ON THE MODIFICATION OF CHARACTERS BY 

 CROSSING 1 



R. RUGGLES GATES 

 University of London 



Ix the early years of Mendelian discovery there was 

 much discussion concerning gametic purity in hybrids, 

 and the question whether unit characters are modified on 

 crossing was keenly debated. Convinced by the numer- 

 ous instances in which Mendelian characters appear to 

 be unmodified by crossing, many writers came to the con- 

 clusion that characters universally segregate without 

 being modified or "contaminated" by association with 

 other characters in the hybrid. That such a conclusion is 

 far too sweeping is, however, indicated by many later 

 results, and there is now a disposition to admit that 

 changes in a character or the breaking up of a character 

 may be effected through crossing. But some writers con- 

 tinue to look upon a unit character as an entity, which is 

 unmodifiable and indestructible by hybridization. 



Notwithstanding the admitted belief of Bateson and 

 others that characters may be modified by crossing, I 

 know of no extensive body of evidence that such modifica- 

 tions take place except the work of Castle and Phillips 

 (1914) whose conclusions have not been fully accepted 

 and are chiefly concerned with modification by selection. 

 It therefore seemed worth while to direct attention to 

 certain experimental results of a somewhat different kind 

 which appear to show beyond cavil that modifications of 

 characters sometimes result from crossing. The matter 

 is an important one because it affects the old question of 

 the swamping of new characters through crossing, as well 

 as various other aspects of evolutionary theory. 



iRead before the American Genetic Association, San Francisco meeting, 

 August 3, 1915. 



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