INTRODUCTION 3 



the more important data on human heredity which 

 have accumulated chiefly in the last twenty years ; 

 but the general principles will be briefly discussed, 

 and reference will be made to experiments, particu- 

 larly with regard to the higher animals, when the 

 results bear directly upon problems of human heredity. 

 It w^ill be seen that a large amount of information has 

 already been gained regarding the inheritance of a 

 multitude of traits, both physical and mental, in 

 mankind. And perhaps the most surprising feature 

 of these results is the minuteness and variety of the 

 differences which are now known to follow definite 

 laws of inheritance. But it is not necessary to rely 

 upon recent work to establish the minuteness and 

 peculiarity of some of the differences which are 

 inherited in man. Darwin, who was unsurpassed 

 as an observer, and, what is equally important, a 

 collator of the observations of others, has a chapter 

 on blushing in his book, The Expression of the Emo- 

 tions, in which (p. 312) he cites, not only a number 

 of cases of the inheritance of a tendency to blush, 

 but also one in which mother and daughter blushed 

 in the same peculiar manner. The tendency to blush 

 excessively is due to a psychological peculiarity, 

 while the distribution of the area over which a blush 

 spreads must have a physical basis. That gait, 

 gestures, voice, and general bearing are inherited, 

 was recognised in the scientific writings of over a 

 century ago, though imitation may also, of course, 

 play a part here, but this is excluded in some cases. 



Further random examples of inheritance in man 

 will not be cited here, but the reader is invited to 

 consider the mass of evidence found in the body of 

 this book. It is believed that, in this way, any reader 

 who is inclined to doubt the universality and import- 

 ance of heredity in mankind will attain a truer per- 

 spective regarding the whole matter. But certain 



