158 THE MALAYAN PAPILIONIDE 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, 
