120 THE AMERICAN NATURALIST [Vol. LIV 



Similarly, the attractions could not be magnetic, nor 

 could they be due to any kind of diffuse ''physical" 

 forces, such as those that emanate from centers of sur- 

 face tension change or from centers of vibrational dis- 

 turbances. Those who deny linear arrangement, while 

 admitting the mathematically linear linkage results would 

 therefore be driven to assume that the linkage attraction 

 depended on the specific chemical nature of the genes, 

 which, by virtue of their chemical composition, exerted 

 a specific attraction at a distance, as the substances of 

 adsorption compounds are sometimes supposed to do. 

 But such a theory, as a method of accounting for linkage, 

 becomes stretched to the breaking point when it is re- 

 membered that each gene must be assumed to have such 

 an attraction for just two of the others, never more nor 

 less, and that when this attraction is broken it is always 

 exchanged for that of the allelomorph. Moreover, it 

 would be exceedingly hard to reconcile this theory with 

 the finding that changes in the nature of the genes— mu- 

 tations— alter in nowise the sequence of their linkage 

 attractions, and very rarely change even the strength of 

 the linkages. And when we come to analyze the linkage 

 relations in detail, and encounter the phenomenon of 

 interference, we find relations that are entirely at 

 variance with all our preconceptions concerning chem- 

 ical attractions or chemical activity in general,— results 

 that would force us to assume (1) that a breakage of the 

 attraction between two genes leads to an increased at- 

 traction between the other genes and (2) that the amount 

 of this increased attraction ("interference") depends 

 solely on the directness of the connection ("distance") 

 between these other genes and the one whose attraction 

 was broken, being not at all influenced by the chemical 

 nature of the broken attraction, or by the chemical nature 

 of the other attractions themselves. The facts of "inter- 

 ference" or "coincidence" are thus diametrically op- 

 posed to a chemical view of linkage, although they, like 

 all the other facts of linkage, are quite in accord with 



