HISTORY OF ZOOLOGY. 19 



Eadiata into two branches differing greatly in structure. Tlie 

 lower forms, in which no special body-cavity is present, the 

 interior of the body consisting only of a system of cavities serving 

 for digestion, the alimentary canal, he called the Ccelentera 

 (essentially the Zoophj-ta of the older zoologists); to the rest, in 

 which the alimentary canal and the body-cavity occur as two 

 separate cavities, he gave the name Echinodcrma. 



The Present System. — Thus there resulted seven classes: 

 Protozoa, Ccelentera, Eehinoderma, Vermes, Arthropoda, Mol- 

 lusca, and Vertebrata. Still this arrangement does not meet the 

 requirements of a natural system and hence is more or less unsat- 

 isfactory. Some zoologists are returning to the Cuvierian classifi- 

 cation to the extent of uniting the segmented worms with the 

 arthropods in a group Articulata. Upon the ground of important 

 anatomical and embryological characters the Brachiopoda, the 

 Bryozoa, and the Tunicata have been separated from the Mollusca; 

 they form the subject of diverse opinions. The relationships of 

 the iirst two groups have not yet been settled : of the Tunicata we 

 know indeed that they are related to the Vertebrata, but the 

 differences are such that they cannot be included in that group. 

 The only way out of the difficulty is to unite vertebrates, tunicates, 

 and some other forms in a larger division, Chordata. The Vermes, 

 too, must be divided, as will appear in the second part of this 

 volume. 



HISTORY OF THE THEORY OF EVOLUTION. 



Importance of the Subject. — Before closing the historical intro- 

 duction we must consider the historical development of a question 

 whose importance might, on a superficial examination, be under- 

 rated, but which from a small beginning has grown into a problem 

 completely dominating zoological research, and has occupied not 

 only zoologists, but all interested in science generally. This is the 

 question of the logical value of the systematic conceptions species, 

 genus, family, etc. 



The Nature of Species. — In nature we find only separate 

 animals: how comes it that we classify them into larger and smaller 

 groups ? Are the single species, genera, and the other divisions 

 wdiich the systematist distinguishes, fixed quantities, as it were 

 fundamental conceptions of nature, or a Creator's thoughts, which 

 fiiul expression in the single forms ? Or are they abstractions 

 which man has brought into nature for the purjiose of making it 



