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 way 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 
L 
