Page:Popular Science Monthly Volume 68.djvu/525

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The fish fauna of Lake Titicaca, about as large as Lake Erie, consists of a catfish, Pygidium rivulatum, and several species of Orestias, a Fundulus-like pœcilid. The former belongs to a genus of mountain catfishes universally distributed in mountain streams of South America. It has succeeded in crossing all sorts of barriers, and has undoubtedly migrated into Lake Titicaca from the streams surrounding it. The genus Orestias, on the other hand, is confined to Lake Titicaca and the streams and lakes immediately surrounding it. The latter have doubtless received theirs from Lake Titicaca, which, on its part, could only have received them at the time it was an arm of the sea, in which its nearest relatives flourish.

The interest in the Patagonian fresh-water fish fauna is entirely out of proportion to its diversity and centers largely in its origin. Only about twenty-five species of fishes are known to live or enter the fresh waters south of the line joining Valparaiso and Bahia Blanca. These few species fall, according to their origin, into four distinct groups.

1. Immigrants from the sea (a) are in the process of acclimatization (species of Menidia and Atherinichthys), or (b) may be looked upon as long established (species of Percichthys and Percilia). Members of (a) are found in all the rivers; members of (b) are found in the north chiefly, but reach the Santa Cruz.

2. Immigrants from the fresh waters on the north: a very small overflow from the extremely rich fauna to the north and still retaining their generic affinity with the northern forms. Here belong the species of the genera Cheirdon and Astyanax, which are very widely distributed in tropical South America and are not known to extend much south of the Rio Negro.

3. Autochthons or of doubtful origin. Here belongs the highly

 9. Astyanax bimaculatus from Panama to Buenos Ayres east of the Andes, close relatives existed in the early tertiary. This photograph of a specimen from Trinidad.