Page:Popular Science Monthly Volume 68.djvu/521

Rh compelled me to abandon the work when but half finished. Occasional collections received from South America for identification have, however, kept up my interest. The entire problem presented by this fauna has been reviewed and will be published in one of the reports of the Hatcher expedition to Patagonia. I am permitted to publish this summary of results through the courtesy of the editor of the Hatcher volumes, Professor W. B. Scott, of Princeton University.

On February 23, 1866, Louis Agassiz wrote to the emperor of Brazil:


 * I estimate the total number of species which I actually possess [from the Amazon] at eighteen hundred, and it may be two thousand.

To this is added a footnote in Agassiz's 'A Journey in Brazil,' p. 383.

To-day I can not give a more precise account of the final results of my survey. Though all my collections are safely stored in the museum, every practical zoologist understands that a critical examination of more than eighty thousand specimens can not be made in less than several years.

Agassiz secured more species from a small lake in the valley of the Amazon than there are in all the fresh waters of Europe.

The number of species he collected was overestimated by Agassiz. While about half of his Amazonian collections have not, after forty years, been examined, it is certain that the species not yet examined will not swell his list to 1,800 species. The total number of species recorded from the Amazon basin up to date is 674.

Although Agassiz's estimate of the number of species he collected is too high, the total number of species found in South America is very great. About ten per cent, of all the known species of fishes have been recorded from the freshwaters of South America. In 1892 I estimated that three fourths of the fauna was known. Now, after examining recent lists, and considering that collections have largely been made in easily accessible and great highways, and that from great river basins like the Purus, Tapajos, Xingu and the Uruguay and the greater part of the Madeira and the Tocantins we have nothing at all, and that even from the great Orinoco and Magdalena we know next to nothing; I doubt very much if we even yet know so much as three fourths of the fauna of the area between the Caribbean Sea and the Argentine Republic.

The tropical American fresh-water fauna, having its center of greatest diversity in the middle Amazon basin, is attenuated northward till it reaches the vanishing point just on the borders of the United States. Southward it extends to somewhere—no one knows where—south of Buenos Aires. The Patagonian fauna and North American fauna are entirely different from the tropical American fauna and from each other.

The key to the great diversity of the tropical American fauna is to be found in the enormous single water system, extending from 10°