I do not propose to examine each of Haeckel's ancestral stages in Man's pedigree, or to estimate the degree of probability with which they may be deduced from the ontogeny; but that Man's ancestry does, in a general way, include such a series of phyletic stages may be admitted, even if we grant that many of these stages are now no longer represented in the ontogeny as stages of the developing organism as a whole, but only by stages of individual organs or group of organs. Thus it may be disputed whether there is still a fish-stage in human development, but it cannot be disputed that the rudiments of 'gill-arches' and 'gill-clefts,' which are peculiar to one stage of human ontogeny, give us every ground for concluding that we possessed fish-like ancestors. As we now know that the history of a given mode of embryogenesis has involved numerous time-displacements of the organ-rudiments, we must attach all the more weight to the developmental history of the individual parts and characters, in which the phylogeny can often be read more clearly than in the stages of the organism as a whole, and we can probably find out important laws in this way. As far back as 1873 Würtemberger investigated the fossil ammonites with special reference to this point. He was concerned even more at that time with finding proofs of the theory of descent in general, and this was the first case in which any one succeeded in demonstrating phyletic transformation-series of species, deposited one above the other in a corresponding series of geological strata, and connected by transition forms lying between these. In studying this interesting material, of which many examples were at his disposal, Würtemberger proved that the variations which had taken place in the spirally coiled shell in the course of ages appeared first on the last whorl, and then subsequently extended to the one before this, and thence to the still younger whorls of the shell. Meanwhile the last whorl not infrequently exhibited another new character. Thus, for instance, protuberances on the shell were shifted in the course of the phylogeny from the last convolution to the second last, and later to the third last, and so on, while at the same time the last convolution showed the protuberance changed into spines. In other words, the new phyletic acquirements first appeared in the mature animal (in the last-formed whorl or chamber of the shell), but were subsequently shifted back in the ontogeny to younger stages in proportion as new transformations of the mature animal appeared. Thus there was, so to speak, a retraction of the phyletic acquisitions of the mature animal deeper and deeper into the germinal history of the species. About the same time--in the seventies--I obtained similar results from living species when I was attempting to work out the ontogeny of the markings on the external skin of the caterpillars of certain butterflies, and I should like to submit a short account of these.