INHERITANCE 731 



an altered environment. But a change in the cytoplasm, it is urged, would 

 in the course of time be overcome and dominated by the unchanged con- 

 stitution of the nucleus, bringing about a return to the original charac- 

 teristics — as actually occurs. These inherited modifications are, on this 

 view, essentially transitory conditions, forming no part of the system of 

 genuinely inherited characters. 



The fact that these modifications usually (though not always) dis- 

 appear at sexual reproduction is likewise held to be due to the fact that 

 they have their seat in the cytoplasm, though the logic of this is not 

 entirely clear. As will be shown later, it is the nucleus, not the cytoplasm, 

 that is directly changed in constitution at conjugation. The disappearance 

 at conjugation is seemingly attributed to the profound making over of 

 the cytoplasm that has been believed to occur at conjugation, producing 

 rejuvenescence. The long-continued inheritance of the modifications dur- 

 ing vegetative reproduction is held to be an example of cytoplasmic in- 

 heritance. 



If we are here indeed dealing with cytoplasmic inheritance, this 

 appears to demonstrate that in the Protozoa the cytoplasm partakes of 

 the essential features of genetic material. These essential features, as 

 before set forth are: (1) the fact that the material in question affects 

 the inherited characteristics, and ( 2 ) the fact that this material multiplies 

 true to type. The environmental modifications are, as we have seen, at 

 times inherited for more than 200 generations after removal from the 

 conditions that induced them. In 10 generations, the originally modified 

 cytoplasm has been diluted to one-thousandth of its volume, in any indi- 

 vidual; in 20 generations, to less than one-millionth of its volume. The 

 rest of the cytoplasm of the individual is a product of cytoplasmic growth. 

 Yet the environmental modification still persists. If the cytoplasm is the 

 seat of the modification, the modified cytoplasm must reproduce itself 

 in its modified condition at fission; otherwise its effect would have dis- 

 appeared under the great dilution that it has undergone. 



On the question as to whether cytoplasmic inheritance indeed occurs 

 in these organisms, evidence will be presented in the section on biparental 

 reproduction. There also a method will be indicated by which it may be 

 determined whether the seat of these modifications is in the cytoplasm 

 or in the nucleus. 



