808 ZOOLOGY capable of being shown by adequate study of a series of intermediate forms to be derived from one and the same part or organ of the "plan-form" or "archetype." It is not easy to exaggerate the service rendered by Owen to the study of zoology by the introduction of this appar ently small piece of verbal mechanism ; it takes place with the classificatory terms of Linnaeus. And, though the conceptions of "archetypal morphology," to which it had reference, are now abandoned in favour of a genetic mor phology, yet we should remember, in estimating the value of this and of other speculations which have given place to new views in the history of science, the words of the great reformer himself. " Erroneous observations are in the highest degree injurious to the progress of science, since they often persist for a long time. But erroneous theories, when they are supported by facts, do little harm, since every one takes a healthy pleasure in proving their falsity " (Darwin). Owen s definition of analogous struc tures holds good at the present day. His homologous structures are now spoken of as "homogenetic " structures, the idea of community of representation in an archetype giving place to community of derivation from a single representative structure present in a common ancestor. Darwinian morphology has further rendered necessary the introduction of the terms "homoplasy" and "homoplastic" 1 to express that close agreement in form which may be at tained in the course of evolutional changes by organs or parts in two animals which have been subjected to similar moulding conditions of the environment, but have no genetic community of origin, to account for their close similarity in form and structure. Owen s The classification adopted by Owen in his lectures > slfi - (1 855) does not adequately illustrate the progress of zoologi cal knowledge between Cuvier s death and that date, but, such as it is, it is worth citing here. Province : Vertebrata (Myclencephala, Owen). Classes : MAMMALIA, AVES, REPTILIA, PISCES. Province ; Articulata. Classes : ARACHXIDA, INSECTA (including Sub-Classes Myria- poda, Hexapoda], CRUSTACEA (including Sub-Classes Ento- mostraca, Malacostraca), EPIZOA (Epizootic Crustacea), AN- NELLATA (Chsetopods and Leeches), CIURIPEDIA. Province : Mollusca. Classes: CEPHALOPODA, GASTEROPODA, PTEROPODA, LAMELLI- BRANCHIATA, BRACHIOPODA, TlJNICATA. Province : Radiata. Sub-Province : Radiaria. Classes: ECHINODERMATA, BRYOZOA, ANTHOZOA, ACA- LEPH^;, HYDROZOA. Sub-Province : Entozoa. Classes : CCELELMINTHA, STERELMIXTHA. Sub-Proi ince : Infusoria. Classes : ROTIFERA, POLYGASTEIA (the Protozoa of recent authors). The real centre of progress of systematic zoology was no longer in France nor with the disciples of Cuvier in England, but after his death moved to Germany. The wave of morphological speculation, with its outcome of new systems and new theories of classification, which were as numerous as the professors of zoological science, 2 was necessarily succeeded in the true progress of the science by a period of minuter study in which the microscope, the discovery of embryological histories, and the all-important cell-theory came to swell the stream of exact knowledge. Johann We have already mentioned Von Baer in this connexion, Her. and given a passing reference to Johann MULLER (q.v.), the greatest of all investigators of animal structure in the pre sent century. Miiller (1801-1858) was in Germany the suc cessor of Rathke (1793-1860) and of Meckel (1781-1833) as the leader of anatomical investigation ; but his true 1 See Lanke.ster, " On the Use of the Terra Homology in Modern greatness can only be estimated by a consideration of the fact that he was a great teacher not only of human and comparative anatomy and zoology but also of physiology, and that nearly all the most distinguished German zoolo gists and physiologists of the period 1850 to 1870 were his pupils and acknowledged his leadership. The most striking feature about Johann Miiller s work, apart from the com prehensiveness of his point of view, in which he added to the anatomical and morphological ideas of Cuvier a consideration of physiology, embryology, and microscopic structure, was the extraordinary accuracy, facility, and completeness of his recorded observations. He could do more with a single specimen of a rare animal (e.g., in his memoir on Amph^ox^ls, Berlin, 1844) in the way of making out its complete structure than the ablest of his contemporaries or successors could do with a plethora. His power of rapid and exhaustive observation and of accurate pictorial reproduction was phenomenal. His most important memoirs, besides that just mentioned, are those on the anatomy and classification of Fishes, on the Coecilians, and on the developmental history of the Echinoderms. A name which is apt to be forgotten in the period J. V. between Cuvier and Darwin, because its possessor occupied Thomp. an isolated position in England and was not borne up by sou> any great school or university, is that of John Vaughan Thompson, who was an army surgeon, and when past the age of forty, being district medical inspector at Cork (1830), took to the study of marine Invertebrata by the aid of the microscope. Thompson made three great discoveries, which seem to have fallen in his way in the most natural and simple manner, but must be regarded really as the oiitcome of extraordinary genius. He showed that the organisms like Flustra are not hydroid Polyps, but of a more complex structure resembling Molluscs, and he gave them the name Polyzoa. He discovered the Pentacrinus europceus, and showed that it was the larval form of the Feather-Star Antedon (Comatidci). He upset Cuvier s retention of the Cirripedes among Mollusca, and his subsequent treatment of them as an isolated class, by showing that they begin life as free-swimming Crustacea identical with the young forms of other Crustacea. Vaughan Thompson is a type of the marine zoologists, such as Daly ell, Michael Sars, P. J. Van Beneden, Claparede, and Allman, who during the present century have approached the study of the lower marine organisms in the same spirit as that in which Trembley and Schaffer in the last century, and Swammer- dam in the 17th, gave themselves to the study of the minute freshwater forms of animal life. It is impossible to enumerate or to give due considera tion to all the names in the army of anatomical and embryo- logical students of the middle third of this century whose labours bore fruit in the modification of zoological theories and in the building up of a true classification of animals. Their results are best summed up in the three schemes of classification which follow below those of Rudolph Leuckart (b. 1823), Henri Milne-Edwards (1800-1884), and T. H. Huxley (b. 1825), all of whom individually contributed very greatly by their special discoveries and researches to the increase of exact knowledge. Contemporaneous with these were various schemes of Single- classification which were based, not on a consideration of fact the entire structure of each animal, but on the variations S 3 st ^ ms . of a single organ, or on the really non-significant fact of uca tion. the structure of the egg. All such single-fact systems have proved to be useless and in fact departures from the true line of growth of the zoological system which was shaping itself year by year unknown to those who so shaped it as a genealogical tree. They were attempts to arrive at a true knowledge of the relationships of animals by " royal
roads " : their followers were landed in barren wastes.