478 ON OUR PRESENT KNOWLEDGE OF THE ORIGIN OF MAN. 



unicellular ovum repeats the unicellular condition of our protozoan 

 ancestors. The blastula germ form corresponds to a Volvosc or Mayo- 

 sphcera, a similar ancestral form; the Gdstrula is the inherited repeti- 

 tion of the Gastrcea, 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 unicellu- 

 lar 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 Gastrcea and Amphioxus. Amphioxus itself, 

 that famous laucelet, or lancet animal, whose fundamental significance 

 had already been recognized by its first exact describer, the great 

 Johannes Miiller, is the most precious document ot 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 posi- 

 tion to obtain any satisfactory insight into the older steps that led to 

 the formation of vertebrates. Above amphioxus stand its near relations, 

 the Gyclostomata or round-mouths. These are the oldest Craniota or 

 skulled animals, the first vertebrates that succeeded in obtaining a 

 skull and brain. These Gyclostomata (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 verte- 

 brates. We may derive these prochordonia, or primitive chorda animals, 

 from the Frontonia, a twig of the Vermalia, or true worms. The iso- 

 lated 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 Gastrwades, a 

 long series of intermediate forms, and we suppose that the older Rota- 

 toria 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 palaeontology 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 



