CHROMOSOMES AND EVOLUTION 93 



rare in a genus like Carex, or perhaps polyploid 

 individuals are considerably less viable than diploid 

 ones. 



Origin of Complex Heterozygote Organisms 



Hypericum punctatum ^^ is the only species of its 

 genus which is a complex heterozygote, all the other 

 St. John's Worts being normal diploids ; its evolu- 

 tionary origin is thus extraordinarily interesting, but 

 cannot be analysed. Bhoeo discolor is the only 

 representative of its genus, so that here again we have 

 no method of determining the origin of the complex - 

 heterozygote mechanism. The genus Oenothera, on 

 the other hand, contains a series of forms ranging 

 from ordinary diploid species to organisms which 

 form a ring of fourteen chromosomes at meiosis (see 

 Chap. V). 



If we take one of the diploid species of Oenothera 

 which must be regarded as ancestral to those which 

 form rings we can consider two pairs of chromosomes : 



abcdefghi and mnopqrst 



abcdefghi mnopqrst 



If a reciprocal exchange (mutual translocation) takes 

 place between one member of each pair of chromo- 

 somes it will give rise to two new chromosomes : 

 abcpqrst and mnodefghi. The four chromosomes : 



abcdefghi mnopqrst 



abcpqrst mnodefghi 



will now form a ring at meiosis ; but there will not 

 be any ' median segment ' in such a ring ; on the 

 other hand, if a second reciprocal interchange takes 

 place it will establish a median segment if it does not 

 correspond in position with the first. The second 

 interchange may result from normal crossing-over if 

 the first one was interstitial instead of terminal. By 

 a repetition of this process rings of 6, 8, 10 and higher 

 numbers can arise. The ring-forming species of 



