INTRODUCTION TO EUGENICS 445 



b) The statistical method. — A great deal can be learned about the 

 heredity of fluctuating and graduated differences, such as those of 

 height, weight, longevity, etc., by determining the degree of correla- 

 tion that exists between a large group of parents and a large group of 

 offspring. Galton and his successor, Karl Pearson, are the chief ex- 

 ponents of this method. 



c) The twin method, — This method is quite distinct from either of 

 those just listed, though it may use statistical methods. Many facts 

 may be determined, by the use of one-egg (monozygotic) and two-egg 

 (dizygotic) twins, about what is inherited in man and what characters 

 are commonly modified by the environment. 



A separate chapter will be devoted to a discussion of each of these 

 methods and the contributions to our knowledge of human heredity 

 that have accrued from their use. 



