[456] The importance of convergence came to be realised after the vogue of phylogenetic speculation had passed--see Friedmann, _Die Konvergenz der Organismen_, Berlin, 1904, and A. Willey, _Convergence in Evolution_, London, 1911. Also L. Vialleton, _Elements de morphologie des Vertebres_, Paris, 1912. [457] From this point of view there is a very profound analogy between artificial and natural selection. Upon the theory of natural selection organisms are lifeless constructs which are mechanically perfected by external agency, just as machines are improved by a process of conscious selection of the most successful among a number of competing models. (_Cf._ passage quoted below, on p. 308.) [458] _Arch. f. mikr. Anat._, xi. (suppl.), 1874; _Morph. Jahrb._, ii., 1876, v. 1879, and vii., 1882. [459] _Vergleich. Anat. d. Wirbelthiere_, i., pp. 200-1, 1898. [460] For a full historical account of work on membrane and cartilage bones (as well as on the theory of the skull) see E. Gaupp, "Altere und neuere Arbeiten ueber den Wirbelthierschaedel," _Ergeb. Anat. Entw._, x., 1901, and "Die Entwickelung des Kopfskelettes," in Hertwig's "_Handbuch vergl. exper. Entwickelungslehre d. Wirbelthiere_," iii., 2, pp. 573-874, 1905. [461] "Les Ancetres des Marsupiaux etaient-ils arboricoles?" _Trav. Stat. zool. Wimereux_, vii., pp. 188-203, pls. xi.-xii., 1899. See also Bensley, _Trans. Linn. Soc._ (2) ix., pp. 83-214, 1903. [462] _Proc. Zool. Soc._, pp. 649-62, 1880. _Sci. Mem._, iv., pp. 457-72. [463] J. F. Gemmill, _Phil. Trans. B_, ccv., p. 255, 1914. CHAPTER XVIII THE BEGINNINGS OF CAUSAL MORPHOLOGY Until well into the 'eighties animal morphology remained a purely descriptive science, content to state and summarise the relations between the coexistent and successive form-states of the same and of different animals. No serious attempt had been made to discover the causes which led to the production of form in the individual and in the race. It is true that evolution-theory had offered a simple solution of the great problem of the unity in diversity of animal forms, but this solution was formal merely, and went little beyond that abstract deduction of more complex from simpler forms, which had been the main operation of pre-evolutionary morphology. Little was known of the actual causes of ontogeny, and nothing at all of the causes of phylogeny; it was, for instance, mere rhetoric on Haeckel's part to proclaim that phylogeny was the mechanical cause of ontogeny.