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In all these stems the gastrula recurs in the same very simple form. The elaborate alimentary canal of the higher animals develops ontogenetically from the same simple primitive gut of the gastrula. This gastraea theory is now accepted by nearly all zoologists.
We see this ontogenetically in every child. The biogenetic law compels us to affirm it phylogenetically. From all these facts sound reason must conclude that the still prevalent belief in the immortality of the soul is an untenable superstition. I have shown its inconsistency with modern science in the eleventh chapter of The Riddle of the Universe.
We understand only half of the psychic impulses, as indeed we do all spiritual development, if we look merely at the root. We have to regard not merely whence we come but also whither we go. Then only can the course of the psyche be comprehended, ontogenetically as well as phylogenetically, according to a dynamic scheme as it were.
We have previously seen how this very rudimentary beginning of the skull in man is formed ontogenetically from the "head-plates," and thus the fore end of the chorda is enclosed in the base of the skull. The phylogeny of the skull has made great progress during the last three decades through the joint attainments of comparative anatomy, ontogeny, and paleontology.
The apparatus for conducting sound which we find in the external and middle ear of mammals develops quite separately from the apparatus for the sensation of sound. It is both phylogenetically and ontogenetically an independent secondary formation, a later accession to the primary internal ear.
Whoever is convinced that each individual, be it fish or bird, springs from its own cell, knows ipso facto that a human cell, however undistinguishable it may be to the human eye from the cell of a catarrhine ape, could never have been the cell of an ape. And what is true ontogenetically, is of course true phylogenetically.
Speaking, however, not phylogenetically, but ontogenetically, at what point does our own cell come in contact with the cell that was intended to become an ape, and that became and remained an ape?
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