Relations of Japan to Mediterranean Explainable by Present Conditions.—We may conclude that the resemblance of the Mediterranean fish fauna to that of Japan or India is no more than might be expected, even had the present contour of the continents been permanent for the period of duration of the present genera and species. An open channel in recent times would have produced much greater resemblances than actually exist.
The Isthmus of Panama as a Barrier to Distribution.—Conditions in some regards parallel with those of the Isthmus of Suez exist in but one other region—the Isthmus of Panama. Here the first observers were very strongly impressed by the resemblance of forms. Nearly half the genera found on the two sides of this isthmus are common to both sides. Taking those of the Pacific shore for first consideration, we find that three-fourths of the genera of the Panama fauna occur in the West Indies as well.
This identity is many times greater than that existing at the Isthmus of Suez. Moreover, while the Cape of Good Hope offers no impassable barrier to distribution, the same is not true of the southern part of South America. The subarctic climate of Cape Horn has doubtless formed a complete check to the movements of tropical fishes for a vast period of geologic time.
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.
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.