TO THE TEACHER vil 
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 NI 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 
