DIFFERENTIAL AFFINITY 207 



pachytene are structural ; (ii) that pairing is limited amongst 

 identical segments in a polyploid when they are not in the same 

 sequence throughout the chromosomes ; and (iii) that pairing at 

 metaphase is determined by the formation of chiasmata in the 

 pachytene thread proportionate in number to the length paired. 

 The causal sequence is illustrated in the diagram (Fig. 73). 



On such assumptions the corresponding chromosomes derived 

 from opposite species might be represented as a series of gene-blocks 

 or segments ahcdefg and adfgbe. When two such chromosomes are 

 free to pair together they may do so with great regularit}^ but in 

 the presence of identical mates (in the normal allotetraploid of the 

 constitution ahcdefg, ahcdefg and adfgbe, adfghe) they will rarely if 

 ever pair, on account of d, h and e being in a different sequence 

 in the chromosomes of the two species. These observations of 

 differential affinity are therefore in accordance with the theories of 

 structural hybridity and chromosome pairing arrived at on other 

 grounds. 



All the different conditions of autosyndesis have not been obtained 

 in one and the same polyploid, but it is possible to associate groups 

 of species according to their behaviour in autosyndesis, since, 

 whatever the conditions, this pairing indicates a certain degree of 

 similarity between the pairing chromosomes. The classification is 

 arbitrary, the more so since the observations on which it is founded 

 have generally been defective in two ways : (i) they have taken 

 little account of variations in the frequency of pairing from cell to 

 cell ; (ii) they have overlooked the formation of multivalents. 

 Comparison, however, shows a close analogy between the results of 

 the synthesis of new species by doubling and analysis of old species 

 by parthenogenesis and hybridisation. It shows in allopolyploid 

 species a range of variation from the Primula kewensis type to the 

 Raphanus-Brassica type. 



5. CLASSIFICATION OF POLYPLOID SPECIES 



Observations of differential affinity show that every gradation 

 occurs in the relationship of the homologous chromosomes in the 

 gametic complement of polyploids. As a rule these chromosomes 

 can be considered in sets, for the same conditions have operated 



