2. _Embryology._—Baer in 1828 was possessed of all the embryological facts which Darwin used in support of his theory of evolution; in particular, he was well acquainted with the most striking fact of all—viz., the presence in embryo mammals and birds of a series of paired clefts along the sides of the neck, between which run vessels arranged as in gill-breathing vertebrates. The vessels had been figured by Malpighi; the clefts had been discovered by Rathke, who had no hesitation in calling them _gill-clefts_ and the vessels _gill-arches_. Nor had Baer, who nevertheless to the end of his long life refused to accept the one explanation which gives meaning to the facts—viz., that remote progenitors of mammals and birds breathed by gills. Few embryologists have since felt such a scruple. The adaptation to gill-breathing is obvious; is gill-breathing _now_ practised by any mammal or bird? Certainly not. Is it destined to be practised by their descendants at some _future_ time? To say nothing of the danger of putting forth any such prophecy, it involves all the consequences of descent with modification. The opponent of evolution may as well admit at once that the gill-breathing was practised in time _past_. As an example of the same kind taken from plants, we may quote the trifoliate leaves of the furze-seedling, which, though absent from the full-grown furze, are frequent in the family (Leguminosæ) to which it belongs. The general similarity of vertebrate embryos, of insect-embryos and of dicotyledonous seedlings, is also worthy of note. We may suppose that early embryos, being largely or wholly dependent on food supplied by the parent, and perhaps protected by the parent as well, escape the pressure of the struggle for existence, and are often not urgently impelled to produce adaptations of their own. In these circumstances it is intelligible that features inherited from remote ancestors should persist. If, however, early independence is demanded by the conditions of life, the embryo may develop temporary adaptations, wanting in the parent and in embryos of allied groups. Larval adaptation is as much a part of the economy of nature as the retention of ancestral structures which have been lost by the adult.