30 THE THEORY OF THE GENE 



plasms, each different from the others in one or another 

 way. 



The individual variations shown by animals are due 

 to the different recombinations of ids. This is brought 

 about by the union of eggs and sperms. The number of 

 ids would become indefinitely large were it not that, at 

 the ripening of the germ-cells, the number of ids is re- 

 duced to half. 



Weismann also formulated an elaborate theory of 

 embryonic development based on the idea that the ids are 

 separated into their smaller elements as the egg divides, 

 until, finally, each kind of cell in the body comes to con- 

 tain one of the ultimate components of the ids, i.e., deter- 

 minants. In the cells destined to become germ-cells the 

 disintegration of the ids does not take place. Hence the 

 continuity of the germ-plasm, or of the colony of ids. The 

 application of his theory to embryonic development lies 

 outside the modern theory of heredity that either ignores 

 the developmental process, or else postulates a view 

 exactly the opposite of that of Weismann, namely, that 

 in every cell of the body the entire heredity complex is 

 present. 



It will be seen without further elaboration that Weis- 

 mann 's ingenious speculation invokes, in order to explain 

 variation, processes that are akin to those we adopt 

 today. Variation, he believed, is due to the recombination 

 of units from the parents. These are reduced to half in 

 the process of maturation of the egg and sperm. The 

 units are wholes and each represents an ancestral stage. 



W^e owe to Weismann largely the idea of the isolation 

 and continuity of the germinal material. His challenge of 

 the Lamarckian theory was of immense service to clear 

 thinking. The theory of the inheritance of acquired 

 characters had obscured for a long time all problems 

 dealing with heredity. Weismann 's writings were also 



