362 LL. Agassiz on Fishes of the Tennessee River. 
oa 
everywhere together in the West. 
If the study of the geographical distribution of animals is ever 
to furnish us any indications respecting the circumstances under 
which organized beings were created, we must, in investigating 
it, turn our attention particularly to those facts which disclose 
differences of structure in connection with the special localization 
of the different representatives of each family within their natu- 
ral boundaries. For years I have been collecting diligently all 
the data within my reach bearing upon this question, and from 
the results of this enquiry already in my possession, Iam satisfied 
that the day is not far distant when we shall know with sufficient 
precision where all the living beings now existing upon earth 
have made their first appearance. ‘This must of course be the 
first step towards a deeper insight into the conditions of that or- 
gin itself. 
In connection with this train of thoughts it is interesting to 
notice how much different families of animals vary from eact 
other in the most prominent features of their geographical distri- 
bution. There are those the representatives of which are almost 
uniformally distributed over the whole range of their natural 
species of Lepidosteus, for the two types of this genus occur 
t 
me is the case with the family of Esoces, which has howevera 
much greater number of species in the fresh waters of North 
America. So are also the Sturgeons, with this difference, that 
upon the continent of America two peculiar genera, Scaphirhyn- 
chus and Polyodon, are added, which have no representatives in 
the old world. The Percoids however present very different 
combinations: some types are common to North America, Europe 
and Northern Asia, as the genera Perea, Lucioperca and Labrax, 
with this difference however, that North America has many fresh 
water representatives of the genus Labrax which are wanting 1p 
the old world; other types are only to be found either 10 North 
America or in the old world,—for instance Grystes, Centrarchus, 
Pomoxis, Amploplites, Calliurus, Pomotis, have no representatives 
in Europe where we find in their stead the genera Aspro and 
Acerina; the balance being in favor of North America as si 
the number and diversity of the fresh-water types of this family 
is concerned, whilst the old world has many more and more Ce 
versified marine representatives. ‘The family of Cyprino™ 
g with that of the Percoids in the features of its geograph 
cal distribution; the types peculiar to each side of the Atlantic 
being however more equally distributed, for whilst in the © 
world we find the genera Cyprinus, Barbus, Tinca, Cobitis, Pele- 
cus, Aspius, Rhodeus, Phoxinus, North America has 1s 
