conclusions from the available facts of embryology, comparative
anatomy, and paleontology, as to the processes in the formation of the
species in the remote past. It is of the same importance to the
student of evolution as the careful distinction between genuine and
spurious texts in the works of an ancient writer, or the purging of
the real text from interpolations and alterations, is for the student
of philology. It is true that this distinction has not yet been fully
appreciated by many scientists. For my part, I regard it as the first
condition for forming any just idea of the evolutionary process, and I
believe that we must, in accordance with it, divide embryology into
two sections--palingenesis, or the science of recapitulated forms; and
cenogenesis, or the science of supervening structures.

To give at once a few examples from the science of man's origin in
illustration of this important distinction, I may instance the
following processes in the embryology of man, and of all the higher
vertebrates, as palingenetic: the formation of the two primary
germinal layers and of the primitive gut, the undivided structure of
the dorsal nerve-tube, the appearance of a simple axial rod between
the medullary tube and the gut, the temporary formation of the
gill-clefts and arches, the primitive kidneys, and so on.* (* All
these, and the following structures, will be fully described in later
chapters.--Translator.) All these, and many other important
structures, have clearly been transmitted by a steady heredity from
the early ancestors of the mammal, and are, therefore, direct
indications of the presence of similar structures in the history of
the stem. On the other hand, this is certainly not the case with the
following embryonic forms, which we must describe as cenogenetic
processes: the formation of the yelk-sac, the allantois, the placenta,
the amnion, the serolemma, and the chorion--or, generally speaking,