INTRODUCTION 11 



It is within the last twenty years or so that the real 

 development of the subject, as an exact science, has 

 begun ; and it is not too much to say that more real 

 progress has been made since 1900 than had been made 

 altogether before. Much certainly remains to be dis- 

 covered, and the answers to many important questions 

 are still unknown or under dispute. It must also be 

 admitted that the very rapidity of progress, especially 

 as it has been made along different lines of investiga- 

 tion, has tended to throw our ideas into confusion. 

 Yet many facts which appeared contradictory have 

 proved to be consistent ; order is appearing amid the 

 jungle of facts. 



It is a fact, which must have been observed very 

 early in the history of humanity, that like tends to 

 produce like. It may also be said that no two creatures 

 are ever exactly alike. It is the object of the study of 

 heredity to discover the nature and the causes both of 

 the resemblances and of the differences between con- 

 secutive generations. The subject is attractive for its 

 own sake, as other sciences are ; it is also important 

 by virtue of its bearing on the improvement of the 

 races of economic plants and animals ; and there is 

 little doubt that the science will ultimately prove of 

 service by showing the road to a betterment in the 

 inborn characters of man himself. 



