Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/366

Rh of Great Britain and Ireland. In those beds we find fossils, and as these fossils are the remains of animals coeval, if my theory be true, with the existing flora of our mountains, an examination of them ought to afford a good test of its correctness.

Before, however, inquiring into this matter, let us take a general glance at the botanical and zoological features presented by the British seas. A correct knowledge of these, and even in certain departments of their details, will be absolutely necessary to any geologist who hereafter may engage in one of the most interesting inquiries in British geology, as yet but very imperfectiy handled,—the history of the northern drift and boulder clays.

The marine flora is of much less importance, as bearing upon these inquiries, than our marine fauna. For as almost all (Zostera and Zanichellia are exceptions) of the plants inhabiting our seas are cryptogamic, and marine cryptogamia seem to be far more widely diffusable, owing to the organization, transportability, and persistent vitality of their germs (spores)—and also to the capacity of cryptogamic plants to endure great vicissitudes of climatal conditions—their zones of distribution are not nearly so definite as those of most seed-producing plants, and a great number of species are cosmopolitan. Thus there are species of Ulva, Enteromorpha, Codium, and other genera, which seem to have an universal distribution, extending from pole to pole, and diffused through both hemispheres. As a general rule, however, the higher the algue, the more limited in distribution; and well marked, though wide algological regions have thus been traced out by botanists. The observations of Lamouroux, that the diffusion of algæ is determined by lines of coast and depth of water, shows that even their distribution has been mainly determined by geological events; and ere long, when it shall have been studied with reference to the latter, I do not doubt we shall gain new and unexpected information respecting the causes which have arranged them in the sea as they now are. From the researches of Dr. Greville, Mr. Harvey, Mrs. Grrifith, and other British algologists, it appears that our marine vegetation presents at least two well-marked types; a southern and a northern, both perhaps admitting of minor subdivisions. Thus the genera Padina and Halyseris do not range further north than the south coast of England, and are met with only in few localities there; and the genera Cystoseira, Sporochnus, Elaionema, Cutleria, certain species of Dictyota, Sphacelaria, Mesogloia, Rhodomenia (R. bifida, R.jubata) and Gigartina, all aid in marking out a southern region, which includes the British Channel, and part of the east coast, the Bristol Channel and the south and west of Ireland. The absence of southern species, the greater abundance and luxuriance of northern forms, and the presence of such fuci as