BREEDING SYSTEMS 



are something quite dijOferent from what arises with ordinary sexual 

 reproduction. The whole set acts as a single linkage group, which 

 is broken into two parts if crossing-over occurs at any point between 

 its middle segments. A new type of gamete is then produced which 

 is only half-hctcrozygous. Thus crossing-over in the middle of dif- 

 ferential segments must have been suppressed in all the ancestors 

 of a complex hcterozygotc during its building up. The differential 

 segments of opposite complexes in the heterozygous Oenothera 

 species have been separated from one another, isolated, during this 

 evolutionary process, just as much as the chromosomes of different 

 species. 



Since the Oenothera species have incomplete chromosome pairing 

 beginning near the ends, crossing-over is localized near the ends. 

 The situation is therefore prepared for the development of the 

 complex heterozygote before any interchange takes place. 



The complexes of Oenothera are differently adapted. Some fail 

 as pollen. Others fail as embryosacs, and are displaced by their more 

 female-vigorous sister segregates. So different, indeed, are the com- 

 plexes that a triploid endosperm with a 2 : i balance would probably 

 have been an embarrassment in building up the system; and we 

 fmd in fact, that the Oenothera family is remarkable for having a 

 diploid endosperm. 



The complex heterozygous species of Oenothera are, in the 

 aggregate differences between their complexes, at least the equivalent 

 of hybrids between pairs of species. But the differences are mechani- 

 cally concentrated in the differential segments and physiologically 

 adjusted to balance one another. They cannot, therefore, have arisen 

 by hybridization in the ordinary sense. Crossing different stocks 

 enables us to build up by steps a stock of Campatmla persicifoUa with a 

 ring of twelve chromosomes. But the two sets of six into which this 

 ring segregates do not constitute two mutually adapted complexes. 

 The complexes of Oenothera have evidently been built up gradually 

 by internal change. Thousands of generations of almost uninterrupted 

 self-fertilization in species which, as we shall see, were previously 

 cross-fertiUzed, accompanied by the continual elimination of homo- 

 zygotes, have resulted in the production of hybrid species. Hybridity 

 in Oenothera is a matter of crossing different gametes, not different 

 zygotes; a distinaion pointed out by Mendel but still not under- 



262 



