G INTRODUCTORY LETTER. 



wliicli have lost their colours ; others also have been in- 

 troduced after a supei-ficial examination, and without 

 ha\dng been compared with allied species. A few words 

 would have sufficed to characterize them, but very often 

 these diagnostics, or even tlie detailed descriptions, con- 

 tain nothing but an enumeration of characters proper to 

 all the species of that genus ; so that, after having ana- 

 lyzed and rejected them individually, there does not remain 

 a single distinguisliing mark for the species. We may say 

 the same of genera admitted often with similar negligence. 

 According to my opinion, a description which is not com- 

 parative, is of no utility. If it be true that a genus re- 

 presents the assemblage of all the species it includes, it 

 must be allowed, that we can never arrive at a knowledge 

 of the latter but by comparing them witli each other, and 

 by stating what is peculiar to each, and common to them 

 all. Assuredly, there will result but little benefit to 

 science by the admission of species, of the whole peculia- 

 rities of which we know nothing but the name that has 

 been imposed on them — of species, the multitude of which, 

 continually increasing, confuses our systems. The study 

 of nature consists not in a supei*ficial knowledge of exist- 

 ences, but it views them under the triple aspect of zoology, 

 anatomy, and physical geography. My principal object 

 in publishing my researches being to expose the relations 

 subsisting between animals and the places they inhabit, I 

 have judged it proper to adopt no species of which the 

 country is unknown, except w hen some conspicuous feature 

 in its structure might render it of real interest for zoology 

 and for j^hysiology. 



It is also necessary to use circumspection in consulting 

 the intimations of the native place of animals, as they are 

 given in most works. Few naturalists have the oppor- 

 tunity of obtaining these objects at the first hand ; and we 

 can rarely trust to the veracity of mariners, who, often 

 deceived themselves, bring back in their voyages objects of 

 natural history from distant countries which they have 

 visited. The specimens of one colony are sometimes car- 

 ried to another; they pass through several hands ; their 

 origin is forgotten, or they are sent to Europe under the 



