DOMINANT FOEMS BECOME MIMICS 176 



unequal degree.^ The resemblance is due to the 

 advantages of a common advertisement. Before 

 the growth of a mimetic likeness, Batesian mimics, 

 it is reasonable to assume, belonged to the immense 

 group of species possessing a cryptic appearance ; 

 Miillerian mimics on the other hand may be 

 assumed to have possessed warning or aposematic 

 colours of their own previous to the adoption of 

 those of another species. This test is more readily 

 applied than might be supposed ; for a comparison 

 with allied non-mimetic species, and with the 

 non-mimetic males of mimetic females, wUl gene- 

 rally indicate whether the ancestral pattern of 

 a species now mimetic belonged to the group of 

 concealing colours or to that of warning. 



The Danaidas invaded North America and 

 entered an assemblage of butterflies of which the 

 dominant species are ancient inhabitants of the 

 northern land-belt. Among them are several, such 

 as the species of Grapta or Polygonia {the 'Comma' 

 butterflies), with beautifully cryptic patterns on 

 the parts of the wing surface exposed in the rest- 

 ing position. No such forms have been influenced 

 by the invaders, but with the whole fauna before 

 them they have only produced changes in the 

 dominant group Limenitis, known throughout the 

 northern belt for a conspicuous under surface and 

 a floating flight ; also believed to be mimicked by 

 other butterflies, e.g. the females of the Apaturas 



' It is probable that relative abundance may determine the 

 relationship of model and mimic in cases where there is no reason 

 for suspecting any difference in the degree of unpalatability. 



