158 THE MALAYAN PAPILIOXIDE AS 



at the end of many generations the men would remain 

 pure white, and the women of the same well-marked 

 races as at the commencement. 



The distinctive character therefore of dimorphism 

 is this, that the union of these distinct forms does 

 not produce intermediate varieties, but reproduces the 

 distinct forms unchanged. In simple varieties, on the 

 other hand, as well as when distinct local forms or 

 distinct species are crossed, the offspring never re- 

 sembles either parent exactly, but is more or less in- 

 termediate between them. Dimorphism is thus seen to 

 be a specialized result of variation, by which new phy- 

 siological phenomena have been developed ; the two 

 should therefore, whenever possible, be kept separate. 



3. Local form, or variety. This is the first step in 

 the transition from variety to species. It occurs in 

 species of wide range, when groups of individuals have 

 become partially isolated in several points of its area 

 of distribution, in each of which a characteristic form 

 has become more or less completely segregated. Such 

 forms are very common in all parts of the world, and 

 have often been classed by one author as varieties, by 

 another as species. I restrict the term to those cases 

 where the difference of the forms is very slight, or 

 where the segregation is more or less imperfect. The 

 best example in the present group is Papilio Agamem- 

 non, a species which ranges over the greater part of 

 tropical Asia, the whole of the Malay archipelago, 

 and a portion of the Australian and Pacific regions. 

 The modifications are principally of size and form, 



