3 02 THE POPULAR SCIENCE MONTHLY 



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 subdi- 

 vision 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 

 Balavoglossvs 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 Verte- 

 brata 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 TJemi- 

 chorda. 



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 sub- 

 ordinate 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 zoolog- 

 ical system has been inaugurated by one or two eminent men of great 

 constructive powers who have been able to see deeper into the signifi- 

 cance of facts than their predecessors and contemporaries and to inter- 

 pret 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. 



