SCOPE OF GENETICS 19 



In such an observation two things are 

 strikingly exemplified, (1) the fact of the 

 permanence of the unit, and (2) the fact that 

 a mixture of types in the family means that 

 one or other parent is cross-bred in some 

 respect, and is giving off gametes of more 

 than one type. 



The problem of heredity is thus a problem 

 primary analytical. We have to detect and 

 enumerate the factors out of which the bodies 

 of animals and plants are built up, and the 

 laws of their distribution among the germ- 

 cells. All the processes of which I have 

 spoken are accomplished by means of cell- 

 divisions, and in the one cell-union which 

 occurs in fertilisation. If we could watch 

 the factors segregating from each other in 

 cell-division, or even if by microscopic ex- 

 amination we could recognize this multitu- 

 dinous diversity of composition that must 



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