FLOATING AND FIXED DISCONTINUITY 



And the result has been to maintain the same degree of hetero- 

 zygosity at the end as there was at the beginning. Inbreeding has 

 fixed the interchanges, but only by virtue of this fixation has the 

 move towards inbreeding been successful. Moreover the inter- 



(1) 



Quercifolia ] ^ 

 and 



ferox 



J 



stramonium 



4 7 



\ / 

 4+7 



(2) 



Basic 



T— J r 



B C 



8(2) 



(3) 



basic: 7(2) 



(4) (4) 



Oenothera 



(California ) 

 or 



Campanula 



(Europe) 



(4)+(4) (6) (6) 



(6)+(4y 



(8) 



(14) 



(14) 



Oe. nutans 



(New York) 



Oe. muricata 

 (.Europe) 



Fig. 8o. — Three types of interchange evolution, as seen in (i) Datura, (2) Campanula, 

 and (3) Oenothera. 



mediate stages of incomplete ring formation are unstable. There 

 are as many species with a ring of 14 chromosomes as there are 

 with all the lower sizes of ring (Fig. 80). In Hypericum and Rhoeo this 

 process has gone further and the incomplete ring types have already 

 disappeared. It only remains to add that in animals fixed ring- 

 formation is hindered by the difficulty of establishing fixed in- 

 breeding. 



317 



