134 REVERSION AND ALLIED PHENOMENA 



to conclude that " true atavism, or reversion caused by an innate 

 latent tendency, seems to be very rare," and that most of the 

 botanical instances are due to crossing. He calls this false 

 reversion " vicinism," as indicating the sporting of a variety 

 under the influence of others in its vicinity. " Crossing and pure 

 variability are wholly distinct groups of phenomena, which 

 should never be treated under the same head, or under the same 

 name." He does not deny in any way the numerous " rever- 

 sions " which gardeners describe ; he simply points out (with 

 much circumstantial evidence to warrant his contention) that 

 nearly all these ordinary " reversions " are due to crosses. He 

 shows, for instance, how a famous case, the reversion of the 

 " Tuscarora " variety of American corn cultivated by Metzger 

 in Baden, may be readily interpreted as a typical instance of 

 vicinism. Why .the offspring of hybrids should revert to the 

 parental type is another question, to which we shall return in 

 the chapter on Mendelism (Chapter X.). 



Two white-flowered sweet-peas are crossed, and the result 

 is a progeny with the wild, purple flowers. Two smooth 

 stocks are crossed, and the result is a progeny with the original 

 hoary, ancestral type. These cases are what Darwin called " re- 

 version on crossing." But, as Mr. Bateson says, " such reversion 

 is nothing but the meeting of two parted complementary 

 elements, which have somehow been separated by variation." 



Thus it is possible that many so-called reversions may be 

 simply Mendelian phenomena in disguise. 



8. Reversion of Retrogressive Varieties 



Within a species it is often possible to distinguish several 

 subspecies or " elementary species " (De Vries), which differ 

 from one another in many characters affecting many organs. 

 Thus in the species called Draba verna, or whitlow grass, there 

 are two hundred or so minor groups, like constellations within 



