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THE AMERICAN NATURALIST [Vol. XLVIII 



characters shall appear; and Hoge (1914) has shown 

 that certain temperatures are necessary for the develop- 

 ment of reduplicated legs in Drosophila. Other examples 

 may be found in the literature of genetics. 



(c) Lethal Characters. — There have been reported sev- 

 eral instances in recent years of animals and plants which 

 are unable to live if homozygous for certain genes. The 



