Similar laws regulate the distribution of marine plants. Groups of algæ, or marine plants, affect particular temperatures or zones of latitude and different depths, though some few genera prevail throughout the ocean. The polar Atlantic basin to the 40th degree of north latitude presents a well-defined vegetation. The West India seas, including the Gulf of Mexico, the eastern coast of South America, the Indian Ocean and its gulfs, the shores of New Holland, and the neighbouring islands, have each their distinct species. The Mediterranean possesses a vegetation peculiar to itself, extending to the Black Sea; and the species of marine plants on the coast of Syria and in the port of Alexandria differ almost entirely from those of Suez and the Red Sea. It is observed that shallow seas have a different set of plants from such as are deeper and colder; and, unlike terrestrial vegetation, the algæ are more numerous in the mean latitudes than either towards the equator or the poles. They vary also with the depth: completely different kinds affect different depths, their seeds being of such specific gravity as to remain and germinate where the parent plant grew. The quantity of algæ in that accumulation known as the sargassa or grassy sea is so great, that the early navigators, Columbus and Lerius, compared it to extensively inundated meadows: it impeded their ships, and alarmed their sailors. It is in the North Atlantic, a little to the west of the meridian of Fayal, one of the Azores, between the 25th and 36th parallels of latitude. A smaller bank lies between the 22nd and 26th degrees of north latitude, about 80 leagues west of the meridian of the Bahama Islands. These masses chiefly consist of one or two species of sargassa, the most extensive genus of the order Fucoideæ. Some of the seaweeds grow to enormous lengths, and all are highly coloured, though many of them must grow in deep water. Light, however, may not be the only principle on which the colour of vegetables depends, since Baron Humboldt met with green plants growing in complete darkness in one of the mines at Freyberg. In the dark and tranquil caves of the ocean, on the shores alternately covered and deserted by the restless waves, on the lofty mountain and extended plain, in the chilly regions of the north, and in the genial warmth of the south, specific diversity is a general law of the vegetable kingdom, which cannot be accounted for by diversity of climate; and yet the similarity, though not identity, of species is such, under the same isothermal lines, that if the number of species belonging to one of the great families of plants be known in any part of the globe, the whole number of the flowering or more perfect plants, and also the number of species composing the other vegetable families, may be estimated with considerable accuracy.