72 EVOLUTION AND GENETICS 



peas when two pairs of contrasted characters are 

 present. 



The possibihty of interchanging characters might 

 be illustrated by hundreds of examples. It holds not 

 only for two pairs of characters but when three, four, 

 or more enter the cross. It is as though two individ- 

 uals were taken apart and their characters were put 

 together again by substituting one part for another. 



Not only has this power to make whatever com- 

 binations we choose great practical importance, it 

 has even greater theoretical significance; for it fol- 

 lows that the individual is not in itself the unit in 

 heredity, but that within the germ-cells there exist 

 smaller units concerned with the transmission of 

 characters. 



The older mvstical statement of the individual as 

 a unit in heredity has no longer any interest in the 

 light of these discoveries, except as a past phase of 

 biological history. We see, too, more clearly that 

 the sorting out of factors in the germ plasm is a very 

 different process from the influence of these factors 

 on the development of the organism. There is today 

 no excuse for confusing these two j^roblems. 



