70 ZOOLOGY 



it might well seem that this could be explained on the 

 supposition that each group of characters was due to a 



r\ 



Drawing by R. W-eber (after Morgan) 



FIG. 1 6. Linkage and crossing-over. A, B, determiners in the same chromosome, 

 are linked. But at synapsis (2) the chromosomes divide, and the upper half of 

 each becomes attached to the lower half of the other (3). Then A and b will go 

 together, no longer A and B. The division may occur at any point, or at more 

 than one point, but the nearer the determiners are together, the less likely are they 

 to be separated. 



single inherited factor, which gave rise to various re- 

 sults. This, however, was negatived by the fact that 

 the characters were not necessarily associated, but only 

 generally so. It gradually became evident that the 

 phenomenon, known as linkage, had to do, not with the 

 identity of the factors, but with their occurrence in the 

 same chromosome. This was confirmed by the dis- 

 covery that the number of such groups in Drosophila 

 corresponded with the number of chromosomes. Should 

 this theory be true, how might we account for the fact 

 that linkage is not absolute, that there are exceptions ? 

 Indeed, not only are there exceptions, but they evidently 

 follow some rule, certain of them being much more fre- 

 quent than others. The idea was suggested that per- 



