ZOOLOGY. 792 ZOOLOGY. of descent. Vicq d'Azyr established the principle of serial homology. Bichat was one of the found- ers of histologj'. France at this time led the scientific world, though Germany had lier Blu- menbach, who founded anthropology, her Dol- lingcr, the teacher of later embryologists, Tiede- niann, Bojanus, and Cams. Meckel, in his time the leading German anatomist, studied at Paris with Cuvier, as did Milne-Edwards, while Owen felt liis influence. The great activity shown by Cuvier in building up the Jardin des Plantes led to the French ex- ploring expeditions sent out from 1800 to 1832 to all parts of the globe, resulting in enlarged views regarding the number and distribution of species, and their relations to their environment. The zoologists of these expeditions were Bory de Saint-Vincent, Savign}', Peron, Lesueur, Quoy, Gaimard, Le Vaillant, Eydoux, and Souleyet. From 1823 to 1850 England fitted out exploring expeditions vmder Beechey,Fitzroy, Belcher, Ross, Franklin, and Stanley, the naturalists of which were Bennett, Owen, Darwin, Adams, and Hux- ley. Russian explorations (1803-29) were ac- companied by Tilcsius, Langsdorff, Chamisso, Eschscholtz. and Brandt, all of them of German birth and education. The United States Explor- ing Expedition under Wilkes (1S3S-42) was, in scientific results, not inferior to any previous ones, the zoologists being Dana, Couthuoy, and Peale. Of a later voyage under Ringgold, Stimp- son was the naturalist, but the rich final re- sults were lost by fire. At or near the close of this period, from Germany, Humboldt, Spix, the Prince of Wied, Natterer, Perty, Reugger, Tschudi, Schomburgk, Burmeister ; f»om France, De Azara, D'Orbigny, Gay, Castelnau ; and from Denmark, Lund, traveled at their own expense, an evidence of the spirit of scientific re- search then dominating the centres of civiliza- tion. Their followers were Wallace, Bates, Sem- per. Belt, Michlucbo-Macleay, Przhevalski. the brothers Sarasin, and many others. The voyage (1872-76). of the Challenger (q.v.) was momen- tous in its results, which are published in 30 quarto volumes, mostly containing reports on the zoological results contributed by the leading systematic zoologists of the last halt century. (3) A third period has been distinguished (1) by the discovery by Schleidcn and b^' Schwann ( 1838) that all organisms are formed of cells, and by the studies of Dujardin, of Mohl, and of Scliultze on the nature of protoplasm (q.v.), proving that the cell is the unit of organization, and that protoplasm is the basis of life; (2) by the application of histological discoveries and methods to enibryological research ; and ( 3 ) by the use of the doctrine of evolution ,as a working theory to account for the common origin of ani- mals from a single simple primitive organism. The first great steps in the explanation of the mode of reproduction and development were the disoovcry of spermatozoa by Ilamen. a student of Leeuwenhook. and that of the mammalian egg by De Graaf in 1073. The old theory of preforma- tion was overthrown and that of epigenesis. or formation by the dilTerentialion of the egg-proto- plasm, was established by Wolff in 17.')0. In 182!) Von Baer showed that all mammals develop from eggs, while Coste, Valentin, and Jones proved that these eggs are homologous with those of the lower vertebrates. The real nature of fertiliza- tion was not finally settled until Barry in 1843 observed the union of ovum and spermatozoon in rabbits, and Kiilliker in 1840 proved that sper- matozoa develop from the cells of the testes. The next important advance was the discovery in ver- tebrates by Von Baer of the germ layers; Hu.vley identified two of these laj-ers in the etelenterates. Later steps were the recognition by the brothers Hertwig of the mesoblast and the cadomic cavity, the observations of Lang and Sedgwick on meta- meric segmentation, and the homology of the blas- topore in the embryos of all many-celled animals. All these discoveries gave an impetus to mor- phology (q.v.) and established it on a thor- ough and broad basis of facts. Von Baer also showed that the tissues arise from em- bryonic cells, their formation going on simul- taneously with the process of differentiation and development of organs. He thus discovered what is known as 'Baer's law.,' i.e. that the develop- ment of the individual is an epitome of that of the class to which it belongs ; and that while the embryos of the animals he studied are at first very similar, they diverge more and more as growth and differentiation advance. These laws did not suggest to him the theory of the blood-relationship of the vertebrate and other classes, but led him, as it afterwards did Agassiz, to support Cuvier's view that the animal king- dom was divided into four distinct, unrelated branches or subkingdoms, and that consequently there was no unity of type, as Cuvier asserted in his famous debate with Geoffrey Saint-Hilaire before the French iVcademy of Sciences. The school of transcendental anatomists founded by Oken (1807) and Goethe (1739- 1832), succeeded by Cams and Owen, resulted in the sp-called vertebrate theory of the skull ; which, though crude and speculative, yet had its philosophical raison d'etre. Although the skvUl of vertebrates, as first shown by Huxley, is not truly a series of modified vertebrse. yet, as in worms and especially in arthropods, the head of vertebrates is segmented, as shown by the se- rial homologv' of the embryonic gill-arches and of the cranial nerves. Moreover, Goethe, as did Lamarck, recognized the significance of vestigial structures, and Goethe made the truly scientific prediction of the presence of the preniaxiMary bone in the head of man, the supposed absence of the homologue of that bone having beenbefore his time supposed to be a decisive featiire separating man from the apes. Owen's Report on the Arche- tape and Homologies of the Vertchratc Skeleton (1840) was the outcome of this prcevolutionary method and point-of-view. The old doctrine of archetypes and plans of creation reached its culmination in this book, in which, without reference to the lowest vertebrates and to the early mode of develojiment of animals of this type, Owen (q.v.) considered that the shoulder and pelvic arches were mo(lifi<'d ribs, the shoulder areli belonging to the 'occipital vertebra,' and the limbs themselves being 'diverging appendages' or 'uncinates.' Yet this theory was the fault or erroneous way of thinking of the period in which he lived, and Owen was the greatest of English comparative anatomists, his works giving a great impetus to zoiilogy. The first epochal sic|) in unraveling the mor- phology of tile arllinipod classes was taken by
