ILLUSTRATIVE OF NATURAL SELECTION. 145 



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 impor- 

 tant fact in nature ; and that the only \> ay to exhibit 

 that fact in its true light 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 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 co-existence 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 their offspring, in- 

 stead of being hybrids, or like the two parents, appear 

 to reproduce all the distinct forms in varying propor- 

 tions. I believe it will be found that a considerable 

 number of what have been classed as varieties are really 

 cases of polymorphism. Albinoism and melanism are 

 of this character, as well as most of those cases in 

 which well-marked varieties occur in company with the 

 parent species, but without any intermediate forms. If 



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