xiv] INDEPENDENT SEGREGATION 221 



The second chief objection brought on genetic grounds 

 against the chromosome hypothesis is that there may be 

 more Mendelian characters in a species than there are 

 chromosomes, and that, nevertheless, these characters ap- 

 pear to segregate independently of one another 1 . The 

 answer to this objection leads up to a most interesting 

 extension of the hypothesis, due to Prof. MORGAN of New 

 York, and provides an important argument in favour of the 

 hypothesis as a whole. 



If there are more Mendelian characters than chromosomes, 

 and if the chromosomes in some way bear the determiners 

 or "factors" for these characters, one of two things must 

 follow. Either the chromosomes must be made up of smaller 

 units which are to some extent interchangeable, or, if the 

 chromosomes behave as indivisible units, all the factors for 

 characters that are borne by one chromosome must be 

 associated with one another in inheritance. In the earlier 

 cases to be discovered, in which the pairs of Mendelian 

 characters were more numerous than the haploid number 

 of chromosomes, no such association of the characters was 

 known, and several suggestions were made with regard to 

 the manner in which the factors were borne by the chromo- 

 somes. One was that the true units were the microsomes, 

 or small particles of which the leptotene threads are com- 

 posed, and that when the chromosomes unite in pairs these 

 units might be interchanged. Another, based on FICK'S 

 "Manoeuvre-hypothesis," was that in the resting stage of 

 the nucleus the constituent elements of the chromosomes 

 became separated, but came together in the same formation 



1 It is pointed out by WINCE (1919), however, that no case is known in 

 which the number of simultaneously and independently segregating pairs of 

 characters existing together in one individual exceeds the haploid chromosome 

 number of the species. 



