vi PREFACE 



it would be to deliver a course on the general characteristics of 

 English Literature, suitably illustrated by " elegant extracts," to a 

 class of students who had never read a single English poet or 



essayist. 



There can be no question as to the vast improvement effected in 

 zoological teaching by the practice of preceding the study of a 

 given group as a whole by the accurate examination of a suitable 

 member of it. With the clear mental image of a particular animal, 

 in the totality of its organisation, the comparison of the parts and 

 organs of other animals of like build becomes a profitable study, 

 and the danger of the comparative method that the student may 

 learn a great deal of the systems of organs in a group without 

 getting a clear conception of a single animal belonging to it is 

 much diminished. 



The method of " types " has, however, its own dangers. Students 

 are, in their way, great generalisers, and, unless carefully looked 

 after, are quite sure to take the type for the class, and to consider 

 all Arthropods but crayfishes and cockroaches, and all Molluscs but 

 mussels and snails, as non-typical. For this reason a course of 

 Zoology which confines itself entirely or largely to " types," or, as 

 we prefer to call them, 1 examples, is certain to be a singularly 

 narrow and barren affair, and to leave the student with the 

 vaguest and most erroneous ideas of the animal kingdom as a 

 whole. This is especially the case when the number of examples 

 is small, each of the Phyla being represented by only one or two 

 forms. 



In our opinion every gr.oup which cannot readily and intelli- 

 gibly be described in terms of some other group should be 

 represented, in an elementary course of Zoology, by an example. 

 We have, therefore, in the majority of cases, described, in some 

 detail, an example of every important class, and, in cases where 

 the diversity of organisation is very great as in Crustacea and 

 Fishes two or more examples are taken. The student is thus 

 furnished with a brief account of at least one member usually 

 readily accessible of all the principal groups of animals. 



By the time the example has been studied, a definition of the 

 class and of its orders will convey some idea to the mind, and will 



1 Following a suggestion for which we are indebted to Dr. Alexander Hill, 

 Master of Downing College, Cambridge. 



