88 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



groups of the young and the somewhat older rats, which agrees with the gen- 

 eral observation that when donors and hosts are very young, the reactions are 

 milder than in older animals. This difference cannot be due to a lack of in- 

 dividuality differentials in the former, because such differentials are present ; 

 but it is due rather to a lesser sensitiveness to strange individuality differen- 

 tials, or to a not yet fully developed mode of reaction in the younger animals. 

 In addition, the fact must be taken into account that younger tissues grow 

 more vigorously than older ones, and this condition is associated with a greater 

 ability to overcome the effect of the antagonistic reactions of the host ; it may 

 also be that tissues growing more rapidly do not give off individuality differen- 

 tial substances in as large amounts as the more differentiated tissues in which 

 the functional activity predominates. In accordance with these considerations, 

 we noticed that in the group of younger rats the grades are higher, even in 

 transplantations from hybrids to an inbred parent strain, where the derivatives 

 of strange genes are introduced into the host. 



A comparison of the reactions observed in these three series of trans- 

 plantations shows that a continuous progress to a homozygous condition has 

 been made. In the first series there was only a slight indication of an improve- 

 ment in grades over the grades of ordinary homoiogenous and syngenesious 

 transplantations. A further slight progress was noted in series II, but the 

 greatest advance was made in the interval between series II and III. This 

 means that after about forty generations, there was only a very slight progress 

 towards an autogenous character of the individuality differentials ; some ad- 

 vance was made after 60 to 67 consecutive brother-sister transplantations ; 

 but the greatest advance had been made when the 102nd generation was 

 reached ; however, even at that time no completely homozygous condition had 

 as yet been attained. This finding is indicated especially by the transplanta- 

 tions into which the hybrids entered ; but it is noticeable also in the transplan- 

 tations within the inbred family A. 



It seems most probable that the slow and imperfect progress in the direction 

 towards a homozygous condition in the inbred rats is due to the occurrence of 

 germinal mutations, leading to the introduction of strange genes into the con- 

 stitution of host and donor and opposing, therefore, the attainment of an 

 identity of the individuality differentials, which continued close inbreeding 

 would otherwise more readily have accomplished. But of these two counter- 

 acting factors, germinal mutations and close inbreeding, the effects of the 

 latter prove to be the more potent, and therefore the individuality differentials 

 continue to progress on their way towards increasing mutual similarity, with- 

 out, however, reaching this goal completely, even after as many as 106 con- 

 secutive brother-sister matings. 



