZARDS 121 



animals than reptiles, notably in amphibians and birds. 

 In the former we find amongst the newts some striking 

 sexual colourings, while it is a matter of common 

 knowledge that in the birds the sexes in very many 

 cases are readily distinguished by the plumage. 



Those who have studied this question in the reptiles 

 will recollect that there is another factor which seems 

 to exert considerable influence, and that is the factor 

 of age. In the case of the adder it is found that the 

 very old specimens are not so brilliantly coloured as 

 are the young adults.^ This applies to both sexes 

 in those reptiles. The next point, therefore, is to 

 inquire into the question of age in the case of the 

 lizards. 



Again, we may note some of the many observa- 

 tions on American species quoted in Professor Cope's 

 book. 



Eeferring again to the genus Sceloporus, the follow- 

 ing instances of the effect of age strike the reader. 



In the species >S'. zosteromus there is a dark, broad, 

 lateral band connecting the groin with the arm in 

 adults ; in younger specimens the prebrachial spot 

 often does not connect with the lateral abdominal 

 band, the pigment in the intervening area apparently 

 not having been yet developed. In S. biseriatus, the 

 throat, middle of the belly, and lower side of the thigh 

 may be black in old males, instead of the greyish or 

 bluish colour of younger specimens. 



^ British Serpents, p. 120. 



