144 THE MALAYAN PAPILIONIDZ AS 
3rd, local forms; 4th, co-existing varieties; 5th, races 
or subspecies ; and 6th, true species. 
1. Simple variability—Under this head I include all 
those cases in which the specific form is to some extent 
unstable. Throughout the whole range of the species, 
and even in the progeny of individuals, there occur 
continual and uncertain differences of form, analogous 
to that variability which is so characteristic of domestic 
breeds. It is impossible usefully to define any of these 
forms, because there are indefinite gradations to each 
._ other form. Species which possess these characteristics 
have always a wide range, and are more frequently the 
inhabitants of continents than of islands, though such 
cases are always exceptional, it being far more common 
for specific forms to be fixed within very narrow limits 
of variation. The only good example of this kind of 
variability which occurs among the Malayan Papilio- 
nid is in Papilio Severus, a species inhabiting all the 
islands of the Moluccas and New Guinea, and exhibit- 
ing in each of them a greater amount of individual 
difference than often serves to distinguish well - 
marked species. Almost equally remarkable are the 
variations exhibited in most of the species of Ornithop- 
tera, which I have found in some cases to extend even 
to the form of the wing and the arrangement of the 
nervures. Closely allied, however, to these variable 
species are others which, though differing slightly from 
them, are constant and confined to limited areas. After 
satisfying oneself, by the examination of numerous 
specimens captured in their native countries, that the 
