iv GENERAL ZOOLOGY 



is sometimes called the ''order of evolution/' beginning 

 with the Protozoa. Although the chapters are interdepend- 

 ent, and biological principles are developed in large measure 

 in the consideration of groups where they find most ready 

 application, yet there is sufficient unity in the individual 

 chapters to make it possible for the teacher to diverge from 

 the sequence which we have employed. 



Whatever the order followed, it is evident that recitations 

 on the chapters in the textbook should be held only after the 

 student has made his study in the laboratory. The textbook 

 in science has its greatest usefulness in connecting, extend- 

 ing, and illuminating the work of the laboratory. Labora- 

 tory work and field observations bring the student in touch 

 with actual things, and if the studies are properly conducted 

 they will aid in developing the power of independent judg- 

 ment. The untrained student cannot build up a conception 

 of the science of zoology from the more or less isolated 

 data of the laboratory and field ; in this fact lies the justi- 

 fication of a textbook in zoology. 



The inductive method of presentation, in necessarily mod- 

 ified form, has been followed in the earlier chapters of the 

 book, as being the natural mode of approach to a new sub- 

 ject based upon laboratory work. After the study of the 

 grasshopper, for example, another animal which has easily 

 recognizable relationship to this form is considered. Not un- 

 til the selected representatives of the Orthoptera have been 

 described are the characters of the order mentioned. By that 

 time the student's mind is ready for the definition of Orthop- 

 tera. The conceptions of the larger groups of invertebrate 

 phyla and classes are developed in the same manner. 



Against the possibility that direct, continuous, page se- 

 quence may be followed in a course covering only a part of 

 the text, some of the fundamental biological material has 

 been introduced relatively early. This explains the position 



