2 28 Experimental Zoology 



extent sterile with the parent type, in the same sense that even 

 closely related species are o]ten sterile. De Vries's statement 

 gives the impression that he regards this property of steriHty as 

 a general property of mutations. But there are other cases of 

 mutations where nothing of the sort is evident. Such types as 

 the ancon ram, the merino sheep, the turnspit dogs, the Japa- 

 nese peacock, the guinea pig with whorled hair, certain breeds of 

 fowls with different combs, show no evidence of infertihty when 

 crossed with the parent type. Furthermore if many of our do- 

 mesticated races owe their origin to the appearance of mutations, 

 and to the introduction of wild elementary species (w^hich in 

 principle may amount to the same thing), it is surprising to find 

 no trace of steriHty between the different breeds, as we should ex- 

 pect if sterility is one of the general peculiarities of mutations. 

 If many Linnaean varieties are elementary species ^ that have 

 arisen as mutations, it is w^ell known that they are, as a rule, per- 

 fectly fertile when crossed. 



It seems likely, therefore, that the sterility of some of the 

 Oenotheras may be the exception and not the rule for elementary 

 species, and that the same principle holds here that Darwin 

 has so ably developed, viz. that fertility is a graded process, 

 not strictly following the hnes of species and varieties, but 

 dependent on the extent of physiological differences between 

 two forms. 



Our examination of the possibihty of new mutations becoming 

 swamped by intercrossing shows that there is no real difficulty 

 to be met in this direction, although it is also apparent that some 

 kinds of mutations may, of course, become lost by crossing.^ 

 The opportunity exists, however, for some of them, especially 

 those that dominate, to find a place in the economy of nature. 

 No one will pretend that all new forms that appear will survive. 



^ Some of them may be "elementary varieties," others local varieties due to 

 external conditions, and others hybrids in which there is a blending of the new 

 and old characters. 



^ Those, for example, in which blending occurs, for this, too, is one of the re- 

 sults of hybridizing even where we have reason to think that the new character 

 arose as a discontinuous variation. 



