At first we might be inclined to answer this question in a _polyphyletic_ sense, by saying that we must assume, for each of the seven great animal tribes, at least one independent primary form completely distinct from the others. On further considering this difficult problem, we arrive in the end at the notion of a _monophyletic_ origin of the animal kingdom, viz., that these seven primary forms are connected at their lowest roots, and that they are derived from a single, common primæval form. _In the animal as well as in the vegetable kingdom, when closely and accurately considered, the monophyletic hypothesis of descent is found to be more satisfactory than the polyphyletic hypothesis._ It is _comparative ontogeny_ (embryology) which first and foremost leads to the assumption of the monophyletic origin of the whole animal kingdom (the Protista excepted of course). The zoologist who has thoughtfully compared the history of the individual development of various animals, and has understood the importance of the biogenetic principle (p. 33), cannot but be convinced that a common root must be assumed for the seven different animal tribes, and that all animals, including man, are derived from a single, common primary form. The result of the consideration of the facts of embryology, or ontogeny, is the following genealogical or phylogenetic hypothesis, which I have put forward and explained in detail in my “Philosophy of Calcareous Sponges” (Monograph of the Calcareous Sponges, vol. i. pp. 464, 465, etc.,—“the Theory of the Layers of the Embryo, and the Pedigree of Animals”). The first stage of organic life in the Animal kingdom (as in the Vegetable and Protista kingdoms) was formed by perfectly simple _Monera_, originating by spontaneous generation. The former existence of this simplest animal form is, even at present, attested by the fact that the egg-cell of many animals loses its kernel directly after becoming fructified, and thus relapses to the lower stage of development of a cytod without a kernel, like a Moneron. This remarkable occurrence I have interpreted, according to the law of latent inheritance (vol. i. p. 205), as a phylogenetic _relapse_ of the cellular form into the original form of a cytod. The _Monerula_, as we may call this egg-cytod without a kernel, repeats then, according to the biogenetic principle (vol. ii. p. 33), the most ancient of all animal forms, the common primary form of the animal kingdom, namely, the Moneron.