PRINCIPLES OF ANIMAL BIOLOGY 61 



STATE MONOGRAPHS: 



BARROWS, Michigan Bird Life. 



FORBES and RICHARDSON, The Fishes of Illinois. 



ORTMANN, The Crawfishes of Pennsylvania. 



RUTHVEN, THOMPSON and THOMPSON, The Herpetology of 



Michigan. 



WALKER, An Illustrated Catalogue of the Mollusca of Michigan : 

 Part 1. Terrestrial Pulmonata. 



To THE STUDENT. Generic characters, that is characters which permit 

 of the breaking up of the families into groups of forms, are generally 

 structural and less variable than the so-called specific characters by means 

 of which the genera are in turn divided into species. The specific charac- 

 ters may be and generally are superficial, such as form and color of the 

 body or its parts, but the characters differ greatly in different groups and 

 may in fact be any difference which is sufficiently constant. As an aid in 

 identification "keys," that is simplified tabulations of characters, are 

 compiled by systematists. These keys do not necessarily show the actual 

 relationships of the forms in the groups which they analyze, but they 

 illustrate the characters used and the methods employed in analytical 

 systematic zoology. 



F. SUMMARY 



State the principles at the basis of classification. What differences 

 in degrees of relationship are expressed by groups of different rank, as 

 orders, families, etc.? How do the characteristics of the groups show 

 these differences? What is the relative age of groups of different ranks, 

 as orders, families, etc.? Give the reasons for the last answer. 



The summary need not directly answer these questions, but the 

 answers should be included in the course of the discussion. 



