body consists, in the simplest case, solely of a round digestive sac, the
thin wall of which is formed by two layers of cells--the two primitive
germinal layers. A corresponding germinal condition, the two-layered
gastrula, occurs transitorily in the embryological history of all the
other Metazoa, from the lowest Cnidaria and Vermes up to man. From the
common stock of the Helminthes, or simple worms, there develop as
independent main branches the four separate stems of the Molluscs,
Star-fishes, Arthropods, and Vertebrates. It is only these last whose
bodily structure and development in all essential respects coincide with
those of man. A long series of lower aquatic Vertebrates (lancelets,
lampreys, fishes) precedes the lungbreathing Amphibians, which appear for
the first time in the Carboniferous period. The Amphibians are followed
in the Permian period by the first Amniota, the oldest reptiles; from
these develop later, in the Triassic period, the Birds on the one hand,
and the Mammals on the other. That man in his whole bodily frame is a
true mammal, becomes obvious as soon as the natural unity of this highest
class of animals is recognised. The simplest comparison must have
convinced the unprejudiced observer of the close constitutional
relationship between man and the ape, which of all the Mammals comes
nearest him. Comparative anatomy, with its deeper vision, showed that all
differences in bodily structure between man and the Anthropoidea
(gorilla, chimpanzee, orang) are less important than the corresponding
differences in bodily structure between these anthropoid apes and the
lower apes. The phylogenetic significance of this fact, first emphasised
by Huxley, is quite clear. The great question of the origin of the human
race, or of "man's place in Nature," the "question of all questions," was
then scientifically answered: "Man is descended from a series of ape-like
Mammals." The descent of man (anthropogeny) discloses the long series of
vertebrate ancestors, which preceded the late origin of this, its most
highly developed offshoot.[13]