No. 631] 



FACTORS OF HEREDITY 



119 



maintains; consequently any system of representing 

 linkage which permitted or showed such values would 

 not be ipso facto inconsistent. The mere fact that all 

 factors hitherto worked with in a single chromosome 

 have less than 50 per cent, of separation, and that those 

 in different chromosomes have just 50 per cent., does not 

 mean that factors can never be found which are so far 

 apart, and which lie in such a rigid chromosome (little 

 double crossing over) that they separate more often than 

 they remain together at segregation. Whether this 

 phenomenon should then be called linkage is but a ques- 

 tion of words ; the chromosomes themselves would have 

 no regard for the 50.0 per cent, mark, or for the idio- 

 syncrasies of our terminology. 



The proof of the law of linear linkage, including all 

 the main aspects of it which have been given above, has 

 been stated on several previous occasions. It seems un- 

 fortunate that the argument has had to be repeated each 

 time that a new "theory of crossing over" has arisen, 

 for the discussion and data given in the original papers 

 supply all the material necessary for a decision of the 

 matter, at least so far as the germ plasm of Drosophila 

 is concerned. 



Before closing, it may be desirable to supplement these 

 arguments for a mathematically linear mode of linkage, 

 by a statement of the considerations which indicate that 

 this mathematically linear linkage can have its basis 

 only in a linear physical connection between the genes. 



If the genes are not spatially arranged, or physically 

 connected, in the same linear sequence as that in which 

 they have been found to attract each other in linkage, 

 then the forces of linkage attraction must be such as to 

 "act at a distance." But, although acting at a distance, 

 these linkage forces must nevertheless be extraordinarily 

 specific— binding each gene directly to just two specific 

 associate genes. Hence the forces could not be of an 

 electrical nature, for, since there are only two kinds of 

 electricity, electric forces could not be specific enough. 



