He constructed a genealogical tree stretching far below the apes. Next to them came the lemurs. The lemur, the ghostly nocturnal inhabitant of Madagascar, came from the Australian marsupial (kangaroo, &c.). The marsupial came from the duck-bill; the duck-bill from the lizard; the lizard from the salamander; the salamander from the dipneust or mud-fish; this from the sturgeon or the shark, and the shark from the lamprey. Below the lamprey, at the lowest limit of the vertebrate kingdom, was the amphioxus (or lancelet). This must have come from the worm—it was not at all clear how, at that time. And so the series ran on down to the unicellular protozoa, the amœbæ and the monera. The construction of this tree would have been impossible for one who had not already done gigantic work. The whole of the new system of animals and plants, conceived in the form of a genealogical tree, had first to be sketched in outline. Then the narrower thread that led up to man, the Ariadne-thread of God-Nature, would gradually come to light. Both ends of the system, the lower one in the monera, the upper one in man, were first thoroughly treated by him in 1865, and in part somewhat later. His exhaustive _Monograph on the Monera_ was not published until 1868. Man’s genealogical tree was privately circulated at Jena in two essays in October and November 1865. They were published in the Virchow-Holtzendorff collection in 1868 (“The Origin and Genealogical Tree of the Human Race”). But in both cases the substance of the work, as an accumulation of facts, is much older. And this work was, of course, only possible in connection with a number of further conclusions: in regard to spontaneous generation, life and death, the crystal and the cell, the mathematical form of organisms, the nature and limits of individuality, the method of research, the new natural philosophy, God, and so on.