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DISPERSION OF FRESH-WATER FISHES. 131 
the White-fish, the Pike, and to arctic and sub- 
arctic species generally, Behring Strait have evi- 
dently proved no serious obstacle to diffusion; 
and it is not unlikely that much of the close re- 
semblance of the fresh-water faunz of northern 
Europe, Asia, and North America is due to this 
fact. To attempt to decide from which side the 
first migration came in regard to each group of 
fishes might be interesting; but without a wider 
range of facts than is now in our possession, such 
attempts would be mere guesswork and without 
value. The interlocking of the fish-faunz of Asia 
and North America presents, however, a number of 
interesting problems, for numerous migrations in 
both directions have doubtless taken place. 
I could go on indefinitely with the discussion of 
special cases, each more or less interesting or sug- 
gestive in itself, but the general conclusion is in all 
cases the same. The present distribution of fishes 
is the result of the long-continued action of forces 
still in operation. The species have entered our 
waters 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 
selection,” and under varying conditions its repre- 
sentatives have undergone many different modifi- 
cations. Each of the six hundred species we now 
know may be conceived as making every year in- 
roads on territory occupied by other species. If 
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 be- 
comes widened. If the surroundings are different, 
