168 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
GENERAL CONCLUSIONS. 
The results of the observations recorded in the present paper are fully in accord 
with the general conclusions as to the geographical distribution of fresh water fishes 
advanced in a paper on the subject in Science Sketches (1888, pp. 83-133). It is evi- 
dent that the question of distribution reduces itself to the question of barriers of vari- 
ous sorts. Each species extends its range in every direction and holds the ground thus 
taken if in the struggle for existence it is able to do so. 
To quote from the work just mentioned : “ The present distribution of fishes is the 
result of the long-continued action of forces still in operation. The species have 
entered our w'aters in many invasions from the Old World or from the sea. Each 
species has been subjected to the various influences implied in the term Natural Selec- 
tion, and, under varying conditions, its representatives have undergone many modifi- 
cations. Each of the six hundred species we now know (in rivers of the United States) 
may be conceived as making each year inroads on territory occupied by other species, 
kf these colonies are able to hold their own in the struggle for possession they will 
multiply in the new conditions and the range of the species will become widened. If 
the surroundings are different, new species or varieties will be formed with time and 
these new forms may again invade the territory of the parent species. Again, colony 
after colony of species after species may be destroyed by other species or by unconge- 
nial surroundings. The ultimate results of centuries on centuries of the restlessness 
of individuals are seen in the facts of geographical distribution. Only in the most 
general way can the history of any species be traced, but could we know it all, it would 
be as long and as eventful a story as the history of the colonization and settlement of 
North America by immigrants from Europe. But by the fishes, each river in America 
has been a hundred times discovered, its colonization a hundred times attempted. In 
these efforts there is no co-operation. Every individual is for himself, every struggle 
a struggle of life and death, for each fish is a cannibal, and to each species each mem- 
ber of every other species is an alien and a savage.” 
The fact of the analogy existing between the fauna of rivers and the land faunse of 
islands is rendered very evident. As the fauna of the islands is limited by the barrier 
of the sea, so that of the rivers is limited by barriers of land, and analogous laws 
determine what species can obtain a hold in either case. 
Additional confirmation has been given to the idea that the lowland swamp fishes 
of the United States are remains of an earlier and, in part, now extinct fauna. To 
such a fauna, it is generally admitted, belong the genera Amia and Lepisosteus. To 
this list I would add JJnibra^ Lucius^ Chologaster, Aphredoderus, Jordanella, Massoma, 
Acantharchus, Pomoxis, PnneacantJms, Mesogonistius, and doubtless Percopsis. The up- 
land fishes seem to be mostly of more recent origin, the species of Notropis and 
Etheostoma probably latest of all. 
