Read it through once

At first sight such swamps appear to be useless, impossible, and dangerous. But that is not the case. No one, of course, would ever willingly reside in mangrove swamps, and the mangroves themselves are of scarcely any use to man, although the bark does sometimes furnish a useful tanning material; but, indirectly, the mangroves are one of the most important of all Nature's geographical agents.