4 Preface 



to employ in different passages diverse designations for the 

 same thing. What we observe in the Protozoa are combina- 

 tions of chemicals ; of matter and energy, with their char- 

 acteristic activities. Technical terms tend to set these apart 

 and render them unintelligible ; what we need is to render 

 them intelligible by showing their community with the world 

 of every-day experience. 



The book deals with heredity, variation, evolution, as 

 present physiology, not as past history. It discusses what 

 now occurs, not what may have occurred in the past. Hence 

 discussion of the origin of the conditions now existing will 

 hardly be found. 



A cknowledgments 



To the Editor and Publishers of Genetics I am indebted 

 for the use of the blocks for Figures 12 and 23, taken from 

 Genetics, volume 1. To my wife, Louise Burridge Jennings, 

 I am indebted for the drawing of the remainder of the 

 figures in the book. 



Johns Hopkins University, 

 Baltimore, Md. 



