228 LECTUEES ON BIOLOGY 



once more for a new period of reproduction. The opinion which 

 I expressed briefly in the beginning that in the mingling of the 

 characters of two different individuals we have to look for the 

 most important meaning of conjugation is hereby corroborated. 



We have already heard (in the fourth lecture) that cogent 

 proofs force us to regard the nucleus as the carrier of heritable 

 characters i.e., as that organ or that part in which all heritable 

 characters of the whole individual are potentially contained in 

 the form of minute ' primary constituents.' In exchanging and 

 mingling their nuclear substance each of the conjugating infusoria 

 transmits to its ' pair ' its own characters. After the fusion both 

 Paramcecia have, therefore, in their body the primary constituents 

 (Anlagen) of their old characters augmented by those of the 

 second animalcule. What increase of variability, or, at least 

 what possibility ? Let us assume, for instance, that both Para- 

 mcecia about to unite differ from each other in the formation 

 of their cilia-dress, which consists in the one of short and strong, 

 in the other of long and delicate cilia, and that the correspond- 

 ing Anlagen were fixed in the nuclear substance. In that case 

 each ' pair ' would obviously have at their disposal after the nucleus- 

 exchange both these different * dispositions/ and we can conceive 

 that in them and their descendants those which reach develop- 

 ment would prove to be best adapted to the existing conditions 

 of life. And what applies to the cilia-dress applies to all other 

 organella and divergencies, so that the two sets of characters 

 of both animals may find in their descendants a development of 

 the most varied combinations. 



But while amphimixis increases on one side the extent of 

 variation it limits on the other side the danger that might 

 threaten from one-sided variation. If, for instance, one part of 

 an animal ' varies ' in the descendants, the variations may either 

 become weaker or, with the same probability, it may tend to 

 * vary ' in a forward direction and become more and more pro- 

 nounced. If now such variations accumulate through several 

 generations we can imagine that the form of the affected part may 

 assume such extreme dimensions that it would constitute a serious 

 danger to its owner. Such a one-sided tendency of variation is 

 entirely prevented by conjugation, because an exchange of 

 characters implies, generally, a simultaneous correction of too 

 pronounced deviations. 



