WATERFALL  , a point in the course of a stream or

river where the water descends perpendicularly or nearly so . Even a very small stream of water falling from any considerable height is a striking object in scenery . Such falls, of small volume though often of immense depth, are common, for a small stream has not the power to erode a steady slope, and thus at any considerable irregularity of level in its course it forms a fall . In many mountainous districts a stream may descend into the valley of the larger river to which it is tributary by way of a fall, its own valley having been eroded more slowly and less deeply than the main valley . Mechanical considerations apart, the usual cause of the occurrence of a waterfall is a sudden change in geological structure . For example, if there be three horizontarl strata, so laid down that a hard stratum occurs between twosoft ones, a river will be able to grade its course through the upper or lower soft strata, but not at the same rate through the intermediate hard stratum, over a ledge of which it will consequently fall . The same will occur if the course of the river has been interrupted by a hard barrier, such as an intrusive dyke of basalt, or by glacial or other deposits . Where a river falls over an escarpment of hard rock overlying softer strata, it powerfully erodes the soft rock at the base of the fall and may undermine the hard rock above so that this is broken away . In this way the river gradually cuts back the point of fall, and a gorge is left below the fall . The classic example of this process is provided by the most famous falls in the world—Niagara . WATER-FLEA, a name given by the earlier microscopists (Swammerdam, 1669) to certain minute aquatic Crustacea of the order Cladocera, but often applied also to other members of the division Entomostraca (q.v.) . The Cladocera are abundant everywhere in fresh water .

One of the commonest

species, Daphnia pulex, found in ponds and ditches, is less than one-tenth of an inch in length and has the body enclosed in a trans-parent bivalved shell . The head, projecting in front of the shell, bears a pair of branched feathery antennae which are the chief swimming organs and propel the animal, in a succession of rapid bounds, through the water . There is a single large black eye . In the living animal five pairs of leaf-like limbs acting as gills can be observed in constant motion between the valves of the shell, and the pulsating heart may be seen near the dorsal surface, a little way behind the head . The body ends behind in a kind of tail with a double curved claw which can be protruded from the dell . The female carries the eggs in a brood-chamber between the back of the body and the shell until hatching takes place . Through-out the greater part of the year only females occur and the eggs develop " parthenogenetically," without fertilization . When the small males appear, generally in the autumn, fertilized " winter " or " resting eggs " are produced which are cast adrift in a case of " ephippium " formed by a specially modified part of the shell . These resting eggs enable the race to survive the cold of winter or the drying up of the water . For a fuller account of the Cladocera and of other organisms which sometimes share with them the name of " water-fleas," see the article ENTOMOSTRACA . (W . T .