2i >• > 



THE 



[Vol. L 



the factors, as arranged in line in order of their linkage, 

 are exchanged in whole sections at a time. 



In the first section, evidence was presented, showing 

 that groups of factors are connected with particular chro- 

 mosomes, and segregate with them at the maturation divi- 

 sions; this was in fact proved to be true in the case of 

 sex-linked factors, which are found always to segregate 

 with the X-chromosome during spermatocyte divisions. 

 Yet it was conceivable that the factors were not actually 

 in the chromosomes, but rather tied to them by some 

 obscure connection (chemical, physical or metaphysical), 

 although the fact that the relative sizes of the groups cor- 

 respond to the lengths of the chromosomes might be taken 

 as evidence against such a view. On that view, a separa- 

 tion of linked factors would be considered not a physical 

 interchange between the chromosomes themselves, but a 

 transference, by a factor, of its invisible bond, from one 

 chromosome to the homologous one. But Sturtevant's 

 evidence just presented shows that however one may have 

 conceived, a priori, the chemical attraction or physical 

 connection that makes linked factors tend to segregate to 

 the same pole in the maturation divisions— this connec- 

 tion binds them in a linear manner, one after another, in a 

 chain. This unique result, then, constitutes specific evi- 

 dence that the factors are actually in the chromosomes, in 

 an order which can be determined by their linkage rela- 

 tions, and that the separation of linked factors is conse- 

 quently a real interchange between parts of the chromo- 

 somes themselves. 1 



i The fact of linear linkage does not connote that frequency of crossing- 



to escape the conclusion that crossing-over occurs equally often in all parts 

 of the chromosome, by assuming that coincidence of separations A-B and 

 B-C usually occurs with not very different frequency from coincidence of 

 separations G-H and H-I, even if different actual lengths are involved, 

 provided the frequencies of separation are the same in the two cases. And 

 on either way of explaining the results, the factors must be linked in line 

 in the chromosomes in the same order as on the diagram. In fact, no 

 matter how great the differences in frequency of crossing-over in -different 

 parts of the chromosome might be, the linkage order of the factors would 

 still be the same as their real order so long as coincident crossing-over in 

 any two regions did not occur as often as single crossing-over in either region. 



