CHAPTER XI 



HISTORY OF THEORIES OF HEREDITY AND INHERITANCE 



" Like leaves on trees the race of man is found, 

 Now green in youth, now withering on the ground ; 

 Another race the following spring supplies, 

 They fall successive and successive rise." 



Iliad {Pope's Translation). 

 [The same may be said of the succession of theories of heredity, but, 

 in both cases, there is a persistent living tree, to whose growth all the 

 leaves contribute.] 



§ I. What is required of Theories of Heredity and 



Inheritance. 

 § 2. The Old Theories of Heredity. 

 § 3. Theories of Pangenesis. 

 § 4. Theory of Genetic or Germinal Continuity. 



§ I. What is required of Theories of Heredity and Inheritance 



The main object of a theory of heredity is to express in as 

 simple terms as possible the nature of the genetic relation which 

 binds generations together, and to interpret the facts of inheri- 

 tance in terms of this relation. 



The Uniqueness of the Germ -Cells. — ^The first and chief pro- 

 blem is to account for the material basis of heredity — i.e. in all 

 ordinary cases, for the germ-cells. What is their origin and 

 history ? what relation have they to the parental body which 

 bears them, from which they are liberated ? what relation have 

 they to the germ-cells of the body into which they develop ? Or, 

 more generally, in what way are they peculiar ? how do they 



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