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THE AMERICAN NATURALIST [Vol. XLIV 



Rosenberg has shown that the irregularity of certain 

 first-generation Hieracium hybrids is accompanied by 

 irregularity in the number of chromosomes present. 



Quite recently Gager,.in his study of the effect of ra- 

 dium emanations on plants, has not only produced mutants 

 apparently of the character of those found by de Vries, 

 but has shown that in mitosis in treated specimens there 

 is irregularity in the distribution of chromosomes. The 

 writer some time ago suggested a similar explanation for 

 the interesting work of McDougall in whicli mutants were 

 produced by chemical stimulants. The behavior of the 

 Rubus hybrid produced by Burbank, and resulting in the 

 Primus berry, is so closely parallel to that of Uviracium 

 hybrids studied by Rosenberg that a similar explanation 

 at once suggests itself. 



Boveri's classic work on the relation of chromosomes 

 to development strongly supports the view that the de 

 Vriesian mutants are due to loss or exchange of chro- 

 mosomes, and this view is consistent with the work of 

 Wilson, Morgan, Stevens and others on the relation of 

 the chromosomes to sex. 



I think, therefore, that we are in position to recognize, 

 at least tentatively, a new type of variation, which is due 

 to irregularities in the distribution of eliromosomes, and 



tions, being hereditary, arc amenable to the action of 

 natural selection, and must therefore he recognized as one 

 method of evolution. In a certain sense these mutations 

 are discontinuous, and the reason for this is apparently 

 clear. 



"When we consider all the facts in the case, which I have 

 not time here to outline, I think that few biologists will 

 contend that all evolutionary changes are due to Men- 

 delian recombination, or to de Vriesian mutation as de- 

 fined above. The enormous diversity in groups having 

 the same number of chromosomes, as well as observable 

 differences in the chromosomes themselves, re 

 such view untenable. I therefore postulate a fourth kind 

 of variation due to fundamental changes in what we may 



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