In fact, not only is embryological homology of no greater value than adult homology as an argument for evolution, but it is, on the contrary, considerably inferior to the latter, as regards cogency. _Differentiation_ pertains to the final or _adult_ stage of organisms. Embryonic structures, inasmuch as they are undeveloped and undifferentiated, present for that very reason an appearance of crude and superficial similarity. “Most of what is generally ascribed to the action of the so-called biogenetic law,” says T. Garbowski, “is erroneously ascribed to it, since all things that are undeveloped and incomplete must be more or less alike.” (“Morphogenetische Studien,” Jena, 1903.) When we consider the fact that the metazoa have all a similar unicellular origin, are subject to uniform morphogenetic laws, and are frequently exposed to analogous environmental conditions demanding similar adaptations, it is not at all surprising that they should present many points of resemblance (both in their embryonic and their adult morphology) which are not referable to any particular line of descent. At all events, these resemblances are far too general in their extension to enable us to specify the type of ancestor responsible therefor. More especially is this true of embryological homologies, which are practically valueless as basis for reconstructing the phylogeny of any type. “That certain phenomena,” says Oskar Hertwig, “recur with great regularity and uniformity in the development of different species of animals, is due chiefly to the fact that under all circumstances they supply the necessary condition under which alone the next higher stage in ontogeny (embryological development) can be produced.” (“Allgemeine Biologie,” 1906, p. 595.) The same author, therefore, proposes to revamp Haeckel’s “biogenetisches Grundgesetz” as follows: “We must leave out the words ‘recapitulation of forms of extinct ancestors’ and substitute for them ‘repetition of forms regularly occurring in organic development, and advancing from the simple to the more complex.’” (_Op. cit._, p. 593.)