No. 509] THE CATEGORIES OF VARIATION 265 



not seem to be an invariable occurrence. Davenport has 

 shown that albinism in poultry is in some cases a non- 

 Mendelian character, and the same is probably true, ac- 

 cording to Castle, for the lop-eared condition in rabbits. 

 In the inheritance of long and short hair in guinea pigs 

 there is also a marked departure from Mendelian ratios. 

 In silkworms Kellogg has shown that while most features 

 are Menaelian, cocoon color in some cases follows Men- 

 delian ratios, but in others it proved to be " inconstant 

 as to dominance and recessiveness and numerical propor- 

 tions, and may even break down and blend." Deaf- 

 mutism also refuses to come under Mendelian categories 

 according to the statistics compiled by Bell. The fore- 

 going are cases of apparent retrograde variations which 

 form an exception to Mendel's law, but it must be ad- 

 mitted that the majority of such variations which have 

 been investigated show a fair approximation to Men- 

 delian ratios. 



In the crosses between the elementary species of 

 (Enothera lamarckiana there is commonly a splitting 

 up in the first generation with absence of splitting in the 

 second and subsequent ones. Hybrids of 0. Lamarckiana 

 and 0. biennis, however, have nearly the aspect of the 

 latter species and remain true in the second and subse- 

 quent generations without reversion or splitting. Crosses 

 between 0. muricata and 0. Mrtella produced hybrids 

 showing the characters of both parents. These were 

 propagated through four generations and remained "true 

 to /this type, showing <&nly slight fluctuations and never 

 reverting or segregating the mixed characters." In- 

 stances of constant hybrids between different species are 

 very common and it is unnecessary to specify them here. 

 Such constancy according to De Vries is ' i one of the best 

 proofs of unisexual unions" or unions between distinct 

 elementary species in which there is always one unpaired 

 pangen. 



The attempt to make a general rule for the hybridizing 

 of elementary species leads to many difficulties. In 



