2 4 o HEREDITARY TRANSMISSION [CH. xiv 



mechanism required for the Mendelian segregation of 

 characters, and when, as in FEDERLEY'S moth-hybrids, the 

 normal segregation of chromosomes did not take place, 

 there was also no Mendelian segregation. This correspon- 

 dence of chromosome behaviour with transmission of 

 hereditary characters is even more striking in the case of 

 the sex-chromosome and characters which show sex-limited 

 (sex-linked) transmission, and perhaps the most remarkable 

 correspondence of all is that seen in the exceptions to normal 

 sex-limited transmission, found by BRIDGES to be associated 

 with "non-disjunction" of the sex-chromosomes. 



(4) Finally, the phenomena of gametic coupling or linkage 

 of characters have been found, in the work of MORGAN and 

 others on Drosophila, to correspond closely with the chro- 

 mosome-condition, for in Drosophila there are four groups 

 of linked characters and four pairs of chromosomes, and all 

 the phenomena of "crossing-over" can be accounted for 

 on a simple hypothesis of chromosome behaviour. 



