It is undoubtedly of the greatest value to be able to penetrate more and more deeply into embryonic development, and to discover more precisely the changes that have taken place throughout its course in the originally existing material of ancestral forms. But it must not be forgotten that, all transformations notwithstanding, so much of the racial history is still very plainly indicated in the germinal history, that this must always remain for us a most important source from which to draw conclusions in regard to the phyletic development of any animal group. I admit that these conclusions have sometimes been drawn with too great confidence, but even if we cannot regard as well founded Haeckel's view that in the ontogeny of Man there are fourteen different ancestral stages recognizable, a protist stage, a gastræa stage, a prochordate, an acranial, a cyclostome, a fish-stage, and so on, we must recognize that the unicellular stage of ontogeny, with which even now the development of every human being begins, undoubtedly repeats the facies of an ancestor, although greatly altered; for we must be descended from unicellular organisms. The essential part of this ancestral stage is thus preserved in the ontogeny, and only what is special and in some measure due to chance, that is, to adaptation to special conditions of existence, has been modified. It has been supposed that the proposition that phylogeny is recapitulated in the ontogeny is disproved, because the ontogenetic stage must always contain within it the primordia of the later stages which have been added since the corresponding phylogenetic stage. It is certain that the egg-cell or the sperm-cell of Man contains, though in a form not recognizable by us, all the determinants of the perfect human body, but this neither affects its nature as a cell nor its particular form as ovum or spermatozoon. It is essentials that are important in this comparison, not accessories. Neither can I agree with Hensen's argument when he says that the 'recapitulation-idea' is erroneous, because the actual course of ontogeny is the 'best and only possible one,' which, apart from previous history altogether, must of necessity be followed. Certainly the actual course is the best, and under the given circumstances the only possible one, but that does not exclude recapitulation, on the contrary it implies it, for ontogeny could at no time have arisen from a _tabula rasa_, but only from what was historically existent.