=Unlikeness of Species on the Shores of the Isthmus of Panama.=--But, curiously enough, this marked resemblance is confined chiefly to the genera and does not extend to the species on the two shores. Of 1400 species of fishes recorded from tropical America north of the Equator, only about 70 are common to the two coasts. The number of shore fishes common is still less. In this 70 are included a certain number of cosmopolitan types which might have reached either shore from the Old World. [Illustration: FIG. 179.--Map of the Continents, Eocene time. (After Ortmann.)] A few others invade brackish or fresh waters and may possibly have found their way, in one way or another, across the Isthmus of Nicaragua. Of fishes strictly marine, strictly littoral, and not known from Asia or Polynesia, scarcely any species are left as common to the two sides. This seems to show that no waterway has existed across the Isthmus within the lifetime, whatever that may be, of the existing species. The close resemblance of genera shows apparently with almost equal certainty that such a waterway has existed, and within the period of existence of the groups called genera. How long a species of fish may endure unchanged no one knows, but we know that in this regard great differences must exist in different groups. Assuming that different species crossed the Isthmus of Panama in Miocene times, we should not be surprised to find that a few remain to all appearances unchanged; that a much larger number have become "representative" species, closely related forms retaining relations to the environment to those of the parent form, and, finally, that a few species have been radically altered. This is exactly what has taken place at the Isthmus of Panama with the marine shore fishes. Curiously enough, the movement of genera seems to have been chiefly from the Atlantic to the Pacific. Certain characteristic genera[54] of the Panama region have not passed over to the Pacific. On the other hand, most of the common genera[55] show a much larger number of species on the Atlantic side. This may be held to show their Atlantic origin. Of the relatively small number of genera which Panama has received from Polynesia[56] few have crossed the Isthmus to appear in the West Indian fauna. =Views of Earlier Writers on the Fishes of the Isthmus of Panama.=--The elements of the problem at Panama may be better understood by a glance at the results of previous investigations. In 1869 Dr. Günther, after enumerating the species examined by him from Panama, reaches the conclusion that nearly one-third of the marine fishes on the two shores of tropical America will be found to be identical. He enumerates 193 such species as found on the two coasts; 59 of these, or 31 per cent. of the total, being actually identical. From this he infers that there must have been, at a comparatively recent date, a depression of the Isthmus and intermingling of the two faunas.[57]