[2] Palingenesis = new birth, or re-incarnation (_palin_ = again, _genesis_ or _genea_ = development); hence its application to the phenomena which are recapitulated by heredity from earlier ancestral forms. Cenogenesis = foreign or negligible development (_kenos_ and _ genea_); hence, those phenomena which come later in the story of life to disturb the inherited structure, by a fresh adaptation to environment.—Translator.
This careful discrimination between the primary or palingenetic processes and the secondary or cenogenetic is of great importance for the purposes of the scientific history of a species, which has to draw 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.[3] 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, the various fœtal membranes and the corresponding changes in the blood vessels. Further instances are: the dual structure of the heart cavity, the temporary division of the plates of the primitive vertebræ and lateral plates, the secondary closing of the ventral and intestinal walls, the formation of the navel, and so on. All these and many other phenomena are certainly not traceable to similar structures in any earlier and completely-developed ancestral form, but have arisen simply by adaptation to the peculiar conditions of embryonic life (within the fœtal membranes). In view of these facts, we may now give the following more precise expression to our chief law of biogeny: The evolution of the fœtus (or _ontogenesis_) is a condensed and abbreviated recapitulation of the evolution of the stem (or _ phylogenesis_); and this recapitulation is the more complete in proportion as the original development (or _palingenesis_) is preserved by a constant heredity; on the other hand, it becomes less complete in proportion as a varying adaptation to new conditions increases the disturbing factors in the development (or cenogenesis).