xiv] CHIASMATYPE HYPOTHESIS 229 



so forth. The evidence for some of these subsidiary hypo- 

 theses is strong, for others less satisfactory, but it remains 

 true that no other hypothesis that has been suggested fits 

 the main groups of facts in at all the same degree as that of 

 MORGAN and his school, and it must be concluded that the 

 "chiasmatype" hypothesis, as MORGAN prefers to call it, 

 adopting the term from JANSSENS, is the only one which 

 seems at all nearly to approach the truth. 



Before leaving this section of the subject it should be 

 noted that in Drosophila all the known Mendelian characters 

 fall into one or other of the four groups of coupled characters, 

 while in all other species hitherto investigated many or all 

 of the characters show independent inheritance. This may be 

 due to the small number (four) of the haploid chromosomes 

 in Drosophila, but another possible explanation is that in 

 Drosophila the chromosomes are perhaps only slightly 

 twisted, so that only one or two breaks occur in each pair. 

 If, however, they were more tightly twisted, so that a con- 

 siderable but varying number of breaks and recombinations 

 took place, it would be an even chance whether two factors 

 not very near together in the chromosome remained together 

 or became separated, and independent inheritance would 

 result. In such a case coupling would only exist between 

 factors that were arranged quite near one another in the 

 chromosome. 



The last and perhaps most important evidence in favour 

 of the theory that the factors for inherited characters are 

 borne by chromosomes is derived from the behaviour of the 

 sex-chromosomes already described in Chapter XI. Here 

 again, some of the most important work has been done in 

 Prof. MORGAN'S laboratory, and although this aspect of the 

 matter is not very intimately related to the chiasmatype 

 hypothesis, the two are nevertheless closely connected. It 



