INTRODUCTION. ix 
In making the foregoing estimates, as regards the number of new species 
brought home by Mr. Darwin, I have been guided almost entirely by my own 
judgment. The difficulty, however, of ascertaining, in a miscellaneous collection 
of this nature, brought from various localities, what are really new to science, is 
very great; and this difficulty is much increased, where an author is situate apart 
from large public museums to which he might have recourse for comparison. 
Possibly, therefore, some of those described as new in the following work, may 
not be so in reality ; and, in one instance, as mentioned in the Appendix, this 
is known to be the case. My excuse, however, must rest upon what has been 
just stated. It is hoped that caution has been generally shown, at least in regard 
to specimens not in a good state of preservation; and, in several such cases, in 
which an accurate description was hardly practicable,—though they could not be 
referred to any known species,—they are not positively declared new, nor any 
names imposed upon them whatever. 
I have, of course, consulted throughout the invaluable volumes of Cuvier 
and Valenciennes, so far as they have yet advanced in the subject ; and in them 
it will be found that a few species, brought by Mr. Darwin from South America, 
and still but little known, had nevertheless been previously obtained from the 
same country by M. Gay. The zoological atlasses of the three great French 
voyages by Freycinet, Duperrey and D’Urville have been also carefully looked 
through ; and, in regard particularly to the fish of South America, the works of 
Humboldt, Spix and Agassiz, and the more recent one, now in course of pub- 
lication, by M. D’Orbigny. 
There is an equal difficulty felt by every naturalist at the present day, in 
distinguishing species from varieties. And in the case of Fish, residing in a 
peculiar element, and so much removed from our observation,—we are almost at a 
loss to know, at present, to what extent their characters may be modified by 
local and accidental causes, or how far we may trust a different geographical 
position for giving permanence and value to a slight modification of form 
different from what occurs in the species of our own seas. Still less easy is 
it to determine the true importance of characters, in instances in which it 
is only permitted to see a single specimen of the kind, or, at most, very few 
individuals. 
Many mistakes, therefore, are liable to occur, in a work of this nature, arising 
from the above sources. The only way to prevent their creating any per- 
manent confusion in the science, is to describe all species of which the least 
doubt is entertained, in such detail, and with such accuracy, that they may not 
fail of being recognized by any observer, to whom they may occur a second time. 
They will not then continue to hold a false position in the system, as spurious 
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