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 distinct phenomena which have 

 been too often confounded. I shall proceed to consider these under the heads of — 1st, 

 simple variability ; 2nd, polymorphism ; Srd, local forms ; -Ith, coexisting 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 narroAv 

 limits of variation. The only good example of this kind of variability which occurs among 

 the Malayan Papilionidas is in Papilio Sever us, 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 Ornitlioptera, 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 

 ditfering slightly from them, are constant and confined to limited areas. After satisfy- 

 ing oneself, by the examination of niimerous specimens captured in their native coimtries, 

 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 foi*ms 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. Severi(s, but which, being exceedingly constant, I have described 

 as a distinct species under the name of Papilio Pertinax. 



2. Pohjmorpliism or dimorphism. — By this term I imderstand 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 



