[357] Writing to Huxley on the subject of the latter's work on the morphology of the Mollusca (1853), he says:--"The discovery of the type or 'idea' (in your sense, for I detest the word as used by Owen, Agassiz & Co.) of each great class, I cannot doubt, is one of the very highest ends of Natural History."--_More Letters_, ed. F. Darwin and A. C. Seward, 1903, i., p. 73. [358] Italics mine. [359] _Das Problem des Lebens. Biologische Studien_. Bad Sacha, 1906. See also E. Radl, _Biol. Centralblatt_, xxi., 1901. [360] See the excellent treatment of the difference between the "realism" of Darwin and the "rationalism" of his critics, in Radl, ii., particularly pp. 109, 135. The most elaborate criticism of Darwinism from the older standpoint was that given by A. Wigand in _Der Darwinismus und die Naturforschung Newtons und Cuviers_, 3 vols., Braunschweig, 1872. [361] In vol. ii. of his _Reden_, St Petersburg (Petrograd), 1876--_Ueber den Zweck in den Vorgaengen der Natur; Ueber Zielstrebigkeit in den organischen Koerpern insbesondere_; and _Ueber Darwin's Lehre_. [362] "Ueber die Darwinische Schoepfungstheorie," _Zeits. f. wiss. Zool._, xiv., pp. 74-86, 1864. Elaborated in _Anat. u. syst. Beschreibung d. Alcyonarien_, 1872. [363] _Cf._ for instance Naegeli's theory of a perfecting principle, first developed in his _Entstehung u. Begriff der naturhistorischer Art_, Muenchen, 1865. [364] _Anatomy of Vertebrates_, iii., 1868. [365] _Rapport sur les Progres recents des Sciences zoologiques en France_. Paris, 1867. CHAPTER XIV ERNST HAECKEL AND CARL GEGENBAUR At the time when Darwin's work appeared there already existed, as we have seen, a fully formed morphology with set and definite principles. The aim of this pre-evolutionary morphology had been to discover and work out in detail the unity of plan underlying the diversity of forms, to disentangle the constant in animal form and distinguish from it the accessory and adaptive. The main principle upon which this work was based was the principle of connections, so clearly stated by Geoffroy. The principle of connections served as a guide in the search for the archetype, and this search was prosecuted in two directions--first, by the comparison of adult structure; and second, by the comparative study of developing embryos. It was found that the archetype was shown most clearly by the early embryo, and this embryological archetype came to be preferred before the archetype of comparative anatomy. It became apparent also that the parts first formed (germ-layers) were of primary importance for the establishing of homologies.