TO THE TEACHER vii 



when more closely examined it is found to consist not only of 

 engine and of separate cars, but also of wheel and axle, brake 

 and drawbar. The whole is actuated by the energy of the coal and 

 controlled by intelligence, acting through steam and compressed 

 air, by means of lever brake and bell cord. 



Chapter VIII introduces a brief study of the variability of a 

 single character, and it serves not only to fix conceptions as to 

 type, but as an introduction to statistical methods of study now 

 much employed in the problems of breeding. This chapter will 

 afford material for an exceedingly valuable class of problems, 

 and its mastery is especially urged. 



In Chapter IX the attempt is made to convey the essential 

 facts of reproduction and lay the foundation for the study of 

 heredity through the medium of the plant. The hope is that 

 here and in Chapters X and XI more is taught by inference 

 than is taught directly. It has been a secondary aim of the 

 author to convey knowledge and make impressions that are 

 applicable to certain human relations as well as to the subject 

 in hand, but which from the nature of the case cannot be 

 conveyed by the direct method. 



Chapters XI and XII are designed to teach rational notions 

 of descent and to correct the prevalent notion that heredity in 

 some way fails unless the offspring is a duplicate of the parent. 

 The old dogma that like begets like, and that the offspring is 

 like the parent, is modified to read, " The offspring is like the 

 parentage," and the succeeding chapter deals with the distri- 

 bution of hereditary family qualities through the various mem- 

 bers of the back ancestry. It is hoped that the careful study of 

 these chapters will prepare the student for the real behavior of 

 characters in transmission, and will enable him to comprehend 

 both regression and progression, as well as reversion and de- 

 generacy. It will also serve to show that transmission and 

 heredity are complicated, not simple, facts. 



Chapter XIV discusses the relative influences of heredity and 

 environment, a discussion that is useful from the standpoint 



