OF THE MALAYAN REGION. 5 
of the whole species, which leads to the more or less frequent intermixture of the inci- 
pient varieties, which thus become irregular and unstable. Where, however, a species 
has a limited range, it indicates less active powers of dispersion, and the process of modifi- 
cation under changed conditions is less interfered with. The species will therefore exist 
under one or more permanent forms according as portions of it have been isolated at a 
more or less remote period. 
What is commonly called variation consists of several distinet phenomena which have 
been too often confounded. I shall proceed to consider these under the heads of—1st, 
simple variability; 2nd, polymorphism ; 3rd, local forms; 4th, coexisting varieties; 5th, 
races or subspecies; and 6th, true species. 
1. Simple variability.— Under this head I include all those cases in which the specifie 
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 Papilionidæ is in Papilio Severus, a species inhabiting all the islands of 
the Moluccas and New Guinea, and exhibiting in each of them a greater amount of in- 
dividual difference than often serves to distinguish well-marked species. Almost equally 
remarkable are the variations exhibited in most of the species of Ornithoptera, 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 satisfy- 
ing oneself, by the examination of numerous specimens captured in their native countries, 
that the one set of individuals are variable and the others are not, it becomes evident that 
by classing all alike as varieties of one species we shall be obscuring an important fact 
in nature, and that the only way to exhibit that fact in its true light is to treat the inva- 
riable local form as a distinct species, even though it does not offer better distinguish- 
ing characters than do the extreme forms of the variable species. Cases of this kind are 
the Ornithoptera Priamus, which is confined to the islands of Ceram and Amboyna, and is 
very constant in both sexes, while the allied species inhabiting New Guinea and the 
Papuan Islands is exceedingly variable; and in the island of Celebes is a species closely 
allied to the variable P. Severus, but which, being exceedingly constant, I have described 
as a distinct species under the name of Papilio Pertinax. 
2. Polymorphism or dimorphism.—By this term I understand the coexistence in the 
same locality of two or more distinct forms, not connected by intermediate gradations, 
and all of which are occasionally produced from common parents. "These distinct forms 
generally occur in the female sex only, and the intercrossing of two of these forms does 
not generate an intermediate race, but reproduces the same forms in varying proportions. 
I believe it will be found that a considerable number of what have been classed as 
