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THE AMERICAN NATURALIST [Vol. LI 



that this phenomenon is one of the fundamental attributes 

 of Mendelian inheritance 



While the linkage relations of genes do not at present 

 have any immediate bearing on our conception of the 

 nature of genes, they have a very important bearing on the 

 problem of the localization of genes in the germ plasm. 



The original evidence that Weismann accepted to show 

 that determiners are carried in the chromosomes, viz., 

 the evidence based on transmission through the proto- 

 plasm-free head of the spermatozoon, was made much 

 stronger from Boveri's evidence derived from experi- 

 mental embryology. 



The argument became still more convincing when the 

 facts of sex-linked inheritance and non-disjunction were 

 established. For, it was found that certain characters 

 have the same distribution as do the sex chromosomes, 

 and secondly by the actual cytological demonstration that 

 the rare exceptions to the rule are due to irregularities in 

 the distribution of the sex chromosomes. 



All of this evidence has played a role in persuading us 

 that the genes postulated for Mendelian inheritance have 

 a real basis and that they are located in the chromosomes. 

 Finally, in Drosophila, where there are four pairs of 

 chromosomes, there are also four great groups of linked 

 genes. This coincidence adds one more link to the chain 

 of evidence convincing a few of us that the gene in Men- 

 delian inheritance has a real existence. 



CoNSroERATION OF CRITICISMS 



I have tried to make clear how the genetic evidence has 

 necessitated tlie assumption of genes in heredity, and I 

 have pointed out wliat seem to me to be some of the at- 

 tributes that it has been desirable to add to the earlier 

 ooiK-ciitioii of the -pjH. as our knowledge has increased, 

 XoNv that th*- -r.mihl i< ch^aivd, h't nic ti'v to answer the 

 obj,.,-tion~ (M- .TiticiHn-^ u-hirh 1 nuMitioncd at the start, 

 that have hvvn advanced against tliis kind of hypothesis. 



