20 LACERTAD &. 
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- 
