Of the 483 genera ascribed to Japan, 156 are common to the Mediterranean also, 188 to the West Indies and Japan, 169 to the Pacific coast of the United States and Mexico. With Hawaii Japan shares 90 genera, with New Zealand 62; 204 are common to Japan and India, 148 to Japan and the Red Sea, most of these being found in India also. Two hundred genera are common to Japan and Australia.
From this it is evident that Japan and the Mediterranean have much in common, but apparently not more than Japan shares with other tropical regions. Japan naturally shows most likeness to India, and next to this to the Red Sea. Proportionately less is the resemblance to Australia, and the likeness to the Mediterranean seems much the same as that to the West Indies or to the Pacific coast of America.
But, to make these comparisons just and effective, we should consider not the fish fauna as a whole; we should limit our discussion solely to the forms of equatorial origin. From the fauna of Japan we may eliminate all the genera of Alaskan-Aleutian origin, as these could not be found in the other regions under comparison. We should eliminate all pelagic and all deep-sea forms, for the laws which govern the distribution of these are very different from those controlling the shore fishes, and most of the genera have reached a kind of equilibrium over the world.
Significance of Rare Forms.—We may note also, as a source of confusion in our investigation, that numerous forms found in Japan and elsewhere are very rarely taken, and their real distribution is unknown. Some of these will be found to have, in some unexpected quarter, their real center of dispersion. In fact, since these pages were written, I have taken in Hawaii representatives of three[48] genera which I had enumerated as belonging chiefly to Japan and the West Indies. Numerous other genera common to the two regions have since been obtained by Dr. Gilbert. Such species may inhabit oceanic plateaus, and find many halting places in their circuit of the tropical oceans. We have already discovered that Madeira, St. Helena, Ascension, and other volcanic islands constitute such halting places. We shall find many more such, when the deeper shore regions are explored, the region between market-fishing and the deep-sea dredgings of the Challenger and the Albatross. In some cases, no doubt, these forms are verging on extinction and a former wide distribution has given place to isolated colonies.
The following table shows the contents, so far as genera are concerned, of those equatorial areas in which trustworthy catalogues of species are accessible. It includes only those fishes of stationary habit living in less than 200 fathoms. It goes without saying that considerable latitude must be given to these figures, to allow for errors, omissions, uncertainties, and differences of opinion.
Distribution of Shore Fishes.—
| A. Japan and the Mediterranean. | |
|---|---|
| Genera[49] chiefly confined to these regions | 2 |
| Genera of wide distribution | 77 |
| Total of common genera | 79 |
| Total in both regions | 399 |
| Genera above included, found in all equatorial regions | 55 |
| Genera[50] found in most equatorial regions | 11 |
| Genera more or less restricted | 13 |
| 79 | |
| B. Japan and the Red Sea. | |
| Genera[51] chiefly confined to these two regions | 2 |
| Genera of wide distribution | 109 |
| Total genera common | 111 |
| Total in both regions | 424 |
| C. Japan and Hawaii. | |
| Genera chiefly confined to these regions | 3 |
| Genera of wide distribution | 79 |
| Total genera common | 82 |
| Total in both regions | 396 |
| D. Japan and Australia. | |
| Genera chiefly confined to these regions | 13 |
| Genera of wide distribution (chiefly East Indian) | 122 |
| Total genera common | 135 |
| Total in both regions | 533 |
| E. Japan and Panama. | |
| Genera chiefly confined to these regions | 2 |
| Genera of wide distribution | 89 |
| Total genera common | 91 |
| Total in both regions | 499 |
| F. Japan and the West Indies. | |
| Genera chiefly confined to these regions | 5 |
| Genera of wide distribution | 108 |
| Total genera common | 113 |
| Total in both regions | 520 |
| G. The Mediterranean and the Red Sea. | |
| Genera confined to the Suez region | 0 |
| Genera of wide distribution (chiefly Indian) | 40 |
| Total genera common | 40 |
| Total in both regions | 295 |
| H. West Indies and the Mediterranean. | |
| Genera chiefly confined to the equatorial Atlantic | 11 |
| Genera of wide distribution | 59 |
| Total | 70 |
| Total in both regions | 373 |
| I. West Indies and Panama. | |
| Genera chiefly confined to equatorial America | 68 |
| Genera of wide distribution | 101 |
| Total genera common | 169 |
| Total in equatorial America | 376 |
| J. Hawaii and Panama. | |
| Genera chiefly confined to the regions in question | 3 |
| Genera of wide distribution | 74 |
| Total genera common | 77 |
| Total in both regions | 323 |
| K. Hawaii and the East Indies. | |
| Genera chiefly confined to Hawaii | 4 |
| Genera of wide distribution in the equatorial Pacific | 123 |
| Genera confined to Hawaii and the West Indies | 1 |
| Summary. | |
| Genera (shore fishes only) in the Mediterranean Sea. | 144 |
| Genera in the Red Sea | 191 |
| Genera in India | 280 |
| Genera in Japan (exclusive of northern forms) | 334 |
| Genera in Australia | 344 |
| Genera in New Zealand | 108 |
| Genera in Hawaii | 144 |
| Genera about Panama | 256 |
| Genera in West Indies | 299 |
Extension of Indian Fauna.—From the above tables it is evident that the warm-water fauna of Japan, as well as that of Hawaii, is derived from the great body of the fauna of the East Indies and Hindostan; that the fauna of the Red Sea is derived in the same way; that the fauna of the Mediterranean bears no especial resemblance to that of Japan, rather than to other elements of the East Asiatic fauna in similar conditions of temperature, and no greater than is borne by either to the West Indies; that the faunas of the sides of the Isthmus of Suez have relatively little in common, while those of the two sides of the Isthmus of Panama show large identity of genera, although few species are common to the two sides. Of the 255 genera recorded from the Panama region, 179, or over 70 per cent., are also in the West Indies, while 68, or more than 30 per cent. of the number, are limited to the two regions in question.
The Isthmus of Suez as a Barrier to Distribution.—With the aid of the above table we may examine further the relation of the fauna of Japan to that of the Mediterranean. If a continuity of shore-line once existed, it would involve the obliteration of the Isthmus. With free connection across this isthmus the fauna of the Red Sea must have been once practically the same as that of the Mediterranean. The present differences must be due to later immigrations to one or the other region, or to the extinction of species in one locality or the other, through some kind of unfitness. In neither region is there evidence of extensive immigration from the outside. The present conditions of water and temperature differ a little, but not enough to explain the difference in faunæ. The Red Sea is frankly tropical and its fauna is essentially Indian, much the same, so far as genera are concerned, as that of southern Japan. The Mediterranean is at most not more than semi-tropical and its fishes are characteristically European. Its tropical forms belong rather to Guinea than to the East Indies. With the Red Sea the Mediterranean has very little in common, not so much, for example, as has Hawaii. Forty genera of shore fishes (and only fifty of all fishes) are identical in the two regions, the Mediterranean and the Red Sea. Of those, every one is a genus of wide distribution, found in nearly all warm seas. Of shore fishes, only one genus in seven is common to the two regions. Apparently, therefore, we cannot assume a passage across the Isthmus of Suez within the lifetime of the present genera. Not one of the types alleged to be peculiar to Japan and the Mediterranean is thus far known in the Red Sea. Not one of the characteristically abundant Mediterranean types[52] crosses the Isthmus of Suez, and the distinctive Red Sea and Indian types[53] are equally wanting in the Mediterranean. The only genera which could have crossed the Isthmus are certain shallow-water or brackish-water forms, sting-rays, torpedoes, sardines, eels, and mullets, widely diffused through the East Indies and found also in the Mediterranean. The former channel, if one ever existed, had, therefore, much the same value in distribution of species as the present Suez Canal.
Geological Evidence of Submergence of the Isthmus of Suez.—Yet, from geological data, there is strong evidence that the Isthmus of Suez was submerged in relatively recent times. The recognized geological maps of the Isthmus show that a broad area of post-Pliocene or Pliocene deposits constitutes the Isthmus and separates the nummulitic hills of Suez from their fellows about thirty miles to the eastward. The northern part of the Isthmus is alluvium from the Nile, and its western part is covered with drifting sands. The Red Sea once extended farther north than now and the Mediterranean farther to the southeast. Assuming the maps to be correct, the Isthmus must have been open water in the late Pliocene or post-Pliocene times.