TWELFTH LECTURE. 



ON THE HEREDITY OF THE MARKING IN 

 FISH EMBRYOS. 



JACQUES LOEB. 



I. 



UNTIL recently heredity has been treated chiefly as a problem 

 for whose solution one single theory or one single principle 

 was considered possible and sufficient. The theories of hered- 

 ity by IJarwin, Weismann, Spencer, and Jaeger are proofs of 

 this. Heredity is explained by Darwin on the assumption 

 that the organs of the parents give off particles from which the 

 organs of the offspring originate. According to Weismann, 

 the egg contains determinants for every organ contained in the 

 later organism and which have a definite arrangement. Accord- 

 ing to Jaeger, the offspring resembles the parents because both 

 consist of the same chemical constituents. None of these theo- 

 ries of heredity have been generally accepted, nor do I believe 

 they ever will be. They overlook the fact that heredity is a 

 collective term for a series of heterogeneous circumstances 

 which cannot possibly be explained by one principle. Thus it 

 happens that while each of these theories is adapted to some 

 of the facts of heredity, other facts can be better explained by 

 some other theory. 



. In contradistinction to these attempts to explain heredity by 

 one single principle and by means of one single theory, a more 

 analytical study of the subject has been undertaken. This has 

 led to the conception that very different circumstances deter- 

 mine the various details in heredity. We thus find- ourselves 

 face to face with the task of investigating in detail which cir- 



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