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T II K . 1 ME RICA N NATURALIST [Vol. XLIII 



poultry, as Davenport has pointed out, such characters 

 as pea and rose comb, extra toes and the presence of 

 muffs and beards on the head, are acquisitions which 

 developed since the domestication of the original an- 

 cestral species. They certainly can not be regarded 

 as the outcropping of latent characters which are rep- 

 resented in allied forms, but are in the line of progres- 

 sive variation and therefore according to theory de- 

 pendent upon the production of new kinds oij pangens. 

 any definite rule of inheritance and none of them follow 

 Mendel's law, the extra toes do not seem to come under 

 any definite rule of inheritance, and none of them follow 

 the rule for the hybridization of elementary species. 



Consider the forms of the common potato beetle studied 

 by Tower. These arise suddenly, breed true to type and 

 differ from the parent form in many characters, some of 

 which are apparently in the line of progressive evolution. 

 They seem to be as truly elementary species as the 

 mutants of Oenothera lanuwcVuma. Yet when crossed 

 with the parental type they produce hybrids which in 

 most cases give a mixed progeny segregating according 

 to Mendel's law. If we can not call these forms ele- 

 mentary species there is no way of distinguishing such 

 except through breeding experiments, and the distinction 

 De Vries draws between elementary species and varieties 

 amounts to nothing more than the fact that crosses be- 

 tween certain groups follow Mendel's law, while crosses 

 between others do not. There is no correlation between 

 any structural criterion of species and the criterion 

 afforded by breeding experiments. 



Now when we attempt to make a classification on the 

 basis of breeding experiments alone we fare little better. 

 With blended inheritance in the first and all subsequent 

 generations, partially blended inheritance, total resem- 

 blance of hybrid to one or another parent with or without 

 subsequent splitting, incomplete segregation of char- 

 acters, splitting of offspring of hybrids in various incon- 

 stant and non-Mendelian ratios, and many other irregular 



