Phenomena of Variation and Geographical Didrihuthn. 167 



New Guinea, and exhibiting in each of thera a greater amount of in- 

 dividual difference than often serves to distinguish well-marked spe- 

 cies. Almost equally remarkable are the variations exhibited in most 

 of the species of Ornlthoptera, 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 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 shjill be ob- 

 scuring an important fact in nature, and that the only way to exhibit 

 that fact in its true life is to treat the invariable local form as a distinct 

 species, even though it does not offer better distinguishing 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 Araboyua, 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. Severns, but which, being exceedingly constant, I have 

 described as a distinct species under the name of Papilio Pertinax. 



2. Polymorphism or Dimorphism.— By this terra I understand the 

 co-existence in the same locality of two or more distinct forms not 

 connected byjintermediate 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 varieties are really cases of pohj- 

 morphism. Albinoisra and melanism are of this character, as well as 

 most of these cases in which well-marked varieties occur in company 

 with the parent species, but without any intermediate forms. Under 

 these circumstances, if the two forms breed separately, and are never 

 reproduced from a common parent, they must be considered as distinct 

 species, contact without intermixture being a good test of specific 

 difference. On the other hand, intercrossing without producing an 

 intermediate race is a test of dimorphism. I consider, therefore, that 

 under any circumstances the term, " variety," is wrongly applied to 

 such cases. 



The Malayan Papilionidre exhibit some very curious instances of 

 polymorphism, some of which have been recorded asS'arieties, others, 

 as distinct species; and they all occur in the female sex. Papilio 



