PREFACE 



This volume is intended for use as a text-book in college courses in 

 vertebrate zoology such as are required of premedical students and 

 others who have had a course in general zoology. The aim of the book 

 is to present those aspects of the subject which are not adequately 

 brought out by laboratory work in comparative anatomy. It is taken 

 for granted that the student who uses this as a text in connection 

 with the lecture and recitational part of a course, shall also pursue a 

 laboratory course in comparative anatomy, using a laboratory man- 

 ual. It is also believed advisable to use as laboratory references the 

 various text-books on comparative anatomy. 



The book is avowedly dynamic in tone, emphasizing the physio- 

 logical, developmental, phylogenetic, and ecological aspects of ver- 

 tebrates. Structural features must of course be dealt with extensively, 

 but purely anatomical details are as a rule subordinated to physio- 

 logic and evolutionary considerations. 



The vertebrates are, moreover, viewed not merely as a group of ani- 

 mals belonging to the present, but, historically, as a very ancient as- 

 semblage of related forms, that arose from simple beginnings many 

 millions of years ago and have passed through many vicissitudes in- 

 volved in the mighty world changes of ancient times. Hence more 

 than the usual attention is given to earlier chapters in the ancestral 

 history of the vertebrate classes, chapters that are often of more dra- 

 matic interest than those of the present and that give to the student 

 a new conception of the significance of modern end-products of evo- 

 lution which, in themselves, are often relatively unattractive and de- 

 void of interest. 



The writer has for some years been much impressed with the far- 

 reaching applicability to problems of animal morphology, of the ax- 

 ial gradient conception of his colleague, Professor Child, and one of 

 the features of the present book is the attempt to interpret verte- 

 brate structures in terms of this conception. In some cases it is 

 probable that the theory has been stretched beyond the limits its 

 author would consider justified; hence the present writer takes en- 



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