Huxley begins these six lectures by noting that "there is a close relation between the organic and the inorganic world---the difference between them arising from the diverse combination and disposition of identical forces, and not from any primary diversity, as far as we can see", and that "not only is the living matter derived from the inorganic world, but that the forces of that matter are all of them correlative with and convertible into those of inorganic nature". He then develops the evidence (now thought much less of) from embryology, stating, "there is a time when the embryos of neither dog, nor horse, nor porpoise, nor monkey, nor man, can be distinguished by any essential feature one from the other; there is a time when they each and all of them resemble this one of the Dog".
