12 Plant Genetics 



In this way individual differences arose in one-celled 

 organisms; and when the sexual method appeared it 

 found varying individuals ready to work upon. The 

 mission of sex, therefore, according to WEISMANN, was 

 to lay hold of these variations and multiply their com- 

 binations, thus providing a wider range of choice for 

 natural selection. In this way he explains the origin of 

 individual variation without the inheritance of acquired 

 characters. 



WEISMANN also devised a mechanism by which 

 sexual reproduction results in multiplying combinations 

 of individual differences. This mechanism is the basis 

 of his theory of germinal selection and is as follows. 



In WEISMANN'S time chromosomes as such had not 

 been recognized, but they were represented in his termi- 

 nology by the term idants. He conceived of each idant 

 as composed of one or more units called ids. An id 

 stood for a complete individual, each id being able to 

 control the complete development of an individual. A 

 child received both paternal and maternal ids in equal 

 numbers; that is, father and mother were each repre- 

 sented by a 50 per cent influence on the offspring; 

 each grandparent would be represented by 25 per cent 

 of the ids in an individual ; and so on, indefinitely. As a 

 consequence, all existing individuals must now contain 

 as many different kinds of ancestral ids as they are 

 capable of containing. Sooner or later, owing to the 

 accumulation of ids, "sex reproduction can proceed 

 only by reduction in the number of ancestral ids, a 

 reduction which is represented in every generation." 



This is WEISMANN'S explanation of the grosser 

 mechanism of heredity, but it was only preliminary to 



