the French, English, Russian and American governments to different parts of the world, especially to the tropical oceans. Of these voyages perhaps the most interesting were that of the Russian ship Rurik from 1818 to 1820, in which Chamisso and Eschscholtz went as naturalists and discovered the alternation of generation of Salpa, that of the English ship Beagle between 1831 and 1835 with Darwin as naturalist, and the American expedition under Captain Wilkes between 1838 and 1842 with James Dwight Dana as the principal naturalist.
The influence of all these investigations, and also of the newly established cellular theory of the structure of plants and animals, on the development of the zoological system led to the third great reform of the latter. In 1845 von Siebold subdivided Cuvier's fourth type, the Zoophyta or Radiata, into three types or phyla, the Protozoa, Zoophyta and Vermes, confining thus the term Zoophyta to the truly radiate animals. He also broke up Cuvier's second type Articulata, removing the Annelida to the new phylum Vermes and creating another new phylum for the Crustacea, Arachnida, Myriapoda and Insecta which he called the Arthropoda. Two years later E. Leuckart broke up the Zoophyta, subdividing it into the phyla Echinodermata and Coelenterata, and emphasized the isolated position of the Protozoa, and a little later Milne-Edwards added still another new type or phylum, the Molluscoidea, in which he included the Bryozoa, Brachiopoda and Tunicata. The animal kingdom was thus in 1850 subdivided into eight phyla, the Protozoa, Echinodermata, Vermes, Arthropoda, Molluscoidea, Mollusca and Vertebrata, an arrangement which is still found in many text books.
Darwin's "Origin of Species" was published in 1859 and the fourth and last important reform of the zoological system of classification was the direct consequence of the doctrines therein promulgated. The theory of the common descent and blood relationship of all animals which Darwin taught was at variance with Cuvier's theory of fixed types and in harmony with Lamarck's theory of the essential unity of the animal kingdom, and was first employed by Haeckel as the basis of a system of classification. In 1877 he called attention to the need of placing the entire system on an evolutionary basis and at the same time subdivided the animal kingdom into the two great groups of the Protozoa and the Metazoa, and the latter into the two great groups of the Coelenterata and the Cœlomata. In still more recent times other authors, notably Hatschek, following Haeckel's lead, have carried the subdivision still further on the same basis. The old idea of types, however, has a very tenacious life and is still the basis of the classification of animals in most text-books—and probably rightly so. For most animals, notwithstanding their ultimate relationships with one another, can as a matter of fact be grouped in a number of distinct types or