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THE AMK NIC AN NATURALIST [Vol.L 



gation may lead to the production of pure recessives, 

 lacking all the plural factors which give the Lamarckiana 

 aspect. These recessives are the supposed mutations. 

 Pure dominants, on the contrary, can not be realized. 



This, in brief, is the Mendelian explanation of muta- 

 bility. It involves the important assumption that the 

 mutations which breed true are Mendelian recessives. 

 The mutations with irregular chromosome numbers have 

 been shown not to belong in this category. The remaining 

 mutations, for many of which the cytological data are 

 lacking, may conveniently be divided into two classes, (1) 

 those which come true when self-pollinated, or, at any 

 rate, do not include the parent species in their progeny, 

 and (2) those which give a mixed progeny consisting of 

 the mutational and parental forms. If there is any possi- 

 bility whatever that the Mendelian explanation of muta- 

 bility is true, it should at least account for the first and 

 simplest of these two cases. We shall therefore confine 

 our attention for the moment to mutations which give a 

 constant progeny. 



De Vries found that certain of the original mutations 

 from (Enothera Lamarckiana were of the Mendelian type. 

 These mutations are assumed by Heribert-Nilsson to be 

 recessives which have corresponding homozygous domi- 

 nants, the latter being the strains of (E. Lamarckiana 

 which do not give rise in every generation to the muta- 

 tions in question. Other mutations, isolated by Heribert- 

 Nilsson himself from (E. Lamarckiana, are produced in 

 every generation, and are therefore, according to this 

 author, recessives which have no corresponding homo- 

 zygous dominants. If this were the case, they would be 

 recessive when crossed with (E. Lamarckiana regardless 

 of which way the cross was made. As a matter of fact, 

 Heribert-Nilsson made his crosses with (Enothera La- 

 marckiana as the pistillate parent, and therefore obtained 

 the results which he expected. If the crosses had been 

 made the other way, there is very good reason to believe 

 that he would have got the most unexpected results, and 

 would never have advanced his Mendelian hypothesis. 



