hybrid offspring resulting from crosses be- 

 tween the mutant and its parent will have the 

 unadapted new form, and when the selection 

 becomes extreme, not only will all the pure- 

 bred specimens of the new form be destroyed, 

 but all the hybrids as well, and in this way 

 ^very vestige of the new form will be entirely 

 lost Assuming, on the other hand, that the 

 mutant is recessive to its parent but that in 

 other respects the conditions are the same as 

 before, the extreme selection that is assumed 

 to destroy all the recessive individuals, leaves 

 the heterozygotes living because they have the 

 successful form possessed by the parent 

 species. These successful heterozygotes give 

 rise to a progeny in the next generation in- 

 cluding the recessive form, and also a con- 

 siderable percentage of heterozygotes that may 

 carry the form on to still another generation, 

 and in this way the recessive mutant may be 

 preserved indefinitely under the protection of 

 the dominant characteristics of its more suc- 

 cessful parent. Such prolongation of the life 

 of a recessive may serve to tide it over times 

 of special stress, or may continue its exist- 

 ence until the various distributing agents 

 have carried it beyond the limits of the habitat 

 in which it is a failure into others in which 

 it may become a success. 



GEORGE H. SHULL 



STATION FOE EXPERIMENTAL EVOLUTION, 

 COLD SPUING HABBOB, L. I. 



