Louis Agassiz, in the Lowell Lectures of 1848, proposed for the first time the theory that the embryo of higher forms resembled not so much lower adult animals living at the present time, as those that lived in past times. Since Agassiz himself did not accept the theory of evolution, the interpretation that he gave to the recapitulation theory did not have the importance that it was destined to have when the animals that lived in the past came to be looked upon as the ancestors of existing animals.[8] But with the acceptation of the theory of evolution, which was largely the outcome of the publication of Darwin’s “Origin of Species” in 1859, this new interpretation immediately blossomed forth. In fact, it became almost a part of the new theory to believe that the embryo of higher forms recapitulated the series of ancestral adult forms through which the species had passed. The one addition of any importance to the theory that was added by the Darwinian school was that the history of the past, as exemplified by the embryonic development, is often falsified. Footnote 8: Carl Vogt in 1842 suggested that fossil species, in their historical succession, pass through changes similar to those which the embryos of living forms undergo. Let us return once more to the facts and see which of them are regarded at present as demanding an explanation. These facts are not very numerous and yet sufficiently apparent to attract attention at once when known. The most interesting case, and the one that has most often attracted attention, is the occurrence of gill-clefts in the embryos of reptiles, birds, and mammals. These appear on each side of the neck in the very early embryo. Each is formed by a vertical pouch, that grows out from the wall of the pharynx until it meets the skin, and, fusing with the latter, the walls of the pouch separate, and a cleft is formed. This vertical cleft, placing the cavity of the pharynx in communication with the outside, is the gill-slit. Similar openings in adult fishes put the pharynx in communication with the exterior, so that water taken through the mouth passes out at the sides of the neck between the gill filaments that border the gill-slits. In this way the blood is aerated. The number of gill-slits that are found in the embryos of different groups of higher vertebrates, and the number that open to the exterior are variable; but the number of gill-openings that are present in the adults of lower vertebrates is also variable. No one who has studied the method of development of the gill-slits in the lower and higher vertebrates will doubt for a moment that some kind of relation must subsist between these structures.