136 DARWINISM AND HUMAN LIFE 



ingredient come in from one side and not from 

 the other, then, on an average, in haK the resulting 

 germ-cells it will be present, and from half it 

 will be absent. " This last phenomenon, which 

 is called segregation, constitutes the essence of 

 Mendel's discovery."' "In this," Mr. Punnett 

 says, " lies the explanation of the facts that 

 hybrids mated together produce a definite pro- 

 portion of the pure forms, which subsequently 

 breed true without ever giving a hint of their 

 mixed ancestry." 



Methods of Studying Heredity. — In studying 

 a difficult problem, such as the weather, there 

 are three possible lines of attack : we may make 

 minute researches, e.g. on the role of dust in 

 forming fog ; we may make experiments, e.g. on 

 the change of a cloud into rain, or on the effect 

 of tree-planting on climate ; or we may collect 

 a multitude of observations of a statistical char- 

 acter, e.g. as to the rainfall in different localities 

 and at different times of year. These are three 

 sound methods, which have been worked with 

 success. They are complementary, not opposed. 



Similarly, we may attack the problems of 

 heredity by the microscopical study of the germ- 

 cells in which Ufe is continued from generation 

 to generation, by breeding experiments, and by 

 the statistical study of the measurable characters 

 of successive generations.^ These three different 

 methods of attacking the problems of heredity 



1 W. Bateson, " The Methods and Scope of Genetics " (Cambridge, 

 1908), p. 15. 



* It is of interest to note that Sir Francis Galton, who may be 

 taken as the representative of the statistical study, and Gregor 

 Mendel, who was the pioneer of the experimental study, were born 

 in the same year (1822). 



