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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 Geological phenomena, in driving a part of the indi- 

 viduals of a species to regions different frQm 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 Bide are different from those of 



