From December, 1872, to May, 1876, the English had conducted a peaceful enterprise that will be for ever memorable. A staff of distinguished naturalists had gone on the ship _Challenger_ to explore the depth, temperature, and bottom of remote seas. With the aid of the best appliances specimens of the mud from the floor of the ocean (sometimes more than a mile in depth) were brought up at 354 different spots. It was known from earlier deep-sea explorations that this slime on the floor of the ocean, from a certain coast-limit into the deepest parts, is composed for the most part of the microscopically small shells of little marine animals. The living creatures that form these shells swim in the water of the ocean, partly at the surface and partly at various depths beneath it. When they die the little hard coat of mail sinks to the bottom, and as there are millions upon millions of them living in the sea, thick deposits are gradually formed at the bottom that consist almost entirely of these microscopic shells. The animals in question are primitive little creatures consisting of a single cell, of the type that Haeckel has called “Protists.” Even in Ehrenberg’s time it had been noticed that amongst the shells in the deep-sea mud there were, besides chalky shells, a number of graceful flinty coats that clearly pointed to the radiolaria. The _Challenger_ expedition now made the great discovery that vast fields at the floor of the ocean, especially of the Pacific, were covered almost exclusively with these flinty shells. It was seen at once that the few hundred species of radiolaria that had hitherto been described by Haeckel and others were only a very small part of the masses of radiolaria found in the ocean. The specimens of the deposits which were carefully preserved and brought home by the _Challenger_ contained such an immense number of unknown species with their flinty shells faultlessly preserved, that it was necessary to reconstruct the whole of this wonderful group of animals. And who could be better qualified for the work than the man who had already made a name by his study of the radiolaria, Haeckel?