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THE AMERICAN NATURALIST [Vol.XLIX 



by recent work on hybrids. We have pictured them as 

 actual chromosome functions, because every part of the 

 description has been actual fact as far as the breeding 

 experiments go. Our picture, it is true, is fictitious, for 

 we do not know absolutely that the heredity mechanism is 

 of this nature. But the facts do fit perfectly all that is 

 known of chromosome behavior. It seems impossible, 

 therefore, that there should be so many coincidences. 



There are also two other pieces of evidence that fit in 

 and round out the case. Bridges has shown that females 

 occasionally occur in Drosophila bearing the sex-linked 

 characters borne by the mother but showing no influence 

 of those borne in the father. Such exceptional females 

 were found to inherit directly from their mother the power 

 of producing like exceptions, and it was proven cytolog- 

 ically after the prediction had he en made from the breed- 

 ing facts that these females resulted from the non-disjunc- 

 tion of the X chromosomes at the maturation of the eggs 

 from which they came, and that one half of their daughters 

 did in fact contain a Y chromosome in addition to two X 

 chromosomes. This appears to be definite proof that sex- 

 linked genes are borne by the X chromosomes. 



The other important basis for regarding the chromo- 

 somes as the material basis for heredity also comes from 

 Morgan's work on Drosophila ampelophila, this being the 

 only species upon which sufficient work has been done to 

 give a reasonable basis for the conclusion. All of the hun- 

 dred and thirty or so mutations in this species upon which 

 Morgan and his students have worked are so linked to- 

 gether in heredity that they form four groups correspond- 

 ing to the four pairs of chromosomes found in the species. 

 If one single character should be found that did not fit into 

 one of these four groups, the whole theory would break 

 down. But no such character has appeared, 



This completes the case for the chromosomes as regards 

 the main facts, and it seems only proper that a fair- 

 minded jury of scientists should render verdict for the 

 plaintiff. Xo case is so bad, however, that a lawyer can 



