2 



THE GENETIC SYSTEM 



7/5 History during the Life of the Individual and in the 

 Production of the Next Generation 



In Chapter I were described the discovery of the materials 

 of heredity, and the methods by which it was proved that 

 they do indeed affect the characteristics of organisms. 

 These materials, it was shown, lie in the chromosomes. 

 And it was seen that removing a chromosome, or substitut- 

 ing one for another, has great and varied effects on indi- 

 viduals. 



The study of genetics is very largely therefore the study 

 of these materials, and particularly the study of their 

 physiological action, in reproduction and heredity. As a 

 foundation for understanding their action, it is necessary 

 to have clear ideas of the main features of their location 

 and history, from the formation of the new individual, 

 through its development to an adult, and thence to the 

 formation of a new generation. This forms the subject of 

 the present chapter. Though here heredity, variation and 

 characteristics are not explicitly considered, it will be found 

 that for their understanding, as taken up in later chapters, 

 knowledge of the matters set forth in this chapter is essen- 

 tial. 



The materials of heredity are gathered into a system of 

 structures, which we may call the Genetic System. This 

 system lies within the cells, and is as well defined in its 

 constitution and functions as is the nervous or muscular 



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