338 THE AMERICAN NATURALIST [Vol. XL VIII 



Occasional fertile crossing of unlike strains that rarely 

 interbreed is a probable source of mutations and new 

 types. 



A suggestion is made that a comparatively simple poly- 

 morphic species (like Abraxas grossnlariata) may break 

 up into a cluster of mutually fertile elementary species 

 (e. g., Adalia in Colorado). Further differentiation, in- 

 volving partial sterility, may be illustrated by the Basi- 

 larchia species-cluster. This may be followed by the 

 establishment, and isolation through complete sterility, 

 of distinct types, or species in the strict sense of the term. 



Evolution of color in the yellow and orange butterflies 

 of the genus Colias involves white, which exists to-day in 

 heterozygous condition in certain females. If the an- 

 cestors of Colias were white, as in Pierids generally, we 

 have only to imagine a mutation in the male-producing 

 germ cells of the original white females, by virtue of 

 which white pigment was replaced by, or transmuted into, 

 yellow. This would make all the males yellow, leaving 

 all the females white, which is true of certain arctic 

 species to-day. 



A similar mutation affecting the germ cells of these 

 white females, but introducing the factor for yellow into 

 only half of them, would produce the heterozygous condi- 

 tion found in C. philodice and C. eury theme. Pure yel- 

 low strains may readily be bred from such mixed stock, 

 and hence, probably, it has come about that four fifths or 

 nine tenths of the females of C. philodice in eastern 

 United States are pure yellow. 



Progressive mutations from yellow to orange and fiery 

 orange, affecting first the male, then the female, have 

 probably occurred in Colias in many part of the world, 

 especially in warmer climates. Climatic conditions deter- 

 mine the amount of orange pigment in the cross between 

 the orange eurytheme and the yellow philodice. This 

 hybrid is larger and contains more orange when raised in 

 summer than when bred in late fall and winter. C. philo- 

 dice in this cross is a Mendelian recessive. 



