Many groups of molluscs are monœcious, and it is not in any way astonishing to find a group of crustaceans which are so. The Cirripedes or barnacles are an example. It is probable that the presence of ovaries and spermaries in the same individual--the monœcious condition--is an advantage to immovable fixed animals. During the voyage of the "Beagle," and making use on his return of the collections then obtained, Darwin carried out a very thorough study of the Cirripedes of all kinds from all parts of the world. He worked out their anatomy minutely, classified the 300 different kinds then known, and described many new kinds. The stalked barnacles often occur in groups, the individuals being of different ages and sizes, the small young ones sometimes fixing themselves by their sucker-bearing heads to the stalks of their well-grown relatives. In all the varied kinds studied by Darwin he found that the full-grown individuals were monœcious--that is, of combined sex--as was known to be the case in those studied before his day. But Darwin made the remarkable discovery that in two kinds of stalked barnacles (not the common ship's barnacles), comprising several species, "dwarf males" were present perched upon the edge of the shell of the large monœcious (bi-sexual) individuals. These dwarf males were from one-tenth to one-twentieth the length of the large normal monœcious individuals, but usually possessed the characteristic details of the shell-valves and other features of the latter. This existence of a sort of supernumerary diminutive kind of male as an accompaniment to a race of normal monœcious individuals was quite a new thing when Darwin discovered it. That all the males in some diœcious animals are minute as compared with the females was known, and has been established in the case of some parasitic crustaceans, in some of the wheel-animalcules, and in the most exaggerated degree in the curious worms, Bonellia and Hamingia. But the existence of "complemental males," as Darwin called them, existing apparently in order to fertilize the eggs should they escape fertilization by the ordinary monœcious individuals, was a new thing. And it was doubted and disputed when Darwin described his observations fifty-six years ago. They were, in fact, by many regarded as a distinct species parasitic upon the larger barnacles on which they were found until Darwin's conclusion as to their nature was confirmed by the report of Dr. Hoek, on the barnacles brought home by the "Challenger" expedition.