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CHAPTER IV 



WEISMANN'S THEORY OF HEREDITY 



Weismann believed that a new type of organism arises only 

 in consequence of the origin of a new type of germ-cell. If he 

 had been asked the ancient riddle, " which was created first, 

 the egg or the hen," he would undoubtedly have answered, 

 " the egg.'' He would have explained that the first bird came 

 from a new type of egg laid by a reptile-like ancestor. 

 Changed structure of the germ-plasm must result, he 

 thought, in changed structure of the organism developing 

 from it; and he would scarcely have admitted that a new 

 sort of organism might arise in any other way. But the 

 experimental study of the development of organisms has 

 shown that the germ-plasm forms only one of two comple- 

 mentary sets of agencies which determine what the adult 

 organism shall be. It is true that the character of the germ- 

 cell determines in part what the character of the ;'dult organ- 

 ism shall be, but so also does the environment. If we plant 

 beans, we must expect to harvest beans not corn, but whether 

 the harvest is large or small will depend upon the soil and the 

 season. Sunlight, moisture, a suitable temperature, and 

 proper chemical substances in the soil are all indispensable 

 conditions to the production of any crop at all, and they con- 

 trol within hmits the size, vigor, and productiveness of the 

 plants grown. Both internal and external agencies influence 

 the form of organisms. These are summarized in the two 

 words, heredity and environment. Weismann emphasized 

 the first almost to the neglect of the second. Lamarck had 

 previously gone to the opposite extreme, emphasizing the im- 

 portance of the environment not only in directly adapting the 

 organism to its surroundings but also in controUing its 



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