the laws of their organization to a warmer temperature than that of the Mediterranean, but among them there must be many whose habitat is of a wider range, many whose powers of accommodation would enable them to acclimate themselves in a colder sea. We may suppose the less numerous aquatic fauna and flora of the Mediterranean to be equally capable of climatic adaptation, and hence there will be a partial interchange of the organic population not already common to both seas. Destructive species, thus newly introduced, may diminish the numbers of their proper prey in either basin, and, on the other hand, the increased supply of appropriate food may greatly multiply the abundance of others, and at the same time add important contributions to the aliment of man in the countries bordering on the Mediterranean. [Footnote: The dissolution of the salts in the bed of the Bitter Lake impregnated the water admitted from the Red Sea so highly that for some time fish were not seen in that basin. The flow of the current through the canal has now reduced the proportion of saline matter to five per cent, and late travellers speak of fish as abundant in its waters.] Some accidental attraction not unfrequently induces fish to follow a vessel for days in succession, and they may thus be enticed into zones very distant from their native habitat. Several years ago, I was told at Constantinople, upon good authority, that a couple of fish, of a species wholly unknown to the natives had just been taken in the Bosphorus. They were alleged to have followed an English ship from the Thames, and to have been frequently observed by the crew during the passage; but I was unable to learn their specific character. [Footnote: Seven or eight years ago, the Italian government imported from France a dredging machine for use in the harbor of La Spezia. The dredge brought attached to its hull a shell-fish not known in Italian waters. The mollusk, finding the local circumstances favorable, established itself in this new habitat, multiplied rapidly, and is now found almost everywhere on the west coast of the Peninsula.] Many of the fish which pass the greater part of the year in salt water spawn in fresh, and some fresh-water species, the common brook-trout of New England for instance, which under ordinary circumstances never visit the sea, will, if transferred to brooks emptying directly into the ocean, go down into the salt water after spawning-time, and return again the next season. Some sea fish have been naturalized in fresh water, and naturalists have argued from the character of the fish of Lake Baikal, and especially from the existence of the seal in that locality, that all its inhabitants were originally marine species, and have changed their habits with the gradual conversion of the saline waters of the lake-once, as is assumed, a maritime bay-into fresh. [Footnote: Babinet, Etudes et Lectures, ii, pp. 108,110.] The presence of the seal is hardly