ANALYSIS OF HYBRIDS 139 



(and, as we shall see later, linkage) but have no recognisable effect 

 on chromosome behaviour. Probably most factor differences 

 such as those found in Drosophila have a negligible effect on the 

 pairing of the affected chromosomes. There is no evidence to the 

 contrary, and in recent experiments in which wild type and mutant 

 Drosophila have been compared genetically they show no difference 

 in the frequency or distribution of crossing-over (Redfield, 1930) 

 (v. Ch. VII). It will be seen, therefore, that cytological observations 

 show many large differences, as in triploidy, which are incapable of 

 mendelian analysis, while mendelian analysis shows many differences 

 which are small in origin and incapable of cytological analysis. 

 The visible spectra of variation shown by the two methods have 

 different ranges which reveal two different aspects of the problem of 

 variation. 



2. MEIOSIS IN STRUCTURAL HYBRIDS 



(i) Method of Analysis. The conclusion has been reached that 

 only identical blocks or segments of chromomeres can pair at 

 pachytene and form chiasmata which keep the chromosomes 

 associated or " paired " at metaphase. On this assumption 

 structural hybrids may be analysed. The differences between 

 their chromosomes may be defined and hence the simple changes 

 that have given rise to these differences specified. 



Where terminalisation is complete in the organisms that have 

 been studied from this point of view, the earlier association at 

 pachytene, which is the characteristic of most importance, is only 

 shown at metaphase by an association of the ends of the chromo- 

 somes. This association arises, however, in the ordinary way, as 

 has been shown in particular cases {v. infra). 



Where, in a diploid structural hybrid, several chromosomes 

 are associated at meiosis it is better to speak of the configuration 

 as a " group of three " or a " ring of four," reserving the terms 

 trivalent, quadrivalent, and so on, for associations of homologous 

 chromosomes in which every segment is represented as many times 

 as there are chromosomes concerned. 



(ii) Fragmentation and Fusion Heterozygotes. The simplest kind 



