174 Fhenomena of Variation and Geographical Distribution. 



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 

 segregated more or less completely. Such forms are very common in 

 all parts of the world, and have often been classed^ as varieties or 

 species alternately. 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 

 Fapilio Agamemnon, L., a species which ranges over the greater part 

 of t)-opical Asia, the whole of the Malay archipelago, and a portion of 

 the Australian and Pacific regions. The modifications are principally 

 of size and form, and, though slight, are tolerably constant in each 

 locality. The steps, however, are so numerous and gradual that it 

 would be impossible to define many of thim, though the extreme forms 

 are 'sufliciently distinct. Papilio Sarpedon, L., presents somewhat 

 similar but less numerous variations. 



4. Co-existing variety. — This is a somewhat doubtful case. It is when 

 a slight but permanent and hereditaiy modification of Ibrm exists in 

 company Avith the parent or typical form, without presenting those in- 

 termediate gradations which would constitute it a case of simple var- 

 iability. It is evidently only by direct evidence of the two forms 

 breeding separately that this can be distinguished from dimorphism. 

 The difficulty occurs in Papilio Jamn, Esp., and P. Evemon, Bd., 

 which inhabit the same localities, and are almost exactly alike in form, 

 size, and coloration, except that the latter always wants a very con- 

 spicuous red spot on the under surface, which is found not only in P. 

 Jason, but in all the allied species. It is only by breedhig the two in- 

 sects that it can be determined whether this is a case of a co-existing 

 variety or of dimorphism. In the former case, however, the difference 

 being constant and so very conspicuous and easily defined, I see not 

 how we could escape considering it as a distinct species. A true case 

 of co-existing forms would, 1 consider, be produced, if a slight variety 

 had become fixed as a local form, and afterward been brought into 

 contact with the pai-ent species with little or no intermixture of the 

 two ; and such instances do very probably occur. 



fact; yet the phenomena liere brought forward as existing in the insect world are still more ex- 

 traordinary; for each mother is capable not only of producing male ofl'spring like the father, 

 and female like herself, but also of producing other females exactly like her fellow-wife, and 

 altogether differing from herself. If an island could be stocked with a colony of human 

 beings having similar physiological idiosyncrasies with Papilio Pammon or Papilio Ormcnus, 

 we should see white men living with yellow, red and black women, and their ottspriug always 

 reproducing the same types; so that 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. 



