Page:Popular Science Monthly Volume 68.djvu/502

which Lake Bonneville was once drained. Other species are left locally isolated, but in one species only (Agosia adobe) a small minnow of the clay bottoms can be shown to have undergone any alteration. But with the tiger beetle (Cicindelœ) a large number of species have been produced by isolation.

From the Bay of Panama 374 species of fishes are recorded in the recent monograph of Gilbert and Starks. Of these species, 204 are recorded also from the Gulf of California, while perhaps 50 others are represented in the more northern bay by closely related forms.

Comparing the fish faunas separated by the isthmus, we find the closest relation possible so far as families and genera are concerned. In this respect the resemblance is far closer than that between Panama and Chile, or Panama and Tahiti, or Panama and southern California. On the Atlantic side, similar conditions maintain, although the number of genera and species is far greater (about 1,200 species) in the West Indies than at Panama. This fact accords with the much larger extent of the West Indies, its varied groups of islands isolated by deep channels, and its near connection to the faunas of Brazil and the United States.

But it is also noteworthy that while the families of fishes are almost identical on the two shores of the isthmus of Panama, and the great majority of the genera also, yet the species are almost wholly different.

Taking the enumeration of Gilbert and Starks, we find that out of 374 species, 43 are found apparently unchanged on both sides of the isthmus; 265 are represented on the Atlantic side by closely related species—in most cases the nearest known relative of the Pacific species—while 64 have no near analogue in the Atlantic. Of the last group, some find their nearest relative to the northward or southward along the coast, and still others in the islands of Polynesia.

The almost unanimous opinion of recent students of the isthmus faunas finds the latest expression in the following words of Gilbert and Starks ('The Fishes of Panama Bay,' p. 205):

The ichthyological evidence is overwhelmingly in favor of a former open communication between the two oceans, which must have become closed at a period sufficiently remote from the present to have permitted the specific differentiation of a very large majority of the forms involved. That this differentiation progressed at widely varying rates in different instances becomes at once apparent. A small minority (43) of the species^[1] remain wholly unchanged so far as we have been able to determine that point. A larger number have become distinguished from their representatives of the opposite coast by minute (but not 'trivial') differences, which are wholly constant. From such representative forms we pass by imperceptible gradation to species much more widely separated, whose immediate relation in the past we can not confidently affirm. . . . It is obvious, however, that the striking resemblances between the two faunas are shown as well by slightly divergent as well as by identical species,

1. 43, or 11 per cent, of the species found on the Pacific side; about 2.5 of the combined fauna.