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- 



e species o ^-^^ which are yearly proved intenable. 

 fishes of North ^ , ...... 



A,^^r,vo Four complete lists of the fresh-water 



fishes of the United States have been 

 published by the present writer and his associates. That 

 of J 876 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 

 e species o ^^ ^^^ Beagle. While in South America 

 the Galapagos. 



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 



