No. 640] SHORTER ARTICLES AND DISCUSSION 459 



tions were valid. But how important is it now to make this 

 decision? Linkage is a fact and must be taken into considera- 

 tion. True, the evidence in support of the chromosome hypoth- 

 esis from maize is not extensive. But hybrid vigor is a wide- 

 spread phenomenon sbown by many organisms. The dominance 

 hypothesis applies 1o DrosophUa as well as 1o maize. 



This failure to look outside of the corn ticl<l bas led Collins 

 to make certain statements to wliirh 1 niust tak(> strong objec- 

 tion. He is inclined to believe tliat tlio su pptvssion of deleteri- 



of wild species"^of botli aniuials an.l plants as well as of naturally 

 self-fertilized varieties of cnltivateil |ilants whidi show an un- 

 mistakable increase in growth after crossing. Take Xaudin's 

 Datura crosses which doubled in height. Koliviitei'"s Mrotiana 

 hybrids which astonished their producer, the hylirid waliuits. 

 both natural and artificial, and Gerschler's fish liybi ids. to name 

 a few notable illustrations. Collins himself luis given us several 

 good illustrations of remarkable vigor shown by hybrids of many 

 varieties of maize from ditrerent pai'ts of the world. Here it i^ 

 clearly not a mattei' of suppressing deleterious characters. The 

 parental types are normal, vigoi'ous and perfectly capable of 



ahles ihe olTspi-ing to lake advaidage of the more favorable 

 factoi-s. This is as tr'ue of doniest iealed races as it is for wild 



Furthermore, there is abundant evi.lence that many factors 

 are without effect unless working in consort. In plants, colors 



