While the fresh waters of the globe have thus acquired from the land an abundant population of higher plants, the presence of salt, in water as on land, has had a deterrent effect. The sea was at first fresh. The primitive ocean derived by condensation from a cooling atmosphere in the early days of the world’s history contained no excess of salts. Whether life arose while this condition still persisted it is not possible to say; but as the sea grew salter owing to the rivers bringing into it incessantly salts derived from the land, the Seaweeds alone of the great groups of plants adapted themselves to saline conditions, and the ocean is now their unchallenged kingdom. The divisions which are represented by the Mosses, Liverworts, Club-mosses, Horsetails, and Ferns, have not, and so far as is known never had, a single representative in the sea. Only one or two Fungi--often symbiotically combined with Algæ to form Lichens--and a very few Flowering Plants, have attempted marine colonization, after long ages spent on land; and they have met with indifferent success. As we pass from fresh to brackish water, the population decreases rapidly, till in the seas surrounding our islands only one Seed Plant--the Grass-wrack, _Zostera marina_--has adopted a habitat which is thoroughly marine, and very few are found in other parts of the world. A study of the meeting-ground of the land and sea plants, such as we may make on rambles along the coast, supplies us with some interesting material. On sandy shores, the wave-trampled beach, shifting under the influence of winds and currents, offers a stretch of “no-man’s-land”--a desert strip untenanted alike by terrestrial or marine plants. The former do not descend below spring-tide mark, if they go so far; the latter cannot obtain foothold on the unstable substratum. The peculiar characters of the terrestrial beach plants has been referred to on a previous page (p. 36). On rocky shores the “desert” strip is much narrowed, and a certain overlap may often be found, for the Lichens--essentially a terrestrial group--descend from the plant-covered slopes into the spray-swept zone below, and on to mix with the Seaweeds which occupy the belt under high-water mark, some of them, species of _Verrucaria_ and _Arthropyrenia_, continuing downward till the low-water mark of spring tides is reached. On steep rocky shores the dividing-line between the Flowering Plants and the Seaweeds is quite narrow, and varies in elevation with the exposure. On cliffy coasts open to the Atlantic waves the uppermost Seaweeds, such as _Pelvetia_, which only asks to be wetted periodically by spray, occur far above high-water mark, the lowest Seed Plants perching on the rocks much higher still--sometimes not venturing to within 100 feet of the water-level. Under such extreme conditions none of the higher land plants venture down towards the unfriendly sea. To see the overlap of the terrestrial