THE 



AMERICAN NATURALIST 



HEREDITY AND VARIATION IN THE SIMPLEST 

 ORGANISMS 1 

 PROFESSOR H. S. JENNINGS 

 Johns Hopkins University 



Unicellular animals present all the problems of hered- 

 ity and variation in miniature. The straggle for ex- 

 istence in a fauna of untold thousands showing as much 

 variety of form and function as any higher group, works 

 itself out, with ultimate survival of the fittest, in a few 

 days under our eyes, in a finger bowl. For studying 

 heredity and variation we get a generation a day, and 

 we may keep unlimited numbers of pedigreed stock in a 

 watch glass that can be placed under the microscope. 



Work in this field, so far as it has yet been carried, gives 

 in simple form results which are typical of the trend of 

 investigation over the entire subject; it gives a sort of 

 diagram of the main facts of heredity and variation. 

 For this reason it appears worth while to present here 

 the main results in their bearing on general questions. 

 Technical accounts of the investigations have been pub- 

 lished elsewhere, 2 but these are rather forbidding, owing 



l A paper read before the Scientific Association of Johns Hopkins 



* Jennings, EL S., "Heredity, Variation and Evolution in Protozoa." 

 I, "The Fate of New Structural Characters in Paramecium, with Special 

 Reference to the Question of the Inheritance of Acquired Characters in 

 Protozoa," Journ. Exper. ZooL, 5, 1908, 577-632. II, "Heredity of Size 

 and Form in Paramecium, with Studies of Growth, Environmental Action 

 and Selection," Proc. Amer. Philosophical Soc, 47, 1909, 393-546. 

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