phyla, each of which has a characteristic plan of structure. Cuvier's belief, however, that these types are fixed and isolated creations, has long since been abandoned.
Very important has been the formation in recent times of the phylum Chordonia or Chordata, which brings under the same subdivision all the animals possessing the essential characteristics of the vertebrate type. The formation of this phylum has been due to the fundamental researches of Kowalevsky, who in 1866, 1867 and 1871 gave the first detailed and accurate description of the anatomy of Balanoglossus and also the first detailed accounts of the embryology of ascidians and of Amphioxus, showing that these animals are related to one another and to vertebrates. The term Chordonia was introduced in 1874 by Haeckel to include the Tunicata, Amphioxus and the Vertebrata and the terms Urochorda and Cephalochorda by Lankester in 1878 for the Tunicata and Amphioxus. In 1884 Bateson, on the basis of his researches on the American form Balanoglossus aurantiacus, added the Enteropneusta to the Chordata and proposed the term Hemichorda.
The system of zoological classification was thus fixed some twenty or thirty years ago and has undergone no important changes in its larger features since. This is not true, however, of many of the subordinate and smaller of its groups, the arrangement of which changes from time to time as the knowledge of the relationships of the animals composing them increases. We find this to be especially true of certain low animals which seem to be isolated side branches of the ancestral tree, the origin of which from the main stem is still obscure.
Each of these four distinct periods of reform of the modern zoological system has been inaugurated by one or two eminent men of great constructive powers who have been able to see deeper into the significance of facts than their predecessors and contemporaries and to interpret rightly those which they have gathered. The first reform was started by Lamarck and the second by Cuvier, the third by von Siebold and Leuckart and the fourth by Darwin and Haeckel.