See Annales des Sciences Naturelles ; Taylor's Ann. Nat. Hist. ; the Linncca, fyc. various volumes. Seaweeds rarely grow on sand, unless when it is very compact and firm. There are, therefore, submerged sandy deserts, as barren as the most cheerless of the African wastes. And when such barrens interpose, along a considerable extent of coast, between one rocky shore and another, they oppose a strong barrier to the dispersion of species, though certainly not so strong as the aerial deserts ; because

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