iv PREFACE 



The first is illustrated by yeasts, bacteria, and protozoa; the 

 second by Hydra and the coelenterates ; the third by the earth- 

 worm. The food of animals and the sources of animal energy 

 are treated in connection with Hydra and illustrated by the 

 unicellular plants and the fern. Further differentiations of 

 organ systems are illustrated by the lobster (or crayfish), and 

 with these are introduced the principles of homology (through 

 study of appendages) and of morphological adaptations. This 

 work is followed by a short study of physiological adaptations as 

 illustrated by parasitism and by some of the phenomena of im- 

 munity. As the general theme works out the fundamental 

 principles of evolution are developed in the mind of the student 

 who is prepared for the discussion of the origin and perpetuation 

 of variations, and the modern principles of heredity discussed 

 in the last chapter. 



For permission to use many of the figures I am indebted to 

 Professors Sedgwick and Wilson; to the Macmillan Company 

 for cliches of figures 12, 24, 31, 88, 89; to Professor Morgan 

 and the Columbia University Press for cliches of figures 91, 93- 

 101; to Lea and Febiger for cliches of figures 22, 27; and to Miss 

 Mabel Hedge for the original drawings reproduced in figures 

 55, 57, 69, 86; finally I wish to express my grateful appreciation 

 to Professor J. H. McGregor for reading the manuscript and for 

 many helpful suggestions and criticisms. 



GARY N. CALKINS 



COLUMBIA UNIVERSITY 

 NEW YORK, Sept., 1914 



