8 INTRODUCTORY LETTER. 



should have, m all its parts, a mathematical exactness, so 

 that one could study the relative proportions of the organs 

 it is for the same reason necessary to avoid fore-shorten- 

 ings as much as possible, and not to confuse the figure by 

 projected shadows. The choice of objects demands equal 

 circumspection : the agony of a violent death often leaves 

 traces in the convulsed features of the face ; some parts 

 sustain accidental injuries in putting up the specimens ; 

 in others, the organs change their relative positions. This 

 is especially the case with Ophidians, of which the bones 

 of the head are susceptible of very considerable movements. 

 In tracing my figures, I have aimed at avoiding all these 

 obstacles : 1 have always made use of individuals in a 

 perfect state of preservation, and I have never copied, ex- 

 cept from nature. I have followed the same rules in my 

 descriptions ; or, when I have not been able to do so, I have 

 expressly indicated it. 



The comparative examination of the writings of my 

 predecessors, presented far more difiicult obstacles to be 

 surmounted. The principal cause, to which I have fre- 

 ({uently alluded, and which appears to have engendered 

 the numerous errors that disfigure our systems, is the 

 multiplication of species and of genera instituted and in- 

 troduced into systems upon isolated characters. The 

 instances that would justify these remarks are innumer- 

 able. Let any one consult my articles Eryx, Naja por- 

 phyritica. Boa Cenchria, Python Peronii and P. bivittatus, 

 Acrochordus, Tropidonotus bipunctatus, and T. fasciatus, 

 Crotalus horridus, Vipera Berus, and several others, and 

 he will be convinced that the same species often bears a 

 dozen of names ; that it often has been divided into several 

 different genera ; that it has even formed the types of 

 different families ; that it figures sometimes among the 

 venomous serpents, sometimes among those that are in- 

 nocuous ! Let any one examine the heterogeneous elements 

 out of which some authors have comjaosed their genera 

 Boa, Hurria, Scytale, Elaps, Trimeresurus, and Vipera ! 

 In following out such views, it was necessary, in adhering 

 to this principle of classification, to separate the Pythons 

 from the Boas, and to place them in another family ; it 



