Not only are embryos more alike than adults, because less differentiated, but it is in points not directly connected with the conditions of existence, not strictly adaptive, that their resemblance is strongest (p. 440)--think, for instance, of the arrangement of aortic arches common to all vertebrate embryos. Larval forms are to some extent exceptions to this rule, for they are often specially adapted to their particular mode of life, and convergence of structure may accordingly result. All these facts require an explanation. "How, then, can we explain these several facts in embryology--namely, the very general, but not universal, difference in structure between the embryo and the adult--of parts in the same individual embryo, which ultimately become very unlike and serve for different purposes, being at this early period of growth alike--of embryos of different species within the same class, generally but not universally, resembling each other--of the structure of the embryo not being closely related to its conditions of existence, except when the embryo becomes at any period of life active and has to provide for itself--of the embryo apparently having sometimes a higher organisation than the mature animal, into which it is developed" (pp. 442-3). Obviously all these facts are formally explained by the doctrine of descent. But Darwin goes further, he tries to show exactly how it is that the embryos resemble one another more than the adults. He thinks that the phenomenon results from two principles--first, that modifications usually supervene late in the life of the individual; and second, that such modifications tend to be inherited by the offspring at a corresponding, not early, age (p. 444). Thus, applying these principles to a hypothetical case of the origin of new species of birds from a common stock, he writes:--"... from the many slight successive steps of variation having supervened at a rather late age and having been inherited at a corresponding age, the young of the new species of our supposed genus will manifestly tend to resemble each other much more closely than do the adults, just as we have seen in the case of pigeons"[355] (pp. 446-7). Since the embryo shows the generalised type, the structure of the embryo is useful for classificatory purposes. "For the embryo is the animal in its less modified state; and in so far it reveals the structure of its progenitor" (p. 449)--the embryological archetype reveals the ancestral form. "Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less complete, of the parent form of each great class of animals" (p. 450)--a prophetic remark, in view of the enormous subsequent development of phylogenetic speculation. We may sum up by saying that Darwin interpreted von Baer's law phylogenetically.