VARIATION IN UNICELLULAR ORGANISMS 99 



Such inherited diversities one finds in any unicellular or- 

 ganism, if it is studied with sufficient thoroughness. And 

 there are similar variations in less conspicuous matters. There 

 are differences in vigor, resistance, vitality. Under poor con- 

 ditions, some races die quickly, others resist long. Some are 

 adapted to one set of conditions, some to another. Some stand 

 higher temperatures than do others. Some are resistant to a 

 chemical to w^hich others quickly succumb. Some multiply 

 rapidly, others slow^ly. Some races of Paramecium conjugate 

 frequently, others rarely. Differences of all these types are 

 inherited. 



What is the origin of these inherited diversities? This is 

 the problem of genetic variation. 



In the unicellular organisms, as in those composed of many 

 cells, several different types of genetic variation occur, some of 

 them show^ing direct correlations vv^ith environmental condi- 

 tions such as have not been demonstrated in higher organisms. 

 It is these that are of special interest for our present purposes. 

 But before examining these, certain other types of genetic 

 variation must be briefly presented. 



In the unicellular organisms, as in higher organisms, bipa- 

 rental reproduction is a source of genetic variation. In such 

 ciliate infusoria as Paramecium, at times tw^o individuals unite 

 temporarily in conjugation and exchange parts of their 

 nuclei, then separate and continue reproduction by fission. 

 Conjugation among the members of a uniform stock pro- 

 duces among the descendants races or stocks differing greatly 

 in their inherited characteristics. They differ in size and in 

 other physical features. This has been show^n by the recent 

 work of Sonneborn and Lynch, and by that of DeGaris.Thus, 

 De Garis produced repeated conjugations between the mem- 

 bers of a uniform stock of Paramecium caudatum, the indi- 

 viduals of which, when full grown, had a mean length of 172 



