SAND LIZARD. 19 
Lacerta stirpium, Daup. Rept. III. p. 155, t. xxxv. f. 2. Ducks in An. des 
Se. Nat. XVI. p. 376, sp. 3, t. Ixxvi. f. 1,2. JENYNS, 
Brit. Vert. p. 291. 
»  arenicola, Daun. 1.c. p. 230, t. xxxviii. f. 2. 
»  dnguiformis, SHEPPARD, in Linn. Trans. XVI. p. 51. 
Lézard des Souches, Daup.1.c. M. Epw. in An. des Se. Nat. XVI. p. 65. 83. 
twit, 4, eft. vin. & 1) 2. 
Lacerta di Linneo, C. L. Bonar. 1. c. 
Because it may appear to many persons not accustomed 
to the use of what are commonly termed essential generic 
or specific characters, that many of those phrases by which 
such characters are expressed are confined in their signifi- 
cation, and the characters themselves of slight value and 
importance, it may not, perhaps, be useless, before we 
proceed to describe the species, to offer a few words 
explanatory of their employment, especially with reference 
to those groups of animals on which the present work pro- 
fesses to treat. It has always appeared to me that generic 
distinctions should, as far as possible, be limited to such 
differences of structure as indicate a difference in the habits 
of the animals. Thus, the absence in one species, and the 
presence in another, of an organ or part of an organ, the 
application of which is of obvious or probable moment in 
any of the habits of life,— as, for example, the degree of 
development of the thumb in some genera of monkeys, or 
the modification of the carnivorous propensity in allied 
groups of carnivora, evidenced by the greater or less degree 
of acuteness in the tubercles of certain teeth,—will form 
good grounds for such a distinction. Now it is evident 
here that what is called the generic character is merely 
the phrase expressive of some point of structure belonging 
to the whole group in which a certain habit obtains. In 
many cases even, the most convenient, or the only tangible 
characters of the group may have no reference whatever to 
the peculiarity of habit which forms the legitimate founda- 
CLD 
a 
