As to the mud itself, it is a horrible, greasy, oozy, black or blue-black slime of bottomless depth. "It is full of organic, putrefying, strongly-smelling material, clearly full of bacteria. The water itself is sometimes covered by a dirty, oily scum, and air-bubbles rising from the bottom, spread out on the surface and let loose their microbes in the atmosphere."[74] There are many crocodiles, which may be seen reposing on the mud above high tide. It is difficult to distinguish them from a rough log of wood, but it is still more difficult to kill them, for their scales turn any ordinary bullet. There is scarcely any experience more exasperating than when, after one has taken a long, careful, and accurate aim, one observes the sleeping brute suddenly wake and scurry down into the water with a hideous leer on its face. Sea-cows or manatees are said to live in these creeks. Little ducks of many kinds rise in hundreds and thousands, but the commonest bird is the "curlew" (either a whimbrel or closely allied to it). During the day they sift the mud with their long curved beaks for insects, and at sunset fly down in vast numbers to the mudbanks near the sea. A miserable little white crane called "Poor Joe" is common, and has the same habit. It is not worth shooting, and it is quite aware of the fact. Herons, cormorants, and other birds are often to be seen. Monkeys sometimes visit the mangroves, probably to eat oysters or crabs. There are several kinds of crablike creatures which climb up the roots and may be seen running about all over them. But during the three weeks spent by the writer in the Mahéla creeks of Sierra Leone, it was the insects that made the deepest impression upon him; as soon as the evening falls the mosquitoes appear in myriads and in millions. Such creeks and mangrove swamps are always feverstricken and dangerous, and probably enjoy the very worst climate in the whole world. Of course nowadays, when Sir Patrick Manson and Dr. Ross have discovered that the mosquito carries the malaria germ, it is possible with great care to guard against malaria. One has also the satisfaction of knowing that the mosquito itself cannot be perfectly at ease with all these tiny parasites attacking its digestive organs. [74] Warming, _Lehrbuch der OEcol Pfl. Geog._ 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. On those horrible, slimy, shifting mudbanks no other plants could manage to exist. If one looks carefully at the seaward side of the last of the mangrove swamps, then it is easy to see that they are colonizing and reclaiming the mud.