MÜLLER, Othon Frederik, an eminent Danish naturalist, was the son of poor parents, and was born at Copenhagen in March 1730. He commenced to study for the church, and supported himself by the exercise of his musical talents. His learning, and the strength of his moral character, soon raised him to notice. He was appointed tutor to the young Count Schulin in 1753. It was the advice of his pupil's mother, a woman of great penetration, that induced him to turn his attention to natural history. With all the determination and concentrative power of his character, he commenced his new studies. He examined patiently both animals and plants, executed exact sketches of them, and wrote a Danish treatise on Fungi, and two Latin works respectively entitled Fauna Insectorum Friedrichsdaliana, and Flora Friedrichsdaliana. At the same time his travels through different countries with his pupil were affording him an excellent opportunity for extending his scientific observations, and for becoming intimate with other naturalists. After his return to Copenhagen in 1761, a high rank in the general estimation was assigned to him. He was honoured with several titles, he was installed in several important offices, and he was appointed to continue the Flora of Denmark, a great work which had been begun by Oeder in 1761 by the command of Frederic V. An advantageous marriage soon afterwards enabled him to resign his official appointments, and to consecrate all his efforts to his favourite study. He first turned his observation upon those annulose animals which are called by Linnaeus "Aphroditeæ and Nereides." His numerous and interesting discoveries on the structures and habits of these creatures were given to the world in a work entitled On Certain Worms found in Fresh and Salt Water, 4to, Copenhagen, 1771. Even more successful were his studies on the Infusoria. By the patient employment of powerful microscopes he discovered many new species, and was the first among naturalists to attempt the extremely delicate task of arranging these animalcules into distinctive genera. His observations in this field of inquiry were published in his Vermium Terrestrium et Fluvialium seu Animalium Infusorium Helminthecorum et Testaceorum non Marinorum succincta Historia, 2 vols. 4to, Copenhagen and Leipzig, 1773-74. The attention of Müller was next occupied with the Hydrachnæ or water-spiders. He was the first who discovered that these animalcules swarm by millions in all our fresh-water streams. His treatise on them was carefully written in lucid and elegant Latin, was copiously illustrated with faithfully-executed plates, and was published at Leipzig in 1781, under the title of Hydrachnæ in Aquis Danica Palustribus detectæ et descriptæ. He was in the midst of his favourite investigations when he was cut off by death on the 26th December 1784. Two posthumous works, the one on the Entomonstraca, small crustaceans belonging to Linnaeus' genus of the Monoculi, and the other on the Infusoria, were published in 1785 and 1786 respectively. Müller had commenced in 1779 the
Zoologia Danica, a gigantic work which was continued first by Abildgaard and afterwards by Rathke. The two parts which he wrote were reprinted in 1788.
"The three works on the Infusoria, Monoculi, and Hydrachnæ," says Cuvier in the Biographie Universelle, "have procured for Müller a place in the front ranks of those who have enriched science with original observations."