INTRODUCTORY 5 



a genetic variation in the hand might in some way cause a 

 genetic variation in the germ cells is another question. But 

 unless the germ cells in some way are affected by the genetic 

 variation, it cannot be carried on to later generations. 



Furthermore, even in any single cell, different parts of the 

 living material play very different roles in heredity and con- 

 sequently in genetic variation. It is know^n that the materials 

 that play the chief role in determining the characteristics of 

 organisms are those located in the central capsule of the cell 

 — the nucleus. By changing these materials in the nucleus of 

 the fertilized egg — leaving the rest of its materials unchanged 

 — the inherited characteristics of the entire individual pro- 

 duced from the egg are altered. To produce an effect on the 

 inherited characteristics, therefore, a genetic variation must 

 affect these materials. The materials within the nucleus con- 

 stitute what has been known in the past as the germ plasm; 

 genetic variations are changes in the constitution of the germ 

 plasm. 



Until recently very little was known of the germ plasm; its 

 existence was assumed but speculative. But it has now been 

 identified, studied minutely, experimented on in many ways. 

 It is found to have an elaborate organization. On the nature 

 of this organization depend in large measure the kind of 

 genetic variations that occur, and their effect in evolution. 

 The first requirement therefore for understanding genetic 

 variation is to have before one a picture of this organization. 

 It is to such a picture that we first turn our attention. 



An organism, as we know, begins its life as a single cell, 

 which we call the fertilized egg. The germ plasm, located 

 within the nucleus of this cell, is constituted of a large num- 

 ber of diverse materials, which we may call genie materials. 

 Each type of genie material is concentrated into a small 

 particle, commonly known as a gene. The entire germ plasm 



