Another consideration which Darwin and his partisans have failed to take into account is the possibility of an _ontogenetic_, as well as a phylogenetic, explanation of rudimentary organs. That is to say, rudimentary organs might, so far as _a priori_ reasons are concerned, be the now useless vestiges of organs formerly developed and functional _in the fœtus_, and need not necessarily be interpreted as traces of organs that functioned formerly in remote racial ancestors. That there should be such things as special fœtal organs, which atrophy in later adult life, is a possibility that ought not to excite surprise. During its uterine existence, the fœtus is subject to peculiar conditions of life, very different from those which prevail in the case of adult organisms—_e.g._ respiration and the digestive process are suspended, and there is a totally different kind of circulation. What, then, more natural than that the fœtus should require special organs to adapt it to these special conditions of uterine life? Such organs, while useful and functional in the earlier stages of embryonic development, will, so soon as birth and maturity introduce new conditions of life, become superfluous, and therefore doomed, in the interest of organic economy, to ultimate atrophy and degeneration, until nothing is left of them but vestigial remnants. The thymus may be cited as a probable instance of such an organ. This organ, which is located in front of the heart and behind the breastbone, in the region between the two lungs, consists, at the period of its greatest development in man, of a two-lobed structure, 5 cm. long and 4 cm. wide, with a thickness of 6 mm. and a maximum weight of 35 grams. It is supplied with numerous lymphoid cells, which are aggregated to form lymphoid follicles (_cf._ Gray’s “Anatomy,” 20th ed., 1918, pp. 1273, 1274; Burton-Opitz’ “Physiology,” 1920, p. 964). This organ is a transitory one, well developed at birth, but degenerating, according to some authors, after the second year of life (_cf._ Starling’s “Physiology,” 3rd ed., 1920, p. 1245); according to others, however, not until the period of full maturity, namely, puberty. (_Cf._ Gray’s “Anatomy,” _loc. cit._) W. H. Howell cites both opinions, without venturing to decide the matter (_cf._ his “Physiology,” 8th ed., 1921, pp. 869, 870). It was at one time classified as a rudimentary or functionless organ. Later on, however, it was thought by certain observers to be an endocrine gland, yielding a secretion important for the growth of young mammals. This took it out of the class of useless vestigial organs, but the recent discovery that it is indispensable to birds as furnishing a secretion necessary for the formation of the tertiary envelopes (egg membrane and shell) of their eggs, has tended to revive the idea of its being a vestigial organ inherited from the lower vertebrates.