12 FOOT-NOTES TO EVOLUTION. 
to year new species of fishes in North American rivers; 
but the number of new forms now discovered each year is 
usually less than the number of old spe- 
cies which are yearly proved intenable. 
Four complete lists of the fresh-water 
fishes of the United States have been 
published by the present writer and his associates. That 
of 1876 enumerated 670 species; that of 1878 contained 
665; the third, in 1885, only 587 species, although up- 
ward of 75 new species were detected in the nine years 
which elapsed between the first and the third list. The 
list of 1896, with 50 more additions, contains 599 spe- 
cies. Additional specimens from intervening localities: 
are found to form connecting links among the nominal 
species, and thus several supposed species become in 
time merged in one, while not unfrequently the sup- 
posed minor variations are the marks of what we must 
finally regard as real species. Their reality consists 
simply in the extinction of the intervening forms. 
We have briefly reviewed the condition of this prob- 
lem and its answers before 1836, when Charles Darwin 
returned to England after the voyage 
of the Beagle. While in South America 
.~ he had been greatly impressed by two 
phases of the question which came to his notice during 
his explorations there. The first of these was the fauna 
of the Galapagos Islands, a rocky cluster lying well out 
to sea some five hundred miles off the coast of Peru and 
Ecuador. The sea birds of these islands are essentially 
the same as those of the shores of Peru. So with most 
of the fishes. We can see how this might well be, for 
both sea birds and fishes can readily pass from the one 
region to the other. But the land birds, as well as the 
reptiles, insects, and plants, are mostly peculiar to the 
islands. The same species are found nowhere else; but 
The species of 
fishes of North 
America. 
The species of 
the Galapagos. 
