478 BREAKDOWN OF GENETIC SYSTEMS 



When we apply the genetical criterion of recombination to 

 forms with sexual reproduction we see that many, if not all, have 

 clonal chromosomes or parts of chromosomes. Thus all the 

 unpaired chromosomes of Rosa canina are clonal, or if they occasion- 

 ally pair they are subsexual. Similarly, the differential segments 

 of (Enothera and of the Y chromosomes of higher organisms are 

 clonal. 



Such clonal parts of the hereditament in sexually reproducing 

 organisms have always the capacity of returning to sexual life as 

 a result of structural change. But if they have become inert 

 before they recover in this way, their change is presumably 

 irreversible. In obligatory apomictics the whole hereditament has 

 made an irreversible step towards asexuality. 



Another special modification of sexual reproduction results from 

 all structural changes like short inversions which inhibit crossing- 

 over in the hybrid^ for such changes establish a condition of 

 endogamy in the segment affected. All the genes within it are 

 prevented from crossing-over effectively with the genes in the 

 unchanged segment. It is therefore separated in evolution. Its 

 descendants become an endogamous community corresponding 

 genetically to an endogamous community of zygotes (D., 1936 a 

 and d). 



By pursuing investigation along these lines with the help of the 

 new genetical interpretation of chromosome behaviour, it will be 

 possible to determine how the genetic system works and changes 

 over a wider range of organisms and a greater space of time than is 

 accessible to experimental enquiry. 



