AGE, DEATH AND CONJUGATION 573 



died out completely in the course of several weeks. Those that had con- 

 jugated showed great variation (as usual); some died very quickly; 

 others multiplied very slowly and finally died out; others multiplied 

 more vigorously than any of the non-conjugants. At the end of six 

 weeks, all those tliat had not conjugated were dead, while certain lines of 

 the others had multiplied and were numerous. The difference between 

 the two sets was in fact very striking. But it is important not to mis- 

 understand the nature of this difference. The lot that had conjugated 

 showed great variation, and many of the lines were not stronger than 

 the non-conjugants, dying out fully as quickly. But a few were 

 stronger, and these multiplied and replaced the rest. Thus after some 

 weeks, all the survivors had come from hut three or four among those 

 that had conjugated. 



But even in these the depressed condition had not been completely 

 overcome; they were still notably less vigorous than the strain which 

 had been kept throughout under more natural conditions and had con- 

 jugated frequently. 



Thus what had happened was this : Conjugation had produced much 

 variation; some few of the variants had been more vigorous and had 

 lived, while the rest died. 



This result when first reached was unexpected and difficult to inter- 

 pret. It seems of such importance that one felt it necessary to try it 

 again. I shall not describe to you the long and wearisome process of 

 providing anew the necessary conditions and repeating the experiment. 

 It will suffice to say that the experiment was repeated and gave the 

 same results as before. 



Thus I believe that we are in position to make certain positive state- 

 ments as to the effect of conjugation. Conjugation does not rejuvenate 

 in any simple, direct way. What it does is to produce variation; to 

 produce a great number of different combinations, having different 

 properties. Some of these are more vigorous, others less vigorous. 

 The latter die, the former survive. This happens equally, whether the 

 animals which conjugate are at the beginning vigorous or weak. If 

 they are vigorous, then one of the most striking effects of conjugation is 

 to produce some lines that are less vigorous than the original ones, so 

 that they die out. If the animals which enter conjugation are weak, 

 then one of the most striking effects of conjugation is to produce certain 

 combinations that are more vigorous than the original ones, so that they 

 survive, while those that did not conjugate die out. In a short time 

 the entire race is replaced by the descendants of a few of those that 

 conjugated. 



jSTow, the relation of all this to certain things that are known in 

 higher organisms seems fairly clear. In higher animals likewise the 

 result of intercrossing is to produce variation. We don't call it varia- 

 tion nowadays, because we know something more about it; we call it 



