VARIATION 421 



that these vital units, which have great persistence of ' in- 

 dividuality ', may exhibit age-changes or periodic reorganisa- 

 tion processes. 



Here may be profitably considered the recent work on 

 the Slipper-Animalcule (Paramecium aurelia) by Professor 

 Woodruff and Miss Erdmann. Woodruff has kept a pure 

 line of this Ciliate healthy for over seven years, through 

 more than 4500 generations. As is usual in a pure line all 

 descended from one there was no conjugation. On an av- 

 erage of once a month, however, a remarkable regulatory 

 process occurs, which the authors call endomixis, which 

 secures the indefinite life of the race. Nuclear changes, 

 comparable to those that precede conjugation in normal 

 wild conditions, set in ; the old nuclear material, both macro- 

 nuclear and micronuclear, is disintegrated and re-organised. 

 But there is no formation of stationary and migratory 

 micronuclei as there is before conjugation. For conjugation 

 is not going to occur ; something that takes its place is occur- 

 ring endomixis. Now it seems probable that such a 

 periodic re-organisation of nuclear material will afford op- 

 portunity for plasmic re-arrangement, and this may imply 

 the origin of variations even within a pure line. Professor 

 Jennings has found in pure lines of non-conjugating Para- 

 mecium evidence of variations about the mean. These might 

 be due to re-arrangements effected in endomixis. It is 

 conceivable, as Woodward and Erdmann point out, that 

 " heritable " variations may result from some rare re- 

 combinations in endomixis. 



This Paramecium is a very complicated organism, as 

 Prof. Clifford Dobell has vividly emphasised, on the non- 

 cellular line of evolution, and we find it in certain conditions 

 exhibiting a monthly re-organisation as part of its life-cycle. 



