The great activity shown by Cuvier in building up the Jardiu 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, Savigny, Peron, Lesueur, Quoy, Gaimard. Le Valliant. Eydoux, and Souieyet. From 1823 to 1850 England fitted out exploring expeditions under Beeehey.Fitzroy. Belcher, Ross, Franklin, and Stanley, the naturalists of which were Bennett, Owen, Darwin. Adams, and Jinx ley. Russian explorations (1803-29) were ac companied by Tilesius, Langsdorff, Chamisso, Eschseholtz. and Brandt, all of them of German birth and edueation. The United States Explor ing Expedition under Wilkes (1838-42) 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 \Vied, Natterer, Perty, Reugger, Tschudi, Sehomburgk, Burmeister; from France, Dc Azara, D'Orbigny, Gay, Castelnan; 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, Miehluebo-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 fast half century.
(3) A third period has been distinguished (1) by the discovery by Sehleiden and by Sehwann (1838) that all organisms are formed of cells, and by the studies of Dujardin, of Mohl, and of Schultze on the nature of protoplasm (q.v.), roving that the cell is time unit of organization, and that protoplasm is the basis of life; (2) by the application of histological discoveries and methods to embryological researeh ; and (3) by the use of the deal rine 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 discovery of spermatozoa by ]f amen, a student of Leeuwenhoek, end that of the mammalian egg by 1)( Graaf in 1073. The old theory of preforma tion was overthrown and that of epigenesis, or forivot ion by the differentiation of the egg•proto pla-m. was established by Wolff in 1759. In 1829 mi Bat r showed that all mammals develop from while coste. Valentin. and Jones proved th-mt till e P?nr? mire 110M1)10gOIN with those of the lower v( rtebrates. The real nature of fertiliza tion was not finally settled until Barry in 1843 observed the union of ovum and spermatozoUn in rabbits, and KOBiker in 1846 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; Huxley identified two of these layers in the ecelenterates. Later steps were the recognition by the brothers Hertwig of the mesoblast and the etelomie cavity, the observations of Lang and Sedgwiek on meta merle 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 'liner'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 Geoffroy Saint-Hilaire before the French Academy of Sciences.
The school of transcendental anatomists founded by Olsen (1807) and Goethe (1739 1832), succeeded by Carus and Owen. resulted in the sp-ealled vertebrate theory of the skull; which, though crude and speculative, yet had its philosophical raison 'rare. Although the skull of vertebrates, as first shown by Huxley, is not truly a series of modified vertebra-, yet, as in worms and especially in arthropods. the head of vertebrates is segmented. as shown by the se rial homology of the embryonic gill-arehes and of the cranial nerves. Slo•eover, Goethe. as did Lamarck. recognized the significance of vestigial structures, and Goethe made the truly scientific prediction of the presence of the premaxillary bone in the head of man, the supposed absence of the homologue of that hone having been-before his time supposed to be a decisive feature separating man from the apes. Owen's Report on the Arche type and Homologies of the Vertebrate Skeleton (18-1(i) was the outcome of this preevolutionary method and point-of-view•. The old doctrine of archetypes and plans of creation reached its culmination in this hook. in which, wit hunt reference to the lowest vertebrates and to the early locale of development of animals of this type, Owen (q.v.) considered that the shoulder and pelvie arches were modified ribs. the shoulder :Ire]) belonging to the `occipital vertebra.' and the themselves being 'diverging or `nneinates.' Yet this theory was the fault or erroneous way Of thinking of the period in whieh he lived. and Owen was the greatest of English VOID Paralive anatomists. his works giving a great impetus to zoOlogy.