Xo. 609] 



THE THEORY OF THE GENE 



527 



new mode ]ias been estahlislicd unless a mutation lias 

 occurred. This lattei' lutci i >rt'tati()ii is indeed in contra- 

 diction to the idea that tlie g-ene is a single molecule, for 

 molecules are not supposed to vary about a mode. At 

 present either interpretation is compatible with the evi- 

 dence, which does not discriminate between them. 



(C) Non-Contamination of Genes 

 At the time when Darwin wrote and for many years 

 afterward it was supposed that any new or unusual traiit 

 of character w^ould become obliterated by repeated cross- 

 ing w-ith the normal or average individual of the species. 

 This was perhaps the most serious difficulty tliat l)ar- 

 w^in's theory of natural selection met W'ith. It will be re- 

 called that in order to overcome it Darwin made a con- 

 cession that in jirinciple amounted to an abandonment of 

 the origin of (4iaracters through natural selection of 

 chance variations, lie admitted that only when a new 

 character appealed in a large nnmher of ind!\idnal> at 



petuation. 



to dominant individna'^. ulimrxn the rliara.-trr .Mnrm«- 

 from the .-lu^^. it ^iiuu^ at c-nc- tiiat it^ u.-n.- iui^ not brrn 

 contaminati'd hy (M.ntact witli other genes. This conclu- 

 sion is an enonnons gain for tlie theory of natural selec- 

 tion based (111 rliance variation, and at the same time is an 

 equal :> -tion- ar-nnu^nt to show that genes remain stable, 

 and are nol infected oi' niix(Ml in the ])resenc(> of other 

 contrasting genes." Let nie illustrate h\ a ease of my own. 



