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 discovered 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 contained 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 creation, has passed away forever. We can no more return to it than astronomers can return to the Ptolemaic notion of the solar system. The same lesson comes up from every hand. It is the common 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 jumble 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."
graphically contiguous."—On the Recognition of Geographical Forms; The Auk, January, 1890, p. 1.