vi PREFACE 



studies. Finally, the last chapter has been devoted to an intro- 

 ductory outline of the principal theories concerning Heredity, 

 Variation, and Evolution. This chapter does not pretend to com- 

 pleteness, for the treatment of this even in an elementary way would 

 require a large volume in itself. It is treated in a historical manner 

 and intended as a preliminary to wider reading, much of which is of 

 importance in the practice of medicine and should be included in any 

 curriculum of education. The many excellent books now available 

 on these topics almost all assume implicitly or explicitly such infor- 

 mation as this chapter contains. 



In arranging the book the plan has been followed of first treating 

 the frog fully as an introduction to morphology, physiology, and 

 histology, and then starting with the Protozoa and working up to the 

 higher forms. This has been done not because it is perhaps the 

 traditional method in ' England since Huxley's time, but because 

 actual experience of other ways has shown it to be the most satis- 

 factory both from the point of view of the teacher and the student. 

 The arrangement of the book, however, allows of almost any altera- 

 tion in the order in which the forms are studied. As the medical 

 student is going on to what is and must be a highly technical training, 

 no attempt has been made to avoid technical terminology, although 

 in all cases where such terms are used they have been defined and 

 also printed in thicker type to facilitate ready reference. No course 

 in Zoology can be considered as adequate unless the reading is 

 accompanied by a satisfactory practical course, and this has been 

 borne in mind in choosing the illustrations. Certain portions are 

 better illustrated by the practical work itself, while others need 

 fuller illustration in the text. 



I have to thank the following publishers for permission to use 

 the following illustrations : 



Messrs. Bell and Sons, for 13 figures from Bourne's "Comparative 

 Anatomy of Animals " ; Messrs. Constable & Co., for 4 figures from 

 Dendy's " Evolutionary Biology " ; Messrs. H. Frowde, for 6 figures 

 from Borradaile's " Manual of Zoology " ; Messrs. H. Holt, for 

 5 figures from Calkin's " Biology," for 3 figures from Kellicott's 

 " Textbook of General Embryology," for 6 figures from Kellicott's 

 " Outlines of Chordate Development," and for 6 figures from Lillie's 

 " Development of the Chick " ; Messrs. Longmans, Green & Co., for 



5 figures from Furneaux's " Elementary Physiology," for 6 figures 

 from Quain's " Anatomy," for 2 figures from Gray's " Anatomy," 

 and for i figure from Owen's " Anatomy " ; Messrs. Macmillan 



6 Co., for 20 figures from Lull's " Organic Evolution," and for 9 

 figures from Marshall and Gamble's " The Frog " ; The Zoological 

 Society of London, for 4 figures. 



