Hitherto we have dealt with his general work only, a résumé of which he gave for many years in a course of thirty lectures before an audience composed of 'all sorts and conditions of men.' Students of biology and of medicine side by side with theologians, incipient and ordained, jurists, political economists, and philosophers, crowded his lecture-room during the 'seventies to hear the master explaining the 'natural history of creation' or the mysteries of anthropogenesis. Another course of eighty lectures during the winter semester was, and still is, devoted to a systematic treatment of zoology, while practical classes are reserved for the more select. His winning personality and fascinating eloquence, combined with a clear and concise delivery, have gained the enthusiastic admiration of many a student who went to the quiet University town in order to learn with his own ears and eyes. _List of Separate Publications by Professor Haeckel._ 'Biologische Studien. I.: Studien ueber die Moneren und andere Protisten.' Leipzig, 1870 (out of print). He was the first to make observations on the natural history of the Monera, living bits of protoplasm, devoid even of a nucleus--_e.g._, _Protogenes primordialis_, _Protomyxa aurantiaca_. 'Monographie der Radiolarien.' Berlin, 1862-88. With 171 plates. 'Entwicklungsgeschichte der Siphonophoren.' Utrecht, 1869. 'Plankton-Studien. Vergleichende Untersuchungen ueber die Bedeutung und Zusammensetzung der pelagischen Fauna und Flora.' Jena, 1880. 'Metagenesis und Hypogenesis von Aurelia aurita.' Jena, 1881. 'Monographie der Geryoniden oder Ruesselquallen.' Leipzig, 1865. 'Generelle Morphologie der Organismen.' 2 vols. Berlin, 1866. 'Anthropogenie oder Entwicklungsgeschichte des Menschen,' 1874; 4th edition, 1891. 'Natuerliche Schoepfungs-Geschichte.' 2 vols. Berlin, 1st edition, 1868; 9th edition, 1898. This work has been translated into most European languages (the first edition in English, under the title 'Natural History of Creation' in 1873; the eighth in 1892). 'Monographie der Kalkschwaemme.' 3 vols. Berlin, 1872 (out of print). With the subtitle, 'An Attempt to solve analytically the Problem of the Origin of Species.' In this work, illustrated by sixty plates, he showed that the Calcispongia are individually so yielding, so adaptive to external influences, that it is practically impossible to break up the whole group into anything like satisfactory species or genera. According to predilection, we can distinguish either 1 genus with only 3 species, or 3, 21, 43 genera, with 21, 111, 181, or 289 species respectively. In this work, in 1872, Haeckel established the homology of the two primary layers, ecto- and endoderm, throughout the Metazoa. The attempt to do the same for the four secondary layers, as made in the second part of his 'Gastræa-theory,' failed. It caused an enormous amount of research, hitherto without a satisfactory solution of the problem.