Nature of Heritable Variations 103 



changes that arc handed on only through vegetative repro- 

 duction he calls modifications ; the two he believes to be of 

 essentially diverse nature. Such a "mutation" Jollos be- 

 lieves that he saw in a single instance in Paramecium. In 

 one of his cultures kept at high temperatures there ap- 

 peared individuals which were much more resistant to heat 

 than most of the animals; they could be cultivated at 39 

 C., which soon killed the others. These individuals retained 

 their high resistance even after conjugation; it had become 

 a permanently inherited character. In no other case was a 

 modification retained through conjugation. Jollos holds 

 that practically all the changes in bacteria and other Pro- 

 tista, which we have described above, are merely instances 

 of these temporary modifications. 



That there is such a difference in principle between the 

 two things, between modifications that are passed on only 

 in fission, but disappear as soon as there is sexual reproduc- 

 tion, so that they cannot be said to form part of the really 

 hereditary characters of the stock, between these and the 

 really hereditary characters, cannot yet be considered es- 

 tablished. If there is such a difference, one can hardly re- 

 frain from bringing it in some way into relation with the 

 two nuclei. Since in fission the new active nuclei are pro- 

 duced by division of the active nucleus of the parent, one 

 might naturally assume that the seat of the temporary 

 modifications is in the active or macronucleus, while the re- 

 serve nucleus (micronucleus) has not been affected. Thus 

 would be accounted for the fact that at conjugation, when 

 the macronucleus disappears and is replaced by the mi- 

 cronucleus, the modifications also disappear; they go with 

 the macronucleus. But we now know from the work of 

 Woodruff and Erdmann that the macronucleus disappears 

 and is replaced from the micronucleus at intervals even 



