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species. This increases very much the difficulties of study, and 
I thought it would be useful to condense in a paper what was 
known on the fishes of the Colony. The Acclimatisation Society, 
always desirous of promoting anything useful to the country, 
entered into these views, and that is the origin of the essay I now 
submit to the public. Before I finish this paper, I think it is 
useful to say a few words on a subject which has always much 
embarassed naturalists, and on which the diversity of their views 
is very great : I mean the question of what is a genus ? — what is 
a species ? 
A genus is, for me, a more or less artificial collection of species 
offering some common characters ; a few appear to constitue 
natural groups ; but I think that in such cases it will generally 
be found that the missing links have not yet been discovered, or 
have entirely disappeared from the recent fauna of the globe. 
The genus, being an artificial division, is, of course, appreciated 
very differently by the various authors. All the so-called 
superior divisions are in the same case, and thus the species seems 
to me to be, of the zoological divisions, the only one to be found 
in nature. But even this, to be such, must be considered as 
a constant variety ; that is, that as soon as a collection of speci- 
mens present the same characters, due to natural circumstances, 
they must be considered as forming a species. I believe that 
accidental or G-eological phenomena, in driving apart of the indi- 
viduals of a species to regions different from those they previ- 
ously inhabited, will, with time, constitute a new species, as it is 
certain that those individuals, having to find a different way of 
living, and to be subjected to different temperatures, will deviate 
from the type, and constitute different sorts, or what is usually 
called, when this process is only beginning, local varieties. Types 
will retain their forms unchanged thousands of years when they 
remain in the same climate and in the same region, but they will 
deviate as soon as these circumstances are changed. I have 
observed elsewhere (Expedition to the Central Parts of South 
America) that the animals who can neither fly nor swim are 
almost all different on one side of the Amazonas to what they 
are on the other, and this has been even observed by the wild men 
who inhabit these regions, the Indians having often told me that 
all the animals on the northern side are different from those of 
