[322] The inversion of the organs shown by Vertebrates as compared with Invertebrates is due to the reversed position of the embryo relatively to the yolk! (pp. 821-6). [323] It is worth while recording that Serres enunciated a "law of symmetry" according to which the embryo is formed by the union of its two symmetrical halves--a law which recalls the "concrescence theory" of His and some modern embryologists. [324] "Embryologie comparee du Brochet, de la Perche, et de l'Ecrevisse," _Ann. Sci. nat._ (4), i., p. 237, 1854; ii., p. 39, 1854. _Mem. Savans etrangers_, xvii. [325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p. 88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863. [326] xx., p. 5, 1863. [327] Particularly in his _Blennius_ (1833) and _Natter_ (1839). [328] In the "preliminary notice" of his Crayfish paper--_Isis_, pp 1093-1100, 1825. [329] "On the Anatomy and the Affinities of the Family of the Medusae," _Phil. Trans._, 1849; _Sci. Memoirs_, i., pp. 9-32. [330] _Phil. Trans._, cxliii., p. 368, 1853. [331] The principle of achromatism was discovered (by Fraunhofer) and achromatic microscopes introduced in the early part of the 19th century. The use of chemical reagents, such as acetic acid, and various hardening fluids, came into fashion not long after. J. Mueller seems to have been one of the first to realise their importance. Remak himself invented one or two fixing and hardening mixtures (pp. 87, 127, 1855), which enabled him to cut excellent hand sections. Section-cutting machines were not invented till later (V. Hensen, 1866, His, 1870). [332] _Untersuchungen ueber die Entwickelung der Wirbelthiere_, folio, pp. xxxvii + 195, 12 plates, Berlin, 1850-1855. CHAPTER XIII THE RELATION OF LAMARCK AND DARWIN TO MORPHOLOGY. It is a remarkable fact that morphology took but a very little part in the formation of evolution-theory. When one remembers what powerful arguments for evolution can be drawn from such facts as the unity of plan and composition and the law of parallelism, one is astonished to find that it was not the morphologists at all who founded the theory of evolution.