6 
INTEODUCTORY LETTER. 
wliich have lost their colours ; others also have been in- 
troduced after a superficial examination, and without 
having been compared with allied species. A few words 
would have sufficed to characterize them, but very often 
these diagnostics, or even the 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 distinguishing 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 with 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 superficial 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 when some conspicuous feature 
in its structure might render it of real interest for zoology 
and for physiology. 
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 
