EVOLUTION AND DISTRIBUTION OF ANIMALS. 513 



What is true of birds is equally the case with other groups of 

 animals. Continued explorations bring to light each year new 

 species of American fishes, but the number of new forms discov- 

 ered each year is usually less than 'the number of old supposed 

 species which are found to intergrade with each other, and have 

 so become untenable. 



I have myself published three complete lists of the fresh-water 

 fishes of North America. The one published in 1876 enumerated 

 670 species ; that of 1878, 665 species ; while the list of 1885 con- 

 tained 587 species, although upward of 75 new species had been 

 found in the nine years which elapsed between the first and the 

 last of these three lists. 



The old idea of a species as a separate entity, a special crea- 

 tion, has passed away forever. We can no more return to it than 

 astronomers can return to the Ptolemaic notion of the solar sys- 

 tem. The same lesson comes up from every hand. It is the com- 

 mon experience of all students of species. We have learned it 

 from Gray and Engelmann and Coulter, and from each of the 

 many students of American botany. We have learned it from 

 Baird and Allen and Coues and Ridgway and Stejneger, and from 

 all who have made life-studies of American birds. We have 

 learned it from Cope and Marsh and Leidy, and from all who 

 have searched the rocks for the bones of our ancestors. 



I do not know of a single naturalist in the world, who has 

 made a thoughtful study of the relations of species in any group, 

 who entertains the old notion as to their distinct origin. There 

 is not one who could hold this view, and look an animal in the 

 face ! The study of the problems of geographical distribution is 

 possible only on the theory of the derivation of species. If we 

 view all animals and plants as the results of special creations in 

 the regions assigned to them, we have, instead of laws, only a jum- 

 ble of arbitrary and meaningless facts. We have been too fully 

 accustomed to the recognition of law to believe that any facts 

 are arbitrary and meaningless. We know no facts which lie 

 beyond the realm of law. I may close with the language of 

 Asa Gray : 



" When we gather into one line the several threads of evidence 

 of this sort to which we have here barely alluded we find that they 

 lead in the same direction with the clews furnished by [other lines 

 of investigation]. Slender indeed each thread may be, but they 

 are manifold, and together they bind us firmly to the doctrine of 

 the derivation of species." 



[Concluded.] 



graphically contiguous." — On the Recognition of Geographical Forms ; The Auk, January, 

 1890, p. 1. 



VOL. XXXTI1. — 37 



