No. 530] PURE LINES IN STUDY OF GENETICS 87 



from that which is typical. But the experimental analy- 

 sis of this matter is still in progress, and conclusions 

 can not yet be drawn. 



It is only in rate of multiplication that I have thus far 

 found hereditary differences arising- within the pure line, 

 and these but rarely. But this encourages one to hope 

 that the same may be found for other characters when 

 these are extensively studied with sufficient minuteness. 

 The negative results thus far reached do not (as many 

 critics have pointed out) exclude the possibility that 

 rare cases of hereditary variation within the pure line 

 will yet be found. What the negative results have 

 demonstrated is that a very large share of the observed 

 variations in organisms are not hereditary, and that se- 

 lection based on these variations leads to no result— a 

 conclusion of such great importance as to make the pure 

 line work epoch-marking in character. 



Finally, what happens when diverse genotypes mix in 

 conjugation? To my disappointment, I have found this 

 much more difficult to determine for the infusorian than 

 I expected. This is owing to the fact that the condi- 

 tions for conjugation differ in the diverse genotypes, so 

 that it is almost impossible to get them to conjugate at 

 the same time. Further, in the rare cases where two are 

 conjugating at once, the assortative mating discovered 

 by Pearl results in the two sets remaining separate. 

 Thus I have not yet been able to get crosses between two 

 genotypes whose characteristics are known beforehand; 

 and this will be necessary before a study of inheritance, 

 exact in the modern physiological sense, can be made. 

 On the other hand, it is possible to get conjugations in wild 

 populations that include many genotypes, and to com- 

 pare the results with conjugations where but a single 

 genotype is involved. Certain most interesting results 

 appear. In these conjugations of mixed populations, a 

 great number of diverse combinations are produced ; the 

 variability increases greatly, in size and in other re- 

 spects. Numbers of the strains produced die, or multiply 

 so slowly that they have no chance in competition with 



