VARIATION IN UNICELLULAR ORGANISMS loi 



long-continued careful selection of differing individuals 

 throughout many generations, in a stock that is multiplying 

 by fission, inherited diversities may be produced. It is true 

 that such selection may be practised for long periods with no 

 apparent results; but if it is persisted in for a very long time, 

 and a suitable basis for selection is employed, there may 

 finally result inherited differences. In DifHugia corona there 

 were produced in this way, in my own work, races slightly 

 diverse in form and size. Middleton and Parker found that 

 by long-continued selection in infusoria, races differing in 

 inherited rate of multiplication could be isolated. 



Just what processes underlie these changes is not known. It 

 appears possible that there occur at times irregularities of 

 nuclear division, analogous to some of those described in 

 Chapter II for higher organisms, resulting in unequal dis- 

 tribution of the genie materials in the two individuals pro- 

 duced, so that they have diverse inherited characteristics. The 

 inherited differences produced in this way have thus far been 

 but slight. 



In addition to these genetic variations resulting from the 

 production of new combinations of genie materials in bipa- 

 rental reproduction, and those presumably due to irregularities 

 in nuclear division, there are others that occur under the 

 action of environmental conditions, and that show special 

 relations to the conditions that induce them. These are of 

 special interest; to them we now turn our attention. 



It is easy to change the characteristics manifested by uni- 

 cellular organisms, by subjecting them to changed environ- 

 ments. In DifHugia the size, form, number of spines, and 

 length of the spines are readily altered by subjecting the 

 animals to certain conditions. By subjecting Paramecium to 

 certain chemicals, they can be altered greatly, in size, form. 



