CHAPTER XI 

 LIMITATION OF THE LINKAGE GROUPS 



It may be questioned whether we are at present justi- 

 fied in speaking of the limitation of the linkage groups 

 to the number of chromosome pairs as one of the funda- 

 mental principles of heredity, since the only species in 

 which a correspondence that is numerically significant 

 between the two has been proved is Drosophila melano- 

 gaster. But despite the absence of other positive evi- 

 dence, the fact that in no other animal or plant does the 

 number of linkage groups exceed the number of the 

 chromosome pairs, may be, I think, legitimately inter- 

 preted in favor of the view. 



It may also be argued, that if the phenomena of linkage 

 are assumed to be due to the genes being carried by the 

 chromosomes,it follows that there could be no more groups 

 of linked genes than there are chromosome pairs ; hence 

 one relation is the direct outcome of the other. But the 

 proof of the linear order that has been developed here 

 rests directly on the linkage data, and is independent of 

 any assumption concerning the chromosomes. It has been 

 shown, secondarily so to speak, that the chromosomes ful- 

 fill all the requirements of the abstract reasoning from 

 the data, and therefore give a mechanism capable of per- 

 forming all that the theory demands. The demonstration, 

 then, that in Drosophila the linkage groups correspond 

 in number to the chromosome pairs may be taken as a con- 

 clusion or a discovery independent of the other relations 

 furnished by linkage. If then, as I anticipate ^\ill be the 

 case, further work in other groups should show that the 

 same relation holds everywhere, w^e should be fully war- 

 ranted in stating the result as one of the general prin- 

 ciples of heredity. 



133 



