ILLUSTRATIVE OF NATURAL SELECTION. 159 



and, though slight, are tolerably constant in each local- 

 ity. The steps, however, are so numerous and gradual 

 that it would be impossible to define many of them, 

 though the extreme forms are sufficiently distinct. Pa- 

 pilio Sarpedon presents somewhat similar but less nu- 

 merous variations. 



4. Co-existing Variety. This is a somewhat doubtful 

 case. It is when a slight but permanent and heredi- 

 tary modification of form exists in company with the 

 parent or typical form, without presenting those inter- 

 mediate gradations which would constitute it a case 

 of simple variability. It is evidently only by direct 

 evidence of the two forms breeding separately that this 

 can be distinguished from dimorphism. The difficulty 

 occurs in Papilio Jason, and P. Evemon, whioh in- 

 habit the same localities, and are almost exactly alike 

 in form, size, and colouration, except that the latter 

 always wants a very conspicuous red spot on the under 

 surface, which is found not only in P. Jason, but in all 

 the allied species. It is only by breeding the two in- 

 sects that it can be determined whether this is a case of 

 a co-existing variety or of dimorphism. In the former 

 case, however, the difference being constant and so very 

 conspicuous and easily defined, I see not how we could 

 escape considering it as a distinct species. A true case 

 of co-existing forms would, I consider, be produced, if 

 a slight variety had become fixed as a local form, and 

 afterwards been brought into contact with the parent 

 species, with little or no intermixture of the two ; and 

 such instances do very probably occur. 



