FLOATING AND FIXED DISCONTINUITY 



that of the other two. Two sequences, the so-called Santa Cruz 

 and Standard types, have floated (together, possibly, with others 

 of which all trace has been lost) in the common ancestor of 

 pseudoohscura and persimilis. They and their derivatives stiU float 

 within pseudoohscura, but persimilis has neither Santa Cruz nor any 

 of its derivates. The Standard type has been fixed in the populations 

 ancestral to this species, even though it has remained floating in the 

 populations ancestral to pseudoohscura. Since the time of this fixation, 

 the lines of variation of Standard have, of course, separated in the 



INVERSION . 



ABCDEF >- AEDCBF 



ABCOeF 



ABcDeF 



AEdCbF 



Fig. 79. — ^The occurrence of an inversion divides the affected chromosomes into 

 two lines, the old and the new, genetically isolated from one another in that recom- 

 bination is prevented in those segments affected by the inversion. Genie heterozygotes 

 which are also structural heterozygotes can give only the parental types; but genie 

 heterozygotes which are structural homozygotes can give rise to new types by 

 recombination. Thus progressive genetical adjustment must proceed independently 

 in the two lines of chromosome descent distinguished by the inversion. 



two Species, so that although this sequence is itself common to them, 

 each of its various derivatives occurs in only one of them (Fig. 78). 



The transition from the floating to the fixed condition is seen 

 also in interchanges, and the plants which show us this transition 

 enable us also to compare its progress in inbreeding and outbreeding 

 forms. For example, in the inbreeding species. Datura stramonium, 

 different interchange types have quickly become fixed, that is to 

 say homogeneous and homozygous, in different parts of the world 

 (Table 26). 



When plants from various places are crossed, rings of 4 and 6 

 chromosomes are produced at meiosis in the hybrids. The different 

 frequencies of the several interchange types in different parts of the 

 world where more than one type is found indicates that selection 

 is favouring one type in one region and another in another. It is 



315 



