The average type of the Theory of Descent of the older or orthodox school, which still lingers in the background with its Darwinism unshaken, is that set forth by Haeckel, scientifically in his “Generelle Morphologie der Organismen” (1866), and “Systematische Phylogenie” (1896), and popularly in his “Natural History of Creation” and “Riddles of the Universe,” with their many editions. We may assume that it is well known, and need only briefly recall its chief characteristics. The “inestimable value,” the “incomparable significance,” the “immeasurable importance” of the Theory of Descent lies, according to Haeckel, in the fact that by means of it we can explain the origin of the forms of life “in a mechanical manner.” The theory, especially in regard to the descent of man from the apes, is to him not a working hypothesis or tentative mode of representation; it is a result comparable to Newton’s law of gravitation or the Kant-Laplace cosmogony. It is “a certain historical fact.” The proofs of it are those already mentioned. What is especially Haeckelian is the “fundamental biogenetic law,” “ontogeny resembles phylogeny,” that is to say, in development, especially in embryonic development, the individual recapitulates the history of the race. Through “palingenesis,” man, for instance, recapitulates his ancestral stages (protist, gastræad, vermine, piscine, and simian). This recapitulation is condensed, disarranged, or obscured in detail by “cenogenesis” or “cænogenesis.” The groups and types of organisms exhibit the closest genetic solidarity. The genealogical tree of man in particular runs directly through a whole series. From the realm of the protists it leads to that of the gastræadæ (nowadays represented by the Cœlentera), thence into the domain of the worms, touches the hypothetical “primitive chordates” (for the necessary existence of which “certain proofs” can be given), the class of tunicates, ascends through the fishes, amphibians and reptiles to forms parallel to the modern monotremes, then directly through the marsupials to the placentals, through lemuroids and baboons to the anthropoid apes, from them to the “famous Pithecanthropus” discovered in Java, out of which _homo sapiens_ arose. (The easy transition from one group of forms to another is to be noted. For it is against this point that most of the opposition has been directed, whether from “grumbling” critics, or thoroughgoing opponents of the Theory of Descent.)