[15] Cf. Nelson’s Encyclopedia, v. 6, p. 452.
[16] Haeckel’s “Biogenetisches Grundgesetz,” which he formulates thus: “Die Ontogenie (Keimesgeschichte) ist eine kurze Wiederholung der Phylogenie (Stammesgeschichte),” 1874.
[17] The objection may be raised that a purely embryonic organ like the pronephros, which is functional in but few vertebrate adults and which originates in vertebrate embryos only to undergo atrophy, can have no other explanation than that of “recapitulation.” The objection, however, fails to take into account the possibility of the organ being serviceable to the embryo, in which it may be a provisory solution of the excretory problem and not a vestige of past ancestry.
[18] See Addenda.