PROF. HAECKEL, of the University of Jena, may be regarded as the most eminent living representative of the doctrine of evolution in Germany. He has won a name for himself during the last ten years as the author of several remarkable works in various sections of Natural History; specially should be mentioned his monograph on the Radiolaria (Berlin, 1862), which is, according to Huxley, one of the most solid and important contributions to zoology that have appeared for a long time. We owe also to him a monograph on the Monads (Journal de Jena pour la Médicine, &c., 1868), the simplest of known organisms, and another on the Geryonidæ or Hydromedusæ (Leipzig, 1865); a history of the development of the Siphonophoræ, a work crowned by the Academy of Utrecht (1869); a paper on the Sarcode bodies of the Rhizopoda (in the Journal de Zoologie Scientifique, Leipzig, 1865); “Considerations on the Division of Labour in Nature and among Men ” (in the collection of scientific treatises of Virchow and Holzendorff, 1869); and an essay on the “Origin and the Genealogical Tree of the Human Race” (in the same collection, 1868; 2nd edition, 1870). There has just appeared a monograph on the Calcareous Sponges (See NATURE, vol. vii. p. 279), on which the author has been engaged for five years. But his principal work is undoubtedly his “Morphology of Organisms,” in which he has condensed the result of all his researches, and unfolded his views on Nature as a whole, its history, its constitution, and its development: it is a learned treatise on natural philosophy, in which the author has adopted out and out the system of Darwin. Indeed, on more than one point he goes much farther than his master, and does not shrink from any of the extreme consequences of principles which are simply stated by the English philosopher: it may with truth be said that he is more Darwinian than Darwin himself. He aims, in fact, at filling up the chasm which separates the organic and inorganic kingdoms, and is inclined to endow with life everything that has being, down to crystals and the smallest molecule of matter. Haeckel, with his comprehensive and philosophic mind, has more than once applied the theory of evolution to certain moral phenomena, and notably to politics, while Darwin has always shown considerable reserve in this direction. With respect, also, to the simian origin of man, he is much more explicit and precise than the English naturalist. In short, as he does not confine himself simply to the exposition of theories and principles, as he seeks to recover the marks of development in the particular genealogy of animal and vegetable organisms, he is compelled to commit himself to a great number of hypotheses, whose boldness it is impossible to deny. We do not speak thus in the way of reproach; we are none of those who think that science can live on experiment alone; hypothesis has always preceded experiment, and has seemed to incite and throw light upon it; it is the torch of induction, and without it the human mind would be doomed to sterility. Goethe has truly said that bad hypotheses are better than none at all. All that we ought to insist on is, that a hypothesis be abandoned the moment it is found to contradict certain facts, or when the same facts are more satisfactorily explained by a new hypothesis. One hypothesis may be better than another in three points—(1) when it accounts for a greater number of facts; (2) when it explains them by a smaller number of causes; and (3) when it makes use only of known causes, and involves a smaller number of accessory hypotheses. This is why Darwinism is preferable to supernatural hypotheses; it only applies to the whole round of natural phenomena causes which undoubtedly explain particular facts—natural selection, adaptation, and heredity.