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THE AMERICAN NATURALIST 



[Vol. XLIX 



— the individual — perish. The experiments of Wood- 

 ruff ( '11, etc.) who in extending the work of Maupas and 

 of Calkins was able to rear several thousand generations 

 of Paramecium without conjugation, as well as the in- 

 vestigations of Harrison subsequently elaborated by 

 Carrel, where human and other animal tissues main- 

 tained cell division for a prolonged time in an artificial 

 medium, are here of much interest. In each case the 

 result is brought about by the favorable artificial environ- 

 ment, and it is made more clear that death itself is wholly 

 or in part due to the unfavorable conditions surrounding 

 an organism. 



IV. A Working Hypothesis of Evolution 



Investigations during the last fifteen years, instead of 

 establishing evolution as the simple process of natural 

 selection conjectured by Darwin and others, have made it 

 evident that the results are due to many factors of much 

 complexity. While the diversity of organisms depends 

 on variation — their inheritance and non-inheritance — it 

 is becoming more and more apparent that the term is too 

 comprehensive and covers variations arising in organ- 

 isms from causes quite different from one another. 



The results reached in the preceding pages indicate the 

 need of extending the older terminology as used by Plate, 

 '13, and others where variations are separated into 

 "somations" or fluctuations induced by the environment 

 and not inherited, and ' 1 mutations" or blastovariations 

 arising in the germ plasm and inherited, if a clearer 

 understanding is to be obtained of evolution and its ap- 

 plication. Therefore the following scheme is proposed. 5 



