No. 592] THE MECHANISM OF CROSSING-OVER 197 



other non-sex-linked group ; these were assigned to Group 

 III or Chromosome III (15). Incidentally, it was evi- 

 dent that these cases are of exactly the same nature as 

 those previously discovered by Bateson and Punnett in 

 the sweet pea, and termed by them "coupling" or "re- 

 pulsion." Moreover, the chromosome interpretation 

 made it clear why the factors should be "coupled" or "re- 

 pelled" according to whether the hybrid received them 

 from the same or from opposite parents. There was only 

 one difference in detail between the facts in the two spe- 

 cies : it was discovered by Morgan that in Drosophila the 

 linkage is always complete in the male, the separation of 

 factors that are linked to each other occurring only in the 

 female (9) ; in the plants, on the other hand, recombi- 

 nation occurs in the genesis both of eggs and of sperm. 



Since that time the inheritance of over one hundred 

 pairs of factors of Drosophila has been studied. This 

 investigation should give an extensive experimental test 

 of the theory of chromosomal interchange, for if linked 

 factors are those carried by the same chromosome, there 

 should be the same number of groups of interlinked fac- 

 tors as there are pairs of chromosomes. There are four 

 pairs of chromosomes in Drosophila— two pairs of long 

 ones, the pair of moderately long sex-chromosomes, and 

 a pair of very small chromosomes. By 1913, work had 

 been done upon a large number of factors, and the results 

 showed that all these factors were linked in one of the 

 three groups already discovered. But in 1914 the author 

 found a pair of factors independent of these (bent vs. 

 straight wing), i. e., constituting Group IV (12), and not 

 long afterwards Miss Hoge found another pair of factors 

 in this fourth group (eyeless vs. normal eye), (3). Ac- 

 cordingly, the number of groups and of chromosomes now 

 correspond, and not only that, but the relative sizes of the 

 groups correspond in a general way with the relative 

 lengths of the chromosomes. Can it be mere chance that 

 one hundred factors fall into this particular grouping? 

 But if it is admitted that these groups are carried in the 



