20 LACERTAD.E. 



tion for the generic distinction. This will be found to 

 be the case, for instance, in some of the genera of saurian 

 reptiles, and even in the generic distinction of our only two 

 native species ; the genus Zootoca, to which our common 

 Lizard belongs, being characterized, as regards habit, by 

 the circumstance that the species of which it is composed 

 are ovo-viviparous. This is a character to which the 

 structure of any of its external organs bears no possible 

 relation; but as it is found that the species having this 

 habit is externally characterized by some peculiarity in the 

 form and situation of certain little scales about the head, 

 having, however, no possible reference to the habit in 

 question, such peculiarities are employed as convenient and 

 permanent artificial characters by which it may be distin- 

 guished. 



It must, however, be acknowledged that there are cases 

 in which a numerous group is found to consist of several 

 divisions, each of them distinguished by some point of 

 form or structure, the use or object of which is absolutely 

 unknown or unintelligible to us, and in which the habits, as 

 far as we know them, are not conspicuously different. 

 For the sake of convenience in some cases, and of con- 

 sistency and harmony of system in others, these groups 

 may, notwithstanding this uncertainty, receive with pro- 

 priety a distinctive generic appellation ; but, wherever it is 

 possible, generic groups ought only to be formed where 

 Nature has herself pointed out their distinction. 



The generic and specific characters of the Lacertine 

 group have only of late years received the degree of atten- 

 tion which they deserved. The divisions and subdivisions 

 of this numerous family had been either overlooked, or 

 so arbitrarily defined as to be detected with difficulty, and 

 their value had been greatly misunderstood. The employ- 



