While we apply this law to the evolution of the soul, we must lay special stress on the injunction to keep _both_ sides of it critically before us. For, in the case of man, just as in all the higher animals and plants, such appreciable perturbations of type (or _cenogeneses_) have taken place during the millions of years of development that the original simple idea of _palingenesis_, or "epitome of history," has been greatly disturbed and altered. While, on the one side, the _palingenetic_ recapitulation is preserved by the laws of like-time and like-place heredity, it is subject to an essential _cenogenetic_ change, on the other hand, by the laws of abbreviated and simplified heredity. That is clearly seen in the embryonic evolution of the psychic organs, the nervous system, the muscles, and the sense-organs. But it applies in just the same manner to the psychic functions, which are absolutely dependent on the normal construction of these organs. Their evolution is subject to great cenogenetic modification in man and all other viviparous animals, precisely because the complete development of the embryo occupies a longer time within the body of the mother. But we have to distinguish two periods of individual psychogeny: (1) the embryonic, and (2) the post-embryonic development of the soul. I. _Embryonic Psychogeny._--The human foetus, or embryo, normally takes nine months (or two hundred and seventy days) to develop in the uterus. During this time it is entirely cut off from the outer world, and protected, not only by the thick muscular wall of the womb, but also by the special foetal membranes (_embryolemmata_) which are common to all the three higher classes of vertebrates--reptiles, birds, and mammals. In all the classes of amniotes these membranes (the _amnion_ and the _serolemma_) develop in just the same fashion. They represent the protective arrangements which were acquired by the earliest reptiles (_proreptilia_), the common parents of all the amniotes, in the Permian period (towards the end of the palæozoic age), when these higher vertebrates accustomed themselves to live on land and breathe the atmosphere. Their ancestors, the amphibia of the Carboniferous period, still lived and breathed in the water, like their earlier predecessors, the fishes.