unicellular ovum repeats the unicellular condition of our protozoan ancestors. The blastula germ form corresponds to a Volvox or Magosphæra, a similar ancestral form; the Gastrula is the inherited repetition of the Gastræa, the common stem form of the entire series of Metazoa. All these typical ancestral forms man shares with all the other Metazoa, that is to say with all other animals except the unicellular Protozoa. Every man, without exception, begins his individual existence in the form of a spherical egg cell, barely visible to the naked eye, as a very small dot, and the special characters of this egg-cell are exactly the same in man as in all other mammals.
The most obscure portion of the genealogical history of man is that part which lies between Gastræa and Amphioxus. Amphioxus itself, that famous lancelet, or lancet animal, whose fundamental significance had already been recognized by its first exact describer, the great Johannes Müller, is the most precious document of vertebrate phylogeny. We should not indeed consider it as a stem ancestor to vertebrates, but rather as a near relation to such, and as a unique living relic of the class of acrania. Had the amphioxus accidentally perished, like so many other links in our ancestral chain, we would hardly be in a position to obtain any satisfactory insight into the older steps that led to the formation of vertebrates. Above amphioxus stand its near relations, the Cyclostomata or round-mouths. These are the oldest Craniota or skulled animals, the first vertebrates that succeeded in obtaining a skull and brain. These Cyclostomata (among whom the well-known lamprey, Petromyzon, belongs) are, at the same time, the presilurian forerunners of fishes. Below amphioxus we find that the agreement between the ontogeny of amphioxus and the ascidians points to an unknown older group of chorda animals, the Prochordonia, from which have developed on the one hand the tunicates, on the other the vertebrates. We may derive these prochordonia, or primitive chorda animals, from the Frontonia, a twig of the Vermalia, or true worms. The isolated Balanoglossus and the old Nemertina are probably closely related to these. There certainly existed, in the Cambrian and Laurentian periods, between these worms and the stem group of the Gastræades, a long series of intermediate forms, and we suppose that the older Rotatoria and Turbellaria belonged in this series. But we can not at this time form any well-grounded hypothesis on this point, and there is indeed here a wide empty space in our genealogical history.
But contrasted with these and other obscure portions of our family history stand out clearly and significantly the conclusions which the rich results of comparative anatomy, ontogeny, and palæontology have given in the investigation of the vertebrate stock, and especially of that of its highest class of mammals. All reliable recent researches have here unanimously confirmed the proposition which Lamarck, Darwin, and Huxley declared to be the most important result of the theory of evolution—the proposition that the immediate placental ancestors of
