Haeckel, ERNST HEINRICH, a distinguished German naturalist, born 16th February 1834, at Potsdam. He studied natural science and medicine at Würzburg, Berlin, and Vienna under Müller, Virchow, and Kölliker; and soon became distinguished for his enthusiasm and originality in zoological studies. After working for a while at Naples and Messina, he became a privat-docent in the university of Jena in 1861, a professor extra-ordinarius in 1862, and an ordinary professor of Zoology in 1865. In this position, in spite of other inducements, Haeckel has remained working indefatigably in his zoological institute, interrupted only by visits to the North Sea shores and the Mediterranean, or by more extended travels—e.g. to Madeira, the Canaries, Morocco, south Spain, Arabia, India, and Ceylon.
The most important of his numerous systematic works are the following: a monograph on the radiolarians (Die Radiolarien, 1862), with a superb atlas of 35 plates; the classic work on calcareous sponges (Die Kalkschwämme, 1872), important both in relation to these animals and in its practical illustration of general problems such as the nature of species; a yet larger work on jelly-fishes (System der Medusen, 1879), with an atlas of 40 plates, which like all Haeckel's work display inborn artistic talent unsurpassed among naturalists; several smaller works, such as that on the development and division of labour of the Siphonophora (1869), or that on the Monera (1870), in which he ranks under the title Protista the lowest forms of life which have not taken a decisive step towards plants or animals, or that on Arabian Corals (1876); and finally his monumental contributions to the Challenger Reports—on Deep-sea Medusæ (1882), with 32 plates; on Siphonophora (1888), and especially on Radiolaria (1887), in three volumes, with 140 plates and 3500 new species.
With the above gigantic descriptive work Haeckel has combined two rarer accomplishments, successful generalisation and popular exposition. His Generelle Morphologie (2 vols. 1866), in its reasoned orderliness and clear generalisations, ranks beside Spencer's Principles of Biology; it is not only one of the very few works of moment on general morphology, but is greater than its name suggests, really including the gist of a series of treatises—e.g. on the commonly avoided subject of organic stereometry—the science of shape or promorphology, on the much-debated problem of individuality, on the various modes of reproduction, on heredity, and on the pedigrees of animals. Besides being one of the first to sketch the genealogical tree (Stammbaum) of animals, Haeckel gave precise and luminous expression to the general fact that the life-history of the individual is a more or less accurate recapitulation of its historic evolution. As a special application of this 'fundamental biogenetic law' his Gastræa-theory (elsewhere stated) is of paramount importance (see EMBRYOLOGY). Among other general works may be noted his Perigenesis of the Plastidules (1876), an ingenious contribution to the theory of Heredity (q.v.), and his speculations on the origin and development of animal tissues (1884).
Apart from detailed zoological work, Haeckel has devoted his life to applying the doctrine of evolution and to making it current coin. Owing much of his motive to Darwin, he stood for a time almost alone in Germany in his championship of a theory not then popular. Before the publication of Darwin's Descent of Man Haeckel was the only naturalist who had clearly recognised the import of sexual selection, and of his Natural History of Creation Darwin says, 'If this work had appeared before my essay had been written, I should probably never have completed it.' His most important expository works are the above-mentioned Natürliche Schöpfungsgeschichte (1st ed. 1868; 8th ed. 1889), which has been translated into twelve languages; 'The Evolution of Man' (Anthropogenie, 1874; 3d ed. 1877); and lectures on development and evolution, Gesammelte populäre Vorträge auf dem Gebiete der Entwicklungslhre (1875–79). Haeckel's popular works are very brilliantly written, but they are not always so careful in statement as Darwin's classics, and offend many by their remorseless consistency, and by their impatience with theological dogma and teleological interpretation. He has always been set against compromise, defending the freedom of science in a famous pamphlet (Freie Wissenschaft und Freie Lehre) written in answer to Virchow. Other works are Die Medusen (1880); Darwin, Goethe, und Lamarck (1882); Der Monismus (1892); and Systematische Phylogenie (1894–95).
Like all other naturalists, he has made a few mistakes; there are hints both in some of his drawings and in some of his arguments of the dangers of artistic and speculative imagination; and it may be doubted whether his early championship of evolution and Darwinism has not resulted in a taint of dogmatism in what is sometimes called 'Haeckelismus.' On the other hand, the thoroughness of his systematic labours, the excellence of his draughtsmanship, the clear generalisations of his Generelle Morphologie, the geniality of his teaching, and perhaps above all the courage, lucidity, and eloquence of his popular expository work on evolution have raised Haeckel to a pre-eminent position among modern naturalists. See BIOLOGY, DARWINIAN THEORY, EMBRYOLOGY, EVOLUTION, HEREDITY, ZOOLOGY.