l8 INTRODUCTION. 



to them are given as a preliminary to the main work. This main 

 work consists in tracing the genesis of dynamic heredity, and the 

 facts revealed by this investigation give us a better understanding 

 of the processes by which character is produced. 



THE GREAT FACTS OF HEREDITY. 



In considering the dynamical side of this question, the first 

 and most important fact is that heredity is the product of two fac- 

 tors, one of which is the length of time elapsing between genera- 

 tions, and the other of which is the degree of activity which char- 

 acterizes the individuals of successive generations. These two 

 factors bear the same relationship to heredity that length and 

 breadth bear to area. No matter how great the length may be we 

 cannot have great area if the breadth is small, and no matter how 

 great the breadth, the area will still remain small if the length is 

 small. In the same way we cannot have a high degree of hereditary 

 development by time alone, nor by activity alone. To produce great 

 development by hereditary action, both factors must be large in 

 the parents, and to produce very great development both factors 

 must be large for two or more generations. 



The second great fact is that each individual during his life 

 undergoes certain physical and mental changes, and that those con- 

 ditions which characterize parents at different ages are transmitted 

 to the offspring which are produced at those ages. This is admira- 

 bly exemplified in mental aptitudes, the children of youthful 

 parents being strongly marked by the characteristics of youth, and 

 the children of old parents being marked by the characteristics 

 of age. 



The third great fact is that the average length of life tends to 

 approximate twice the average age at which reproduction takes 



